MXPA00005193A

MXPA00005193A - Process and device for producing a high-strength steel strip

Info

Publication number: MXPA00005193A
Application number: MXPA/A/2000/005193A
Authority: MX
Inventors: Andre Bodin; Thomas Martinus Hoogendoorn
Original assignee: Hoogovens Staal Bv
Priority date: 1997-12-08
Filing date: 2000-05-26
Publication date: 2001-12-04

Abstract

Process for producing a high-strength steel strip, in which liquid steel is cast in at least one continuous-casting machine (1) with one or more strands to form a slab and, utilizing the casting heat, is conveyed through a furnace device (7), undergoes preliminary rolling in a preliminary rolling device (10) and, in a final rolling device (14), is finishing-rolled to form a steel strip with the desired final thickness, and, in a continuous, endless or semi-endless process, the slab undergoes preliminary rolling in, essentially, the austenitic range in the preliminary device (10) and, in the final rolling device (14), is rolled in the austenitic range or, in at least one stand of the final rolling device (14), is rolled in the two-phase austenitic-ferritic range, the austenitic or austenitic,ferritic rolled strip, after leaving the final rolling device (14), is cooled rapidly in order to obtain the desired structure.

Description

PROCESS AND DEVICE TO PRODUCE A HIGH STRENGTH STEEL BAND DESCRIPTION OF THE INVENTION The invention relates to a process for producing a high strength steel strip and to a device which is suitable for carrying out the process. In a known process for producing a high strength steel strip, the starting point is a hot rolled strip which has been manufactured in conventional manner and undergoes a two stage cooling on the rolling table. In a first stage, the austenitic band is cooled until it is in the mixed range au s t i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i I i I i I i I i I i I i I i I i I i I i I i I i I i I i I i I i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i Then, the band is cooled to a high cooling speed in order to obtain a martenite structure in the band. A high-strength steel of this nature is known as the Dual Phase steel. An object of the present invention is to provide a process which provides greater flexibility in the production of high strength steel. Another object which the invention seeks to achieve is to provide a process which can be carried out using simple means. These objects and other advantages are achieved by means of a process to produce a high strength steel strip, in which liquid steel is cast in at least one continuous casting machine with one or more strips to form a plate and, using the heat of casting, is transported through a furnace device, undergoing a preliminary lamination in a preliminary rolling device, and in a final rolling device, is finished rolling to form a steel strip with the desired final thickness, and, a continuous, endless or semi-endless process, the plate undergoes a preliminary lamination and, essentially, the Semitic range in the preliminary lamination device and, in the final lamination device is laminated in the austenitic range, or so minus one position of the final rolling device, it is laminated in the range of double phase au t eni ti - ferriticay the austenitic or autnitic - ferritic laminated band, after d After leaving the final rolling device, let it cool quickly to obtain the desired structure. The process is based on a continuous, endless or semi-endless process. In a process of this nature, very good temperature control is possible both over the length, width and thickness of the plate or band. On the other hand, the 'temperature homogeneity as a function of time is very good. A device for carrying out this process is generally equipped with cooling means, so that the temperature profile as a function of the location in the installation and / or as a function of time can also be easily controlled and adjusted. An additional advantage that can be mentioned is that the process is particularly suitable for the use of a vacuum refractory tundish in order to adapt the steel composition to the desired properties which will be obtained. Due to the high level of temperature homogeneity, it is very possible to carry out the lamination in a predictable manner with accuracy in the double phase range at u s t i n i t i c a - f e r r e t i c a. Almost no or no difference in the percentage at u s t i n i t o - f e r r i t o occurs through the cross section of the band and along the length of the band. The conventional process can only comply with the level of temperature homogeneity which is required in order to obtain homogeneous properties to a limited extent or by means of special measures. Consequently, the high strength steel strip manufactured in a conventional manner has dehomogeneities in both the cross section and the longitudinal direction. One embodiment of the process according to the invention is characterized in that the strip is laminated, in the final rolling device, at a temperature at which a desired amount of ferrite is present, and in which the band leaving the final rolling device is rapidly cooled to a temperature below Ms (initiates martensite) inside of the temperature range at which martensite is formed. Due to the very good level of temperature homogeneity, it is possible to establish and maintain a desired ratio of power to the final rolling device. After leaving the final rolling device, the band is cooled rapidly, during. whose cooled austenite is transformed into martensite, resulting in a high strength band. It will be clear to the person skilled in the art that it is also possible to carry out the process in such a way that the band is fully laminated in the austenitic range and leaves the final rolling device as an austenitic band. The band laminated in this manner will also exhibit a very good level of temperature homogeneity both in the cross section and in the longitudinal direction. The conventional method for producing Dual Phase steel by means of a two stage cooling may advantageously be produced in a band of this nature. Another embodiment of the process according to the invention is characterized in that the strip is laminated, in the final rolling device, to a temperature at which a desired amount of ferpta is present, in which the strip leaving the final rolling device Cool quickly to a temperature above Ms (start of martensite) and at a cooling rate at which bain ta is formed. In this embodiment of the invention, a desired ratio between austenite and ferrite is once again created and, due to the good level of temperature homogeneity, is distributed evenly throughout the band. The selection of the cooling rate and the cooling temperature means starting from the austenite are converted to bamite, among which residual austenite remains. During the subsequent reformation of the steel band to the byproducts, the austenite generates dislocations which provide the high strength steel with the property of de orbit. The result is a steel band with high strength and high ductility. Due to these properties, these grades of steel are also known as steel TRIP (plasticity induced by transformation). The steel band is rolled in the bainita range. The total process of bainite formation and the formation of residual austenite depends on the alloying elements. It is therefore particularly advantageous, when this type of steel is produced, to make use of a vacuum refractory tundish, which allows the steel composition to be adapted to match the desired properties until the last moment before the plate is casting in the continuous casting machine. In order to obtain not only a good level of temperature homogeneity, but also a good distribution of the deformation in the cross section of the band, an additional embodiment of the process according to the invention is characterized in that in at least one position, preferably all the positions, of the preliminary rolling device and / or in at least one position, preferably each position of the final rolling device, the rolling is carried out by lubrication. Rolling by lubrication ensures that the reduction applied by the rollers is evenly distributed through that part of the steel strip or the steel plate which is located between the rollers. EP-A-0 750 049 describes a hot rolling process for the production of a dual phase steel. A combination of alloy with specific elements and the use of specific winding and cooling temperatures is described. The use of an individual online process is not described in this document, starting from the continuous casting of the liquid steel. Similar comments apply to the descriptions in US 4,790,889; US 5,470,529 and US 4,316,753. EP-A-0 370 575 also describes a method in which a steel strip is produced in a single line, starting from a continuous casting of liquid steel. This document, however, does not describe the production of a high strength steel band. The cooling of the strip is also carried out before the final rolling action instead of after and before the winding of the steel strip. The invention is also exemplified by a device for producing a steel strip, suitable in particular for carrying out a process according to the invention, comprising at least one continuous casting machine for casting thin plates, a kiln device for homogenizing a plate, which optionally has undergone a preliminary size reduction, and a rolling device for laminating the plate to form a band with the desired final thickness, and a winding device for winding the band, which is characterized in that a device Cooler with a cooling capacity of at least 2 MW / m2 is placed between the final laminator position of the rolling device and the winder device. The invention will now be explained in more detail with reference to the non-limiting mode according to the drawings, in which: Figure 1 shows a schematic side view of a device with which the process according to the invention can be carried out cape; Figure 2 shows a graph illustrating the temperature profile in the steel as a function of the position in the device; Figure 3 shows a graph illustrating the thickness profile of the steel as a function of the position in the device. In Figure 1, the reference number 1 indicates a continuous casting machine for casting thin plates. In this introductory description, this term is meant to mean a continuous casting machine for casting thin steel plates with a thickness of less than 150mm, preferably less than 100mm, and more preferably less than 80mm. The continuous casting machine may comprise one or more strips. It is also possible for a plurality of casting machines to be placed next to each other. This embodiment is within the scope of the invention. The reference number 2 indicates a ladle in which the liquid steel to be cast is fed to a refractory trough 3, which in this design is in the form of a tundish refractory to vacuum. The refractory tundish is preferably provided with means, such as measuring means, mixing means and means of analysis, for establishing the chemical composition of the steel to a desired composition, since in the present invention the composition is important. Underneath the refractory trough 3, there is a casting mold 4 inside which the liquid steel sneaks and at least partially solidifies. If desired, the casting mold 4 can be equipped with an electromagnetic brake. The standard continuous casting machine has a casting speed of approximately 6 m / min; Additional additions, such as the vacuum refractory tundish and / or the electromagnetic brake, provide the casting velocity leaflet of 8m / min or more. The solidified thin layer is introduced into a tunnel furnace 7 which has a length of, for example, 250-330m. As soon as the cast plate has reached the end of the furnace 7, the plate is cut into plate sections in a semi-endless process using the cutting device 6. A semi-endless process is understood as a process in which a number of rolls preferably more than three, more preferably more than five rolls of the standard roll size are laminated from a single plate or a plate section, in a continuous rolling process at least in the final rolling device, to provide the final thickness. In an endless rolling process, the plates, or after the preliminary rolling device, the bands are coupled together so that an endless rolling process can be carried out in the final rolling device. In a continuous process, a plate moves through the path between the continuous casting machine and exits on the side of the rolling device without interruption. The invention is explained here based on a semi-endless process, but obviously it can be used for a continuous or endless process. Each plate section represents a quantity of steel that corresponds to five or six conventional rolls. In the furnace, there is place to store a number of plate sections of this nature, for example to store three plate sections. As a result, these parts of the installation falling downstream from the furnace can continue the operation while the pouring ladle of the continuous casting machine is being changed and the casting of a new plate is about to begin, or while the machine Continuous casting has a fault, and also ensures that the continuous casting machine can continue its operation if a downstream fault arises. Storage in the furnace increases the residence time of the plate sections within it, resulting in homogenization of improved temperature of the plate sections. The speed at which the plate enters the oven corresponds to the casting speed and is therefore approximately 0.1 m / sec. Downstream of furnace 7, there is an oxide removal device 9, in this case in the form of high pressure water jets with a pressure of about 400 atmospheres, to remove by means of the jet the oxide that has formed on the surface of the plate. The speed at which the plate passes through the rust removal facility and enters the oven device 10 is approximately 0.15 m / sec. The rolling device 10, which fulfills the function of preliminary rolling device, comprises two positions of four heights, which are preferably equipped with a device for cylinder lubrication. If desired, a cutting device 8 can be included for emergency situations.

It can be seen in Figure 2 that the temperature of the steel plate, which is about 1450 ° C on leaving the refractory tundish, falls into the rolling position at a level of about 1150 ° C, and the plate is Generation in the oven device at that temperature. Intensive spraying with water in the oxide removal device 9 causes the temperature of the plate to fall from about 1150 ° C to about 1050 ° C. In the two rolling positions of the preliminary rolling device 10, the temperature of the plate drops, with each rolling step, by approximately another 50 ° C, such that the plate, the thickness of which was originally about 70mm and which was formed in two steps, with an interior thickness of 42mm, in a steel strip with a thickness of approximately 16.8 mm, it is at a temperature of approximately 950 ° C, that is in the austenitic range. The thickness profile as a function of location is shown in Figure 3 for two situations, one in which a web with a final thickness of 0.8 mm is being laminated and one in which a web with a final thickness of 1.0 mm is being laminated. The numbers indicate the thickness in mm. A cooling device 11, a set of roll boxes 12 and, if desired, an additional oven device (not shown) is accommodated downstream of the preliminary rolling device 10. The band emerging from the rolling device 10 can be temporarily stored and homogenized in the roll boxes 12, and if a further increase in temperature is required, it can be heated in the heating device (not shown) which is placed downstream. of the roll box. It will be obvious to the person skilled in the art that the cooling device 11, the roll boxes 12 and the oven device which is not shown can be in different positions one with respect to the other of those mentioned above. As a result of the reduction in thickness, the rolled strip enters the roll boxes at a speed of approximately 0.6 m / sec. . By means of the cooling device 11, the band is cooled until they are in the range of two phases at u s t in i t i ca s - f e r r i t i ca s. It is also possible that the band is not cooled, or that it is only cooled to a limited extent, or that it is heated, in order to obtain a laminated band at the end of the final rolling device 14. This cooling device can also be accommodated between the rolling positions of the final rolling installation. It is also possible to use natural cooling, optionally between the mill positions. A second oxide removal installation 13, with water pressure of about 400 atmospheres, is placed downstream of the cooling device 11, the roll boxes 12 or the oven device (not shown), for the purpose of once again remove an oxide layer which may have formed on the surface of the laminated strip. If desired, another cutting device can be included with respect to the upper part and later a band. The web is then fed into a rolling mill which can be in the form of six four-level rolling positions which are placed one behind the other and are preferably designed with a device for roller lubrication. When an austenitic band is produced, it is possible to achieve the desired final thickness of between, for example, 1.0 and 0.6 mm using only five rolling positions. The thickness which is achieved for each laminating site is indicated, for a plate with a thickness of 70 mm, in the upper row of Figure 3. After leaving the rolling train 14, the laminated band austenitic-f erritic ame nte , which then is at a final temperature of about 850 ° C and has a final thickness of 0.6 mm, it is intensively cooled by means of a cooling device 15 and wound in a winding device 16. The speed at which it enters the winding device is approximately 13-25 m / sec. A cooling device as described in the ECS final report 7210-EA / 214 can be used for cooling purposes. The content of this report is hereby construed as incorporated in the present invention. The significant advantages of this cooling device are the wide range of control, the high cooling capacity per unit surface area and the homogeneity of the cooling. The cooling 15 is adjusted and controlled depending on whether it is desired to form martensite or bainite. It is possible to start with an austenitic band and to cool it using a two-stage cooling, in which case in the first stage, the cooling is carried out until a desired amount of ferrite has formed, followed by a rapid cooling to form martensite . It is also possible for a band that has been laminated in the two phase range to rapidly cool to form martensite (curve m). It is also possible to cool an austenitic band until a desired amount of ferrite has been formed and then continue cooling in such a manner that bainite is formed with residual austenite. In addition, it is possible to laminate the band in the two-phase range, and then if necessary, continue cooling in such a way that bainite is formed with residual austenite (curve b). If appropriate, the oxide of the strip is removed in the rust removing facility 13. If the exit temperature of the rolling mill 14 is too low, it is possible, by means of an oven device 18 which is located downstream of the rolling mill, to carry a ferritic rolled strip until a desired winding temperature is reached. . The cooling device 15 and the oven device 18 can be placed next to one another or one behind the other. It is also possible to replace a device with another device depending on whether an austenitic or autenitic-ferritic band is going to be produced. A cutting device 17 is included to be able to cut the band to a desired length, corresponding to standard roll dimensions. By properly selecting the different components of the device and the process steps carried out by means of the device, such as homogenization, rolling, cooling and temporary storage, it has been proved that it is possible to operate this device using a single continuous casting machine, while that in the prior art two continuous casting machines are used in order to achieve the limited casting speed up to much higher rolling speeds which are usually used. The device is suitable for webs with a width which is between 1000 and 1500 mm and a thickness, in the case of an aust band in laminated meat iti, of approximately 1.0 mm or a thickness, in the case of a ferriti band laminated meat and approximately 0.5 to 0.6 mm. The homogenization time in the oven device 7 is approximately 10 minutes to store three plates of the length of the oven. The roll case is suitable for storing two full bands in the case of austenitic lamination.

Claims

1. Process to produce a high strength steel strip, in which the liquid steel is cast in at least one continuous casting machine with one or more strips to form a plate and, using the casting heat, is transported through of a furnace device, undergoes preliminary lamination in a preliminary rolling device and, in a final rolling device, the rolling is completed to form a steel strip with a desired final thickness, and, in a continuous process, if so or semi-endless, the plate undergoes a preliminary lamination in, essentially, the austenitic range in the preliminary lamination device and, in the final lamination device is laminated in the austenitic range, or at least in a position of the rolling device Finally, it is laminated in the range of two phase aust in iti ca - ferriti ca and the austenitic or ferritic laminated strip, after leaving the final rolling device, is cooled. quickly to obtain the desired structure.

2. Process according to claim 1, characterized in that the strip is laminated, in the final rolling device, to a temperature at which a desired amount of ferrite is present, and because the strip leaving the final rolling device is rapidly cooled at a temperature below Ms (martensite onset) within the temperature range at which martensite is formed.

3. Process according to claim 1, characterized in that the strip is laminated, in the final rolling device, to a temperature at which a desired amount of ferrite is present, and because the strip leaving the final rolling device is rapidly cooled at a temperature above Ms (martencite onset) at a cooling rate at which bainite is formed.

4. Process according to one of the preceding claims, characterized in that in at least one position, preferably all the positions, of the preliminary rolling device and / or in at least one position, preferably each position, of the final rolling device, carries out the lubrication of the rollers.

5. Device for producing a steel strip, suitable in particular for carrying out a process according to one of the above indications, comprising at least one continuous casting machine for casting thin plates, an oven device for homogenizing a plate , which optionally has passed through a preliminary size reduction, and a rolling device for rolling the plate to form a band with the desired final thickness, and a winding device for winding the band, characterized in that a cooling device with a capacity of cooling at least 2 MW / m2 is placed between the final rolling position of the rolling device and the rolling device.