Method and device for controlling the temperature of steel from the surface of the bath of a continuous casting installation up to the furnace tap

Abstract

The invention relates to a method, which is used in the area of secondary metallurgy (4) in its entirety with a final temperature-determining process step (4.1) such as a ladle furnace, for controlling the temperature of steel from the surface of the bath of a continuous casting ingot mold (1) up to the furnace tap (5) of a steel producing process. According to the invention, the temperature of the steel in the surface of the bath is controlled based on the equation TML=TLI+X° C. (X=5-15° C. and while respecting the same such that a jump in temperature (9) between the surface of the bath in the ingot mold (1) and a distributor (3) is detected according to the pouring rate (6) for a predetermined billet size. In addition, the ladle history (7) with regard to the time intervals, such as ladle full, ladle empty and ladle state, such as ladle bricking up and ladle age is detected as well as a jump in the temperature of steel between the distributor (3) and the last ladle furnace temperature (LF-ex).

Description

[0001] The invention pertains to a method and a device for controlling the temperature of the steel in a steel production process between the meniscus of a continuous casting mold and the tapping of the furnace and thus over the entire course of secondary metallurgical processing with a final temperature-determining process stage such as a ladle furnace.

[0002] For the successful casting of strands in any type of continuous casting machine with or without oscillation and preferably for strands of steel, the temperature of the steel at the meniscus is an essential factor in determining the quality of the steel (both surface quality and internal quality) and also the reliability of the casting operation.

[0003] Controlling the temperature of the steel at the meniscus of the continuous casting mold is especially important when the casting speeds are increased to 10 or 12 m/min. There are also many other important factors, however, which are important for obtaining the desired casting result, which will be discussed below.

[0004] The invention is based on the task of creating a method and a device which make it possible, regardless of these numerous factors, to plan and to control on-line the change in temperature of the melt between the temperature of the steel at the meniscus and the furnace tap.

[0005] An unexpected solution which lies outside standard technical understanding is described in the claims.

[0006] FIGS. 1-4 present the invention by way of example and in highly schematic fashion:

[0007] FIG. 1 shows the process and production chain extending between the furnace tap and the meniscus in the continuous casting mold;

[0008] FIG. 2 shows the change in the temperature campaign or the temperature curve of the steel over the course of processing in the processing chain, starting from the meniscus in the mold and proceeding via the ladle furnace to the furnace tap; and

[0009] FIG. 3 shows an example of a melt plan in the form of the two FIGS. 3a and 3b.

[0010] It is assumed, as illustrated in FIG. 1, that, depending on:

[0011] the casting speed,

[0012] the casting width,

[0013] the casting thickness,

[0014] the casting format (round, profiled, etc.),

[0015] the casting output,

[0016] the tundish design,

[0017] the degree to which the tundish is filled,

[0018] the lining of the tundish,

[0019] the technology of the tundish, and

[0020] the condition of the ladle, regardless of whether casting is carried out with

[0021] exposed casting steel and oil lubrication or with

[0022] an immersed outlet nozzle/immersion tube and casting flux, the temperature of the steel at the meniscus (2) in the mold (1) must be controlled and kept constant. In all casting situations, this steel temperature (2.1) at the meniscus of the melt in the mold (2) must be:

TML=Tli+X° C. (X=5-15° C.), Tli=f (grade of steel)  (2.1)

[0023] at a defined point which is symmetric to the shape of the mold.

[0024] In addition, it should be mentioned here that this condition of a controlled meniscus temperature (2.1) can be applied to any grade of steel.

[0025] To ensure the casting condition (2.1) regardless of the possible casting situation, exact data are collected concerning the temperature losses of the steel, starting from the meniscus (2) and proceeding via the tundish (3), the secondary metallurgy (4) with the ladle furnace (4.1) and, for example, a vacuum treatment station (4.2), all the way to the tapping (5) of a basic oxygen converter (BOF)(5.1), for example, or of an electric furnace (EAF)(5.2).

[0026] The loss of temperature which the steel experiences over the course of the processing and production chain is strongly affected by the times and temperatures of the steel and of the vessels with their refractory linings, specifically by the type and condition of those linings (exposure time, wear, specific heat transfer, etc.).

[0027] Thus, for example, the tundish (3) and the ladle (4.3) can be considered heat exchangers, the specific thermal data of which are determined by the type, age, or wear condition of their linings.

[0028] It is also taken into consideration that, at the start of casting, which is a non-steady-state phase of the process not yet in heat flux equilibrium, the heat contents of the empty tundish (3) and of the ladle (4.3), characterized by the heat-up time, the heat-up temperature, and the ladle history (7), have a significant effect on the temperature losses of the steel during its holding time in the ladle, during the transfer from the ladle to the tundish, and in the tundish itself.

[0029] In the case of a second ladle all the way to an n-th ladle within a casting sequence, the tundish is in thermal equilibrium with a constant heat loss, whereas the ladle, which can be considered an “individual”, continues to lose heat at various rates.

[0030] In the past, these temperature losses along the route from the tapping of the furnace (5) via the secondary metallurgy (4) with the ladle furnace (4.1) as the interface to the tundish (3) and finally to the area of the continuous casting machine (1.1) have been estimated on the basis of practical experience and taken qualitatively into account in the process.

[0031] For controlled casting, therefore, the starting point is the desired steel temperature TML=Tli+X° C. (2.1), and on this basis the desired steel temperature in the tundish (3.1) is determined for a planned casting speed and a planned casting format. The tundish temperature (3.1) is adjusted appropriately on the basis of the knowledge of the temperature loss (4.1.1) which occurs between the ladle furnace and the tundish. A melt plan is shown in FIG. 3; it starts with the temperature of the steel at the meniscus (2.1), expressed as the equivalent liquids temperature in the tundish T°li (3.1) with auxiliary temperatures of +5, 10, 15, 20° C. (3.1.1), as a function of the casting speed (6)—FIG. 3a—up to the delivery temperature from, for example, the ladle furnace LF-ex (4.1) in the area of the secondary metallurgy (4). The delivery temperature of the steel from the ladle furnace is determined by the ladle history (7) with its two essential factors:

[0032] the holding time of the steel in the ladle (7.1) between the time it is tapped from the furnace and its delivery from the ladle furnace for the start of casting, i.e., the “ladle full” time, and

[0033] the period of the time when the ladle is not holding any steel (7.2), i.e., the “ladle empty” time, and by the tundish history (3.2) with its important factors:

[0034] the tundish preheating temperature (3.2.1),

[0035] the tundish preheating time (3.2.2), and

[0036] the tundish turnaround time (3.2.3) with its specific preparation steps,

[0037] FIG. 4 shows a suitable ladle identification system (8) based on the encoding of the natural radiation of the ladle wall by a perforated metal plate (8.1.1) and the detection of that radiation by a radiation sensor (receiver) (8.1.2).

[0038] FIG. 1 shows the entire process and production chain, which extends between the tapping of the furnace (5) and the continuous casting mold (1). So that the temperature of the melt, starting from the desired temperature of the steel at the meniscus (2.1), can be planned and controlled on-line, first for the tundish and then for the ladle furnace LF-ex, it is necessary to know:

[0039] the casting parameters, which here are:

[0040] the dependence of the temperature of the steel in the tundish on the casting speed (6), described in the TT/VC system (10) (see FIG. 2), for a given casting output or holding time of the steel in the tundish;

[0041] the history (3.2) of the tundish, comprising its preheating temperature (3.2.1), preheating time (3.2.2), and the tundish turnaround time (3.2.3) with the various possible preparation steps such as relining, spraying, drying, etc.; and

[0042] the history of the ladle (7), comprising the holding time of the steel in the ladle (7.1) and the remaining ladle time (7.2), during which no steel is in the ladle, and the condition (7.3) of the ladle, which is determined by:

[0043] the age of the ladle lining (wear) and

[0044] the type of ladle lining.

[0045] This history of the ladle (7) thus consists of:

[0046] the “ladle full” time, i.e., the time between the start of the tap and the delivery from the ladle furnace with a maximum duration until the start of casting of, for example, 25 minutes (7-1);

[0047] the “ladle empty” time (7.2), and

[0048] the ladle condition (7.3), where the

[0049] “ladle full” time (7.1) and

[0050] “ladle empty” time (7.2) constitute the standard ladle cycle time (7.4).

[0051] During the “ladle empty” time (7.2), the ladle passes through various treatments such as cleaning (7.2.1) and the preparation of the slide valve for the next use (7.2.2). In addition, the ladle can also be taken out of the ladle cycle so that it can be heated on a heating stand (7.5) during long empty periods or placed on a ladle lining stand (7.6) so that it can be given a new lining.

[0052] To record all the numerous factors which act on the ladle and thus ultimately determine the temperature loss (4.1.1) between the ladle furnace and the tundish, the ladle identification system (8) is installed in the ladle cycle (7.4) at the locations relevant to the determination of the ladle history (7). The detection system works preferably by means of the encoding of the ladle radiation (8.1) and is based on the natural radiation of the ladle. A measuring system of this type can record the movement and the differentiated time sequences in the “ladle full” (7.1) and “ladle empty” (7.2) phases, so that the temperature losses (4.1.1) between the LF and the tundish can be put into functional relationships with each other on-line on the basis of the data thus acquired.

[0053] A procedure similar to that for the ladle is also appropriate in the area of tundish management (3.2), where special attention should be paid to the start of casting of a casting sequence consisting of several ladles. Here the history of the tundish (3.2) is superimposed additionally on the temperature loss (4.1.1) between the LF (4.1) and the tundish (3); this history consists of:

[0054] the preheating temperature of the tundish (3.2.1),

[0055] the tundish preheating time (3.2.2),

[0056] the tundish turnaround time (3.2.3),

[0057] the tundish drying time and temperature (3.2.4), and

[0058] the tundish lining (3.2.5).

[0059] FIG. 2 shows the change in the temperature of the melt as it travels from the electric furnace (5.2) or BOF (4.1) via the ladle furnace LF-ex (4.1) to the meniscus (2) in the mold (1). The desired temperature of the steel at the meniscus (2.1) is the basis on which the temperatures in the tundish and in the ladle furnace LF-ex (4.1) are planned.

[0060] The discontinuity in temperature between the meniscus and the tundish (9), i.e., the decrease in the temperature of the steel between the-tundish and the meniscus, is determined essentially by the casting speed (6) and the casting output, that is, the solidification thickness and casting width.

[0061] The discontinuity in the temperature between the tundish and the ladle furnace (4.1.1), however, is determined essentially by the ladle history (7) and the tundish history (3.2).

[0062] FIG. 3 is divided into parts 3a and 3b. FIG. 3a shows the tundish temperature (3.1) TT versus the casting speed (6) VC for the equivalent liquidus temperature of the steel (3.1.1) in the tundish T°li with the auxiliary temperatures (3.1.1.1′) T°li+5, 10, 15, 20° C.

[0063] If it is planned to cast at a rate of, for example, 5 m/min, then a tundish temperature (10) of 1,560° C is obtained, which is to be considered the ideal steel temperature (10.1)

TT=Tli+10° C. (VC=5 m/min)

[0064] in the tundish for a grade with Tli=1,530° C.

[0065] To arrive at the ideal temperature (10.1) of, for example, 1,560° C., the discontinuity in the temperature (4.1.1) of the steel between the tundish and the ladle furnace LF-ex (4.1) is determined as shown in FIG. 3b.

[0066] Here the temperature loss (4.1.1) is shown versus the time factor “steel in the ladle (7.1)” for the start of casting in the case of

[0067] a tundish preheating temperature of 1,100° C. (3.2.1.1),

[0068] a tundish preheating temperature of 1,200° C. (3.2.1.2), and also for a 2nd and an n-th ladle (3.2.1.3) of a sequence.

[0069] The figure makes it clear how the temperature losses change as a function of the holding time of the steel in the ladle (7.1) and the tundish preheating temperature (3.2.1), both for the first ladle of a sequence and for all the subsequent ladles. These temperature losses also depend on the remaining part of the ladle history (7) and can be determined by means of the on-line method described here.

[0070] FIG. 4 shows the ladle identification system (8), which works with a perforated metal plate (8.1.1) to encode the natural radiation (8.1). The encoded radiation is then detected by one or more radiation sensors (8.1) located at strategic points in the ladle cycle (7.4).

[0071] These descriptive process-related and device-related features lead to controlled casting, which is characterized by:

[0072] good, maximized product quality,

[0073] high casting reliability with

[0074] high casting speeds and thus

[0075] high productivity.

List of Reference Numbers

[0076] 1 1 Continuous Casting Mold 1.1 continuous casting machine 1.2 measuring means 2 Meniscus in the Mold 2.1 planned temperature window of steel at the meniscus, TML = Tli + x° C. (x = 5-15° C.) 3 Tundish 3.1 temperature of the steel in the tundish, TT 3.1.1 equivalent liquidus temperature in the tundish, T°li 3.1.1.1 auxiliary temperatures T°li + 5, 10, 15, 20° C. 3.2 tundish history, tundish management 3.2.1 tundish preheating temperature 3.2.1.1 tundish preheating temperature of 1,100° C. 3.2.1.2 tundish preheating temperature of 1,200° C. 3.2.1.3 2nd to n-th ladle of a sequence 3.2.2 tundish preheating time 3.2.2.1 tundish preheating stand 3.2.3 tundish turnaround time 3.2.4 tundish drying time and temperature 3.2.5 tundish brick lining 3.2.6 tundish spray lining 3.n measuring means 4. Secondary Metallurgy 4.1 final temperature-determining process stage such as a ladle furnace, LF-ex 4.1.1 temperature loss or jump between ladle furnace and tundish 4.2 vacuum treatment stand 4.3 ladle 4.4 computer 5 Tapping from Furnace to Ladle 5.1. converter, BOF 5.1.1 oxygen lance 5.2 electric furnace 6 Casting Speed, VC in m/min 7 Ladle History 7.1 holding time of the steel in the ladle from the tapping of the furnace to the delivery of the ladle furnace (LF-ex) or until the “opening” of the ladle (start of casting), “ladle full” time 7.2. time during which the ladle contains no steel, “ladle empty” time 7.2.1 cleaning of the ladle, bottom, nozzle bricks, slide valve 7.2.2 preparation of the slide valve, inspection, packing the slide valve sand, etc. 7.3 ladle condition, age and type of lining 7.4 ladle cycle time 7.4.1 ladle cycle, ladle turnaround 7.5 ladle heating status 7.6. ladle brick lining status, new spray lining 8 Ladle Identification System 8.1 individual ladle with encoded radiation system, based on the natural radiation of the ladle body, individual ladle 8.1.1 coded metal plate 8.1.2. radiation sensor 9 Temperature Jump steel at the meniscus/steel in the tundish or temperature loss of the steel between the tundish and the meniscus, function of the casting speed (VC) (6), tundish design, etc. 10 Distributor Temperature, TT in ° C. 10.1 ideal target temperature TT-ideal = T°li (3.1.1) + 10° C. 10.1.1 ideal target temperature TT-ideal = T°li (3.1.1) + 10° C. for VC = 5 m/min 10.2 real temperature window TT-real = T°li (3.1.1) + 10° C. ± 5° C. 10.2.1 real temperature window TT-real = T°li (3.1.1) + 10° C. ± 5° C. for VC = m/min 11 Rolling Mill

Claims

1. Method for controlling the temperature of the steel in a steel production process between the meniscus of a continuous casting mold (1) and the tapping of the furnace (5) and thus over the entire course of secondary metallurgical processing (4) with a final temperature-determining process stage (4.1) such as a ladle furnace, characterized in that

the temperature of the steel at the meniscus of TML=Tli+X° C. (X=5-15° C.) is controlled and maintained under consideration of the fact that there is a discontinuity (9) in the temperature between the meniscus in the mold (1) and a tundish (3) as a function of the casting speed (6) for a strand of predetermined format; and in that

the ladle history (7), comprising the time periods “ladle full” and “ladle empty”; the ladle conditions such as the ladle lining and the age of the ladle; and the discontinuity in the temperature of the steel between the tundish (3) and the last ladle furnace temperature (LF-ex) is recorded.

2. Method according to claim 1, characterized in that the ladle history (7) is recorded in a time-differentiated manner by an individual ladle identification system (8), which is installed in the ladle cycle.

3. Method according to claim 1 or claim 2, characterized in that the natural radiation of the ladle skin, which is relatively hot in comparison with the ambient temperature, is used as a means of detecting the individual ladles.

4. Method according to one of claims 1-3, characterized in that the ladle identification system (8) differentiates at least between the “ladle full” time, the “ladle empty” time, and the “ladle with new lining” time.

5. Method according to one of claims 1-4, characterized in that the history of the tundish is differentiated with respect to the tundish preheating temperature, the tundish lining, the tundish drying time, and the tundish service life data to allow for the more accurate determination of temperature discontinuities between the meniscus and the tundish and between the tundish and the ladle furnace (LF-ex).

6. Device for controlling the temperature of the steel in a steel production process between the meniscus of a continuous casting mold (1) and the tapping of the furnace (5) and thus over the entire course of secondary metallurgical processing (4) with a final temperature-determining process stage (4.1) such as a ladle furnace, especially for implementing the method according to claim 1, characterized in that measuring means (1.2 or 4.3) are provided for detecting the temperature of the steel at the meniscus of the mold (1) in a temperature window (2.1) of TML=Tli+X° C. (X=5-15° C.) or for detecting the temperature of the steel in the tundish (3) from TT-ideal=T°li+10° C. (10.1) or TT-real=T°li+10° C.±5° C. (10.2) as a function of the casting speed (6), measuring means (3.n) also being provided for detecting the tundish preheating time (3.2.2) and the tundish preheating temperature (3.2.1); in that a ladle identification system (8) provided inside the ladle cycle (7.4) or the ladle turnover (7.4.1) records the ladle history (7), where the measuring means are connected to a computer (4.4), which uses the measurement values to determine on-line the temperature discontinuities (9 and 4.1.1) between the meniscus (2) and the tundish (3) and between the tundish (3) and the ladle furnace (4.1), respectively.

7. Device according to claim 6, characterized in that the ladle identification system (8) works with a coded metal plate (8.1.1) and makes use of the natural radiation of the ladle, where the metal plate is mounted a certain distance away from the outside surface of the ladle (4.3) and conveys the time and position data of each individual ladle (8.1) (ladle as individual entity) to at least one radiation-measuring device (8.1.2) installed in the ladle cycle (7.4.1).

8. Device according to claim 6 or claim 7, characterized in that the ladle detection system (8) is designed to detect at least the “ladle full” time (7.1), the “ladle empty” time (7.2), and the “ladle with a new lining” time.