Technical field

The present invention relates to a device and method for determination of thermal behaviour of liquid steel during its solidification in steel plants using a continuous casting process.

Prior art

Characteristic temperatures (liquidus and solidus temperatures) are crucial for an optimal management of the continuous casting process. The terms liquidus and solidus temperatures are important thermodynamic quantities which strongly depend on a particular chemical composition. Steel qualities with modified chemical compositions also change their thermodynamic behavior, including the shifts in characteristic temperatures to lower or higher values. Devices to determine phase transitions in metals are well known. Such devices are usually utilized in foundry applications, i.e. determination of carbon content in cast iron, estimation of alloy composition, etc. In the applications where they are used, huge overheating of the melt is present and a measurement can be easily conducted. This is especially true for the casting of non- ferrous alloys. WO 03/064714, WO 201 1/136062 and EP 2 674 749 A2 disclose such methods.

The problem arises when the working temperatures reach around 1400 °C or more. The integrity of the measuring cell becomes an issue because of thermal shocks and a high solubility of metal-made sensors (thin wires of Pt-PtRh) in the steel melt. Additionally, low wettability of liquid steel is also a problem because of its high surface tension. Surface tension of liquids depends mostly on the surface chemical composition rather than on the bulk composition and makes collecting the sample of various liquid steels always problematic.

This problem is detoured by measuring the temperature already in the scooping cup. Such scooping coups were disclosed for instance in WO 96/23206, WO 86/04678 and US 5,033,320.

The problem of the known cups is the fact that most of time, the macro porosity that occurs due to the presence of directional solidification in the measuring zone of the cell can lead to a faulty measurement. Porosity can also occur due to saturation of the subsurface of liquid steel with the evolved gases from disintegrated cell walls.

The problem with the determination of characteristic temperatures in the industrial environment of a melt shop is a limited access to the steel melt. When steel is still in the ladle, it is covered with a thick layer of slag, preventing a direct access to the melt. The sampling of the melt through the slag is dangerous due to the reactions occurring in the ladle and it is not easy to sample a sufficient volume of steel from the ladle, to conduct reliable measurements. So the next process where the steel melt could be accessed is during casting. The melt flows from the ladle to the tundish and further on to the mould. When in tundish, it can be most easily accessed; a thin layer of slag covering the melt has to be removed and the melt can be sampled using a scooping device.

The problem in the tundish is that superheating of the melt above its liquidus temperature is relatively low compared to superheating in the ladle. This is a problem for an accurate measurement since the melt solidifies too fast.

Shrinkage porosity is a phenomenon that occurs due to the fact that the atoms in the solid state are packed closer together in comparison with the liquid state. The problem with the common scooping cups occurs when the solidification begins in the exactly opposite direction of the highest thermal flow. This means that in most cases the shrinkage porosity occurs in the proximity of the measuring point which may result in unreliable measuring results.

Since the materials for the thermocouples are expensive (thermocouple wires), it is desired that the device for determining characteristic temperatures of the steel or at least the essential parts thereof are reusable. After the measurements are finished, the scooping device should be able to provide a sample of the solidified metal that can be used for further processing. The sample should be removed from the scooping device with ease and should not be contaminated with other unwanted elements.

Measurements taken directly from the tundish are conducted in an extremely narrow temperature frame. This is normally not the case in classic thermal analysis measurements in foundries where the transfer of the melt from the ladle to the measuring cell is possible. In this case the samples of the melt are normally heavily superheated before measuring the course of cooling. When this is not the case, it is hard to conduct a successful measurement of liquidus and solidus close to its thermodynamic equilibrium.

Solidification of the metal melt performs with a more or less pronounced temperature gradient. This impacts the state of the liquid as well as microstructural evolution. Normally, the crystal growth is opposite to the direction of the heat flow resulting in columnar crystals pushing the low melting elements and impurities towards the centre of the measuring cell. The length of the columnar grains varies with the temperature of the metal melt, the intensity of the solidification interval, the number of nuclei, etc. A homogenous structure is desired on the location of the thermocouple junction in order to study near equilibrium thermodynamic quantities. With classic thermal analysis apparatuses, as discussed above, the porosity can appear as part of insufficient melt supply, sometimes also on the lower side of the thermal junction and works as an undefined instrument time constant and changing the heat transfer to a given area of the thermal junction.

To determine the valid liquidus and solidus temperatures and with that the equilibrium solidification interval, a representative mass is needed which is a common problem when using thermo-analytical methods such as Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC). This is especially true in the cooling segments, where the problem of supercooling is often observed. Such methods are also time consuming and cannot offer on-site measurements.

Technical Problem

The technical problem solved by the present invention is how to assure accurate and repeatable continuous temperature measurements of steel melt during its solidification when sampling from a tundish.

Solution to the Technical Problem

The above mentioned technical problem is solved with the device and the method according to the present invention. The geometry of the device enables accurate measurements of the most important characteristic temperatures of the steel needed for controlling the process of continuous casting, the so-called liquidus and solidus temperatures. Too fast cooling of the steel melt in the measuring cup after sampling which leads to faulty measurement is prevented by a step of preheating of measuring cup prior to sampling. Application of exothermic powder to the top of the sample in the measuring cup decreases the cooling rate on the top of the measuring cup and shifts the formation of the shrinkage cavity away from the measuring point. The purpose of the preheating of measuring cup and the application of the exothermic powder is not to overheat the steel in the measuring cup with regard to the existing steel melt temperature in the tundish, but rather to decrease the cooling rate. Crossing the steel melt temperature could change the nucleation nature. The nucleation of the melt occurs on already existing sites of solid phases, oxides, etc. The thermocouple readings of the device are therefore virtually free of supercooling effects. The measuring point of the thermocouple is located in the central portion of the measuring cup and enables measurements with a sufficient time interval to accurately determine the needed temperature parameters.

The application of the device and the method according to the present invention allows for a low temperature gradient in the region surrounding the measuring point and the uniform chemical composition. A given geometry of the device makes it possible that a majority of the metal melt solidifies with equiaxed grains. An outwardly inclined wall of the measuring cup, the use of a cup protection element and the funnel contribute to the fact that most parts of the device can be reused.

The advantage of the present invention over the prior art is that it enables sampling from the tundish. It enables accurate measurements of the characteristic temperatures of the steel when sampling from the tundish. The invention enables accurate measurements in spite of very low superheat of the melt.

A further advantage of the device according to the present invention is its ability to be used many times with only a small amount of parts that have to be changed. The repeated usage is important since this increases the repeatability of the measuring process, which is a crucial part of any in-situ measuring system. Since the most parts of the device according to the present invention are reusable, the measurements are cost efficient. The device and the method for determining characteristic temperatures of the liquid steel taken directly from the tundish are described in more detail in the continuation.

Fig. 1 is a top view of the device.

Fig. 2 is a top view of the measuring cup with a holder and a connection of the holder to a housing for thermocouple connector cables.

Fig. 3 is a cross-sectional view of the measuring cup, with a funnel, the cup protection element and a protecting element with thermocouple junction.

Fig. 4 is a side view of the measuring cup showing the thermocouple installation from the measuring point to the holder.

Fig. 5. is a schematic presentation of a solidified sample with a shrinkage cavity location.

Fig. 6. is a diagram showing example of preheating phase and cooling curve for the measuring cup according the present invention.

Fig. 7. is a magnification of the solidification part of the cooling curve presented on the Fig. 6.

Marked parts in the figures:

1 - device for determining characteristic temperatures of the liquid steel

2 - measuring cup

2a - bottom of the measuring cup

2b - wall of the measuring cup

2c - first through hole

2d - second through hole

2e - canals

3 - protecting element

4 - fire resistant cement

5 - cup protection element

6 - funnel

7 - holder 8 - housing

9 - thermocouple

9a - junction point of the thermocouple (measuring point)

9b - wires of the thermocouple

9c - ceramic connector

11 - rod

12 - compensating cable

14 - plastic connector

15 - cardboard tube

16 - shrinkage cavity

17 - layer of exothermic powder

18 - columnar grains

19 - equiaxed grains

A device 1 for determining characteristic temperatures of the liquid steel taken directly from a tundish comprises:

- a measuring cup 2 formed with a flat bottom 2a and a cylindrical wall 2b, the inner side of which leans outwardly to enclose an angle a with the axle of the measuring cup, wherein a first through hole 2c is formed in the flat bottom 2a of the measuring cup, arranged substantially centrally to the bottom, and wherein second through holes 2d are formed through the wall 2b of the measuring cup on the opposite sides of the wall, such that the axis of the holes substantially intersect the axle of the measuring cup, and wherein canals 2e are formed on the outer side of the wall,

- a protecting element 3 in the form of a tube which is arranged in the second through holes 2d of the measuring cup, wherein the gaps between the protecting element 3 and the measuring cup 2 are filled with fire resistant cement 4, - a cup protection element 5 which is formed from a steel sheet in the shape to fit to the outer side of the wall of the measuring cup 2 and is arranged to engage the outer side of the wall 2b of the measuring cup, wherein the cup protection element 5 projects over the lower side of the measuring cup to form a space encircled by said projection which is filled with fire resistant cement 4,

- a funnel 6 that is formed from a steel sheet and arranged to engage the inner side of the wall 2b of the measuring cup and extends outwardly from it, wherein the gaps between the funnel 6 and the measuring cup 2 are filled with fire resistant cement 4,

- a holder 7that is formed with a hollow profile, preferably with a square cross- section, and is connected with the first end to the upper portion of the measuring cup 2,

- a housing 8, preferably with a circular cross-section, which is detachably connected to the second end of the holder 7,

- a thermocouple 9 further comprising a junction point 9a arranged in the central portion of the protecting element 3, wires 9b of the thermocouple, which run through the protecting element 3, through the canals 2e of the measuring cup and through the holder 7 into the housing 8, and a ceramic connector 9c arranged in the housing 8 and connected to the wires 9b of the thermocouple,

- a rod 1 1 that is formed with a hollow profile, preferably with a circular cross- section, and

- a compensating cable 12 with a connector 14 to be connected to a known data logger, wherein the compensating cable 12 is arranged in the rod 11 and connected to the ceramic connector 9c of the thermocouple.

The measuring cup 2 is made from structural steel grade. The measuring cup 2 is dimensioned in a way that the volume of the measuring cup is 0.08 1 to 0.3 1, preferably 0.1 1 to 0.2 1. The protecting element 3 is made from quartz. The aim of the funnel 4 is to change the position of the shrinkage cavity from the measuring point 9a to the upper part of the measuring cup 2 as shown in Fig. 5. The thermocouple wires 9b emanating from the protecting element 3 are protected with a fiberglass braided sleeve. The cup protection element 5 serves as a cup and thermocouple protection against the molten steel. All gaps around the cup protection element and funnel are sealed with fire resistant cement 4 to prevent the entry of the molten steel into the thermocouple area. The rod 11 functions as an extension which allows the sampler to reach the melt in the tundish. At the end of the rod, the compensating cable is connected to the data logger via additional plastic connector. The exterior of the housing 8 is additionally protected by a cardboard tube 15 to protect the connection part of the thermocouple 9 and the compensating cable 12.

A given geometry of the device makes it possible that a majority of the metal melt solidifies with equiaxed grains 19. Columnar grains 18 are only present in the region adjacent to the wall and to the bottom of the measuring cup 2.

The method for determining characteristic temperatures of the liquid steel taken directly from the tundish comprises the following steps:

- a) providing at least one device 1 for determining characteristic temperatures of the liquid steel taken directly from the tundish;

- b) connecting said device 1 to a data logger;

- c) preheating the measuring cup 2 of the device to a temperature from about 400 °C to about 900 °C, preferably from about 500 °C to about 700 °C;

- d) sampling the steel melt out of the tundish by means of the device 1 ;

- e) covering the steel melt in the measuring cup 2 with a layer 17 of exothermic powder;

- f) taking measurements of temperatures in said measuring cup over a certain period of time; and

- g) evaluating the measuring results.

The measuring cup 2 is preheated to prevent too fast cooling of the melt inside of it after sampling. The target temperature of the steel melt in the tundish is between 25 - 35 °C above liquidus temperature and if the cooling of the melt is too fast, it is impossible to get the whole cooling curve because the beginning of the solidification process might be too fast to be measured. The function of the exothermic powder is to keep the surface temperature of the steel melt in the measuring cup 2 high in order to decrease the cooling rate of the melt and to postpone the start of surface solidification. This enables the shrinkage cavity to be relocated to a higher position of the measuring cup 2 and away from the measuring point 9a in the measuring cup as shown in Fig. 5. This is very important in obtaining a reliable measuring result because the measurement can be incorrect if the shrinkage cavity 16 is located on the measuring point.

After the solidification of the steel sample in the measuring cup 2, said sample can be removed from the measuring cup by a stroke through the first trough hole 2c provided in the bottom of the measuring cup. The hardened sample can be cut near the measuring point and the chemical analysis is done near the measuring point.

To prepare the device 1 for the next sampling the protecting element 3, the cup protection element 5 and the funnel 6 need to be replaced. Wires of the thermocouple are pulled out from the housing which houses the excessive length of the wires, so the ends of the wires can be rewelded to the junction point 9a. The gaps between the measuring cup 2 and the respective replaced part 3, 5, 6 is resealed with fire resistant cement 4. The device 1 for determining characteristic temperatures of the steel is thus ready to be reused.