DE102009052172B4 - Method for controlling the temperature profile and the speed of a cast strand in a continuous casting plant and continuous casting plant for carrying out the method - Google Patents

Abstract

Method for determining properties, in particular surface properties, of a cast strand (4) in a continuous casting plant using a laser source (13) for generating a laser beam directed at the surface of the cast strand (4) and a measuring device (10) for evaluating signals reflected from the surface of the cast strand (4), characterized in that the laser source (13) shines a laser beam through an inner region of a transmitting coil (17) filled with a dielectric (16) for generating an alternating magnetic field for inducing eddy currents in the cast strand (4), and that the laser beam reflected from the surface of the cast strand (4), after passing through the dielectric (16) and a beam splitter (14) and a polarization filter (18) again, generates a signal in an optical sensor (19) which provides information about the surface properties of the cast strand (4) under the influence of the alternating magnetic field, and that the sensor (19) based on the measured laser light intensity Generates output signals which it forwards to a controller (11) of a control loop for controlling process variables so that the temperature or the speed of the cast strand (4) are controlled accordingly, wherein the reflected laser beam reproduces information from the cast strand (4) which is obtained by the generation of the eddy currents.

Description

The invention relates to a method for determining properties, in particular surface properties, of a cast strand in a continuous casting plant using a laser source for generating a laser beam directed onto the surface of the cast strand and a measuring device for evaluating signals reflected from the surface of the cast strand.

Furthermore, the invention relates to the quality optimization during the ongoing production operation of continuous casting plants based on the determined surface properties in the cast strand.

After a cast product has left the continuous casting plant, the slab or block or billet separated from the cast strand is then fed to the next processing step in the further course of production, particularly after a cooling process that can take several days. The products are usually subjected to an inspection to check the quality produced.

This allows defects to be identified such as surface cracks, internal cracks and segregation cracks that are on or below the product surface. The defects are documented so that the product can be repaired if necessary. The inspection is carried out using various methods, such as a visual inspection, sampling or surface treatment in order to carry out a surface or internal quality control. The product to be examined must have cooled to a certain temperature, for example below 100 °C, in order to be able to carry out the necessary inspection processes. An inspection is not possible with the CSP process (= Compact Strip Production), i.e. the process for casting thin slabs in which the product produced is fed directly to the next processing step.

From the DE 29 11 578 A1 A method and a device for checking the surface of a moving metal strip in a continuous casting process have become known. The aim is to provide a compact device and a method for determining overlaps of the cast strand that occur on a surface of a steel strip, such as a steel plate or a steel bar, at any desired location or preferably in a cooling chamber during the continuous casting process, without any significant impairment by cooling water or cooling steam. The known device contains a light generating device with a light source that emits light rays with short wavelengths or other waves. A light detector with a filter is also provided, which allows the passage of light rays that only have short wavelengths. In addition, a flat hood is provided, which surrounds a light path running between the light generating device and the light detector. Finally, an image storage device is provided, to which an electrical signal is fed. This characterizes the shadow of a steel strip in the continuous casting process, which is fed to the light detector in the case that the areas of a light path are cleaned by compressed air which is introduced into the hood, the shadow of the steel strip being continuously displayed on a television monitor system after processing by the image storage. In this way, any overlaps present on the surface of the steel strip are determined. The main part of a strip surface inspection device can be housed in a cooling chamber. The outer end of the flat hood is located between two adjacent support rollers which face a steel strip which is produced in the continuous casting process.

This known method is intended to display the surface condition of a steel strip and to perform continuous monitoring of the wide surface area of the steel strip on the television monitor system. Moreover, the surface condition of the steel strip is to be examined at an initial stage of the steel strip movement, whereby the causes leading to the occurrence of overlaps can be quickly eliminated by feeding data resulting from the inspection to a control section in order to adjust the casting speed of the steel strip. The strip surface inspection device is intended to enable the surface of a steel strip to be inspected during the continuous casting process.

From the WO 91 17009 A1 A method for continuously inspecting a metal cast strand has become known. In this method, a laser source is used to generate a pulsed laser beam that is irradiated onto the strand pulled out of the continuous casting mold and generates an acoustic wave in the strand or on the surface. Means are also provided that detect the interaction of the acoustic wave with the strand and generate corresponding signals. A device is also provided for analyzing the waves and generating signals that characterize the condition of the cast strand. Several laser sources can also be arranged on the periphery of the strand. The detector can also comprise an interferometer.

According to the WO 91 17009 A1 It is proposed to control the water flow in the continuous casting mould area, the secondary cooling, the casting speed, the roller contact pressure and the energy consumption based on the information obtained from the measurement of the surface waves.

Out of JP H08 62185 A and CN 2 720 440 Y Non-destructive methods for defect detection using eddy currents are known.

It is the object of the invention to provide a method for use in a continuous casting plant and a continuous casting plant itself in which the material properties of the cast strand can be determined in a simple manner and the casting process is continuously optimized on the basis of the data obtained.

In a method of the type mentioned above, this object is achieved in that the reflected laser beam reproduces information from the cast strand which is obtained by generating an eddy current in the cast strand.

The aim of the invention is to present a novel method with corresponding devices for direct quality determination, even under so-called hot conditions, which uses a contactless measuring method to provide information about the quality produced, i.e. the surface or internal quality, of a steel strand for the process-automated control system or the operator directly during production or shortly after production, in order to then define the further production steps without the need for a material-destructive inspection. Furthermore, timely indications of a drop in quality with the defect thus localized with the corresponding location and material assignment should be obtained.

In the last-mentioned application, which also applies to CSP plants, a direct adjustment of product parameters, for example an adjustment of the casting speed, the change of the cooling water quantities, the adjustment of the casting powder allocation, the adjustment of the soft reduction parameters, is to be carried out in a targeted manner and based on experience, using an action plan according to the invention, which can also be carried out partially automatically in the automation system.

This results in several advantages: Compared to the state of the art, fewer manual inspections of both the cooled product and the running strand are required. Immediate and early quality control is possible, which leads to fewer losses and fewer product rejects. Material depreciation occurs to a lesser extent.

Process steps for further processing can be initiated without any time delay, such as that caused by a quality inspection that is only possible after cooling. This is especially true for hot charging of the products. The surface quality of the preliminary product can be improved at an early stage, particularly when used in CSP systems.

The invention is characterized by a non-contact and non-destructive measuring method for determining the product surface and product interior quality at a surface temperature of the product that is, for example, in the range between 400 and 1000 degrees Celsius.

The measurement is carried out on the material passing by during production; it is not necessary to stop the product for testing or measurement. The measurement is preferably carried out on the broad sides of the product, i.e. on slabs, thin slabs, billets and block systems. In the case of profiles, the measurement is carried out on the surfaces that are accessible for measurement. In the case of round or multi-edge products, the measurement can take place on all accessible surfaces.

The recorded error situations are visualized for the operator at the control center or at appropriately designated operating stations, particularly in comparison to recorded good situations or to a still acceptable quality result.

Process engineering specifications, known as routing, are derived for further processing, e.g. sorting out products with identified frequent defects or direct further processing of the products, defined treatment steps for minor defects, direct further processing for defect-free products, depending on the determined test status. From this, an action plan for systematic quality improvement is derived for the operator when defects are identified.

The action plan for error detection, which can also be partially automated, includes systematic quality improvement for the operator, for example adjustment of the casting speed, cooling or hydraulic adjustment values for the segment adjustment.

According to the invention, a preferably stationary measuring device and a likewise preferably a stationary measuring system is installed. Both are arranged, for example, behind the strand guide in an area where the cast product has not yet passed through the separating or cutting device, for example behind a bending driver or in or behind the straightening driver or immediately before the separating device.

Alternatively, the stationary measuring device and the measuring system can be arranged in a subsequent finishing device, with the product moving past the measuring device while still hot.

The number of measuring sensors used to measure one side of the product depends on the product dimensions and the detection range of the sensor. This means that several sensors arranged next to each other can cover an extended measuring range.

Preferably, the continuous casting plant also includes an automation system for data acquisition and data processing, for quality display and for recording and processing the inspection data or the measurement signals from one or more measuring sensors. Electronic storage devices can also be provided for the operator's data backup and measurement data archiving, with which the values measured during production can be fully documented.

From the data obtained from the cast strand, an automated execution specification for the further processing of the product can be derived if necessary.

An automated action plan can also be derived to improve product quality, preferably the gradual adjustment of the casting speed, the gradual adjustment of the cooling or the gradual adjustment of the soft reduction parameters.

Advantageous developments of the invention emerge from the subclaims and the description.

Overall, the invention provides a method according to which signals are generated in a detector on the basis of the reflected laser signals and fed to a controller of a control loop for controlling process variables, in particular the cooling water flow, the roller contact pressure, the mold powder feed, the energy supply, the temperature and/or the speed of the cast strand, and the temperature or the speed of the cast strand are controlled accordingly.

Preferably, the method is characterized in that the laser source shines a laser light beam through an inner region of a transmitting coil filled with a dielectric to generate an alternating magnetic field for inducing eddy currents in the cast strand, and that the laser beam reflected from the surface of the cast strand, after passing through the dielectric and a beam splitter and a polarization filter again, generates a signal in an optical sensor that provides information about the surface properties of the cast strand under the influence of the alternating magnetic field, and that the sensor generates output signals based on the measured laser light intensity, which it forwards to the controller.

This detection method uses the Faraday effect, as used in magneto-optical eddy current testing systems. In a conventional manner, an alternating magnetic field is generated without contact by an excitation and transmission coil that is operated with an alternating current and placed near an electrically conductive substance. This penetrates the material and induces eddy currents in it. The distribution and strength of the eddy current depend on the electrical and magnetic properties of the substance to be examined. The eddy current also generates an alternating magnetic field, which weakens the field of the transmission coil according to Lenz's law. This weakening could be detected with a receiving coil placed inside the transmission coil. However, the receiving coil can be replaced by another sensor with a high lateral resolution, i.e. a high resolution in the plane of the cast strand to be examined, which shows the Faraday effect. This property is possessed by an optical component that rotates the polarization direction of a light wave depending on a magnetic field. The rotation of polarization is a property of some crystals. The Faraday rotator consists of such a crystal in conjunction with an electromagnet surrounding it, the transmitting coil. In the sense of the invention, the resolution-increasing effect is that the rotation experienced by a polarized light beam passing through the crystal is always doubled when it passes through the rotator a second time, regardless of the direction of travel of the light.

The invention also relates to a further method for determining properties, in particular surface properties, of a cast strand in a continuous casting plant using a laser source. In this case too, laser pulses are directed onto the surface of the cast strand; and a measuring device for evaluating of signals reflected from the surface of the cast strand.

According to the invention, this method is characterized in that acoustic surface waves are generated in the cast strand, which are received by a detector. The method is based on the excitation of acoustic surface waves in samples to be examined. They are generated by short laser pulses, in particular those that impinge perpendicularly on the sample surface, and after traveling a selectable distance are received by a suitable detector, preferably in the frequency range up to 200 MHz. The resulting sound dispersion can then be used to draw conclusions about the elastic properties in the layer being examined. This means that layers with a thickness of 500 nm or less can be examined on substrates with low sound attenuation. Only the layer thickness and the Poisson's ratio of the material of the cast strand must be known.

Preferably, a piezoceramic detector is used to receive the acoustic surface waves. This measuring method can be used to examine the elastic modulus of the cast strand up to a thickness of several micrometers. This means that not only the surface quality of the cast strand is determined, but also the internal quality and condition. The sound travel times are measured. The edge layers of the cast strand can be characterized in this way, and in particular an elastic anisotropy of the bulk material can also be determined if it exists.

Advantageously, the values measured by the measuring device or the detector are displayed on a monitor in a control room of the continuous casting plant.

Advantageously, the method is also characterized in that the temperature is regulated by the supply and the amount of coolant in the region of the strand guide segments.

The invention also relates to a continuous casting plant for carrying out a method as described above. According to the invention, the continuous casting plant is characterized in that the continuous casting plant comprises a control device for controlling the temperature or the speed of the cast strand based on the signals generated by the measuring device or the detector.

• 1 eine schematische Darstellung einer Stranggießanlage zum Gießen eines Metallstrangs mit einem integrierten Messgerät und
• 2 das Messgerät im Detail.

The invention is explained in more detail below in an embodiment with reference to the drawings. These show:

• 1 a schematic representation of a continuous casting plant for casting a metal strand with an integrated measuring device and
• 2 the measuring device in detail.

A continuous casting plant for casting thin slabs comprises a ladle 1 ( 1 ), from which liquid metal, in particular liquid steel, is transferred via an intermediate container 2 into a mold 3. A hot strand 4 is produced from the mold 3 and is guided through a strand guide 5 with segments and rollers (not shown here). The hot strand is guided further via driver units 6, 7 to a separating device 8 in the direction of an arrow A. In the separating device 8, individual slabs 9 are produced from the hot strand 4.

For example, between the last driver unit 7 and the separating device 8, a measuring device 10 for measuring the material or structural properties of the hot strand 4 is arranged without contact with the strand surface and preferably also with an adjustable angle relative to the hot strand 4. The data obtained by the measuring device 10 is fed to a data acquisition, processing, evaluation and archiving as well as control or regulating unit 11. Process data and/or operating parameters are fed to this from a control station (not shown here) via data lines, so that the control or regulating unit 11 evaluates these taking into account the data transmitted by the measuring device 10 in order to determine operating variables such as the supply of liquid metal in the area of the ladle 1 and the intermediate vessel 2, the cooling in the area of the mold 3, the secondary cooling in the strand guide 5 or in a downstream cooling unit (not shown), the contact pressure of guide or driver rollers in the driver units 6, 7 and other operating variables.

Data from the control or regulating unit 11 are forwarded to a monitor 12 and visualized for the operator.

The measuring device 10 ( 2 ) comprises, for example, a Nd:YAG laser 13 with a wavelength of 532 nm as a source for generating polarized light. The laser beam generated by the laser 13 passes through a beam splitter 14 and then into a Faraday rotator 15, which comprises an optically active crystal 16 (dielectric) and a coil 17 surrounding it. A particularly high-frequency alternating current is applied to the coil 17 to generate a magnetic field, which in turn generates eddy currents in the hot strand 4 passing under the measuring device 10. The eddy currents in turn generate magnetic fields, which together with the magnetic field of the coil 17 polarize the light reflected by a separate mirror layer or from the surface of the hot strand 4 itself reflected laser beam.

The laser beam reflected back then passes through the beam splitter 14 through a polarization filter 18, which only allows light of a specific polarization direction to pass through. Only the component of the reflected laser beam that has this polarization direction is allowed through and detected by an optical sensor 19, for example a photodiode or a CCD camera. This converts the magnetic field distribution inside the coil 17, i.e. in the crystal 16, into a light intensity distribution that provides information about the properties of the hot strand 4 in the area of the surface and below the surface. It is also possible to tilt the entire measuring device or just the Faraday rotator 15 at a specific angle relative to the surface of the hot strand 4 in order to carry out investigations for different angle ranges.

List of reference symbols

1

ladle

2

Intermediate container

3

Mould

4

Hot strand

5

Strand guide

6

Driver unit

7

Driver unit

8th

Separator

9

Slabs

10

Measuring device

11

Control and regulation unit

12

monitor

13

Nd:YAG laser

14

Beam splitter

15

Faraday rotator

16

crystal

17

Kitchen sink

18

Polarizing filter

19

sensor

Claims (7)

Method for determining properties, in particular surface properties, of a cast strand (4) in a continuous casting plant using a laser source (13) for generating a laser beam directed at the surface of the cast strand (4) and a measuring device (10) for evaluating signals reflected from the surface of the cast strand (4), characterized in that the laser source (13) shines a laser beam through an inner region of a transmitting coil (17) filled with a dielectric (16) for generating an alternating magnetic field for inducing eddy currents in the cast strand (4) and that the laser beam reflected from the surface of the cast strand (4) generates a signal in an optical sensor (19) after passing through the dielectric (16) and a beam splitter (14) as well as a polarization filter (18) again, which signal provides information about the surface properties of the cast strand (4) under the influence of the alternating magnetic field and that the sensor (19) on the basis of the measured laser light intensity Generates output signals which it forwards to a controller (11) of a control loop for controlling process variables so that the temperature or the speed of the cast strand (4) are controlled accordingly, wherein the reflected laser beam reproduces information from the cast strand (4) which is obtained by the generation of the eddy currents. Procedure according to Claim 1 , characterized in that the cooling water flow, the roller contact pressure, the casting powder supply, the energy supply, the temperature and/or the speed of the casting strand (4) are controlled as process variables. Procedure according to Claim 1 or 2 , characterized in that acoustic surface waves are generated in the cast strand (4) and are received by a detector. Procedure according to Claim 3 , characterized in that the acoustic surface waves are received by a piezoceramic detector. Procedure according to one of the Claims 1 until 4 , characterized in that the values measured by the measuring device (10) or by the detector are displayed on a monitor (12) of a control station of the continuous casting plant. Method according to one of the Claims 1 until 5 , characterized in that the temperature is regulated by the supply and the quantity of coolant in the region of the strand guide segments. Continuous casting plant for carrying out a process according to one of the Claims 1 until 6 with a laser source (13) for generating a laser beam directed onto the surface of the cast strand (4) and with a measuring device (10) with a Detector for evaluating signals reflected from the surface of the cast strand (4), characterized in that the detector is an optical sensor (19), that the laser beam can be radiated through an inner region of a transmitting coil (17) filled with a dielectric (16) for generating an alternating magnetic field for inducing eddy currents in the cast strand (4), and that the laser beam reflected from the surface of the cast strand (4) can, after passing through the dielectric (16) and a beam splitter (14) and a polarization filter (18) again, generate a signal in the optical sensor (19) that provides information about the surface properties of the cast strand (4) under the influence of the alternating magnetic field, and that the sensor (19) generates output signals based on the measured laser light intensity, which are sent to a control device (11) included in the continuous casting plant for controlling the temperature or the speed of the cast strand (4) based on the signals generated by the measuring device (10). can be supplied.

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