EP3488948A1 - Method for analysing causes of errors in continuous casting - Google Patents

Abstract

A method for determining the cause of casting defects in products (S) cast in a continuous casting installation (100), in particular steel slabs, which comprises: a) determining an error (r1-r10) in a continuously cast product (S); b) determining process parameters under which the product (S) was cast; c) recalculating the casting according to the process parameters using a simulation model; d) Determining one or more positions in the continuous casting installation (100) at which the error (r1 - r10) probably occurred.

Description

Technical area

The invention relates to a method for determining the cause of casting defects in cast in a continuous casting plant products, in particular slabs of steel.

Background of the invention

In the continuous casting of metals, especially steel, cracks in the material can occur during the cooling and solidification of the strand, which leads to quality losses or even rejects. The FIGS. 1a and 1b schematically show different types of material defects, which can be roughly distinguished in internal and surface cracks.

In the FIGS. 1a and 1b reference symbol S denotes a cast strand, slab, billet, billet or round. The FIG. 1a shows various forms of internal cracks, such as so-called gusset cracks and triple-point r1, narrow-side cracks r2, half-way cracks r3, segregation cracks r4, off-corner / corner cracks r5 and near-surface cracks r6. Typical surface cracks are in the FIG. 1b shown, such as transverse Edge cracks r7, longitudinal edge cracks r8, lateral cracks r9 and longitudinal cracks / depressions r10.

Analyzing causes of faults leading to the above and other material defects is difficult and time consuming with continuously molded products. The WO 2009/149680 A1 describes a method for predicting the formation of longitudinal cracks in continuous casting. The EP 2 150 362 B1 describes a method for detecting and classifying surface defects on continuously cast slabs.

However, detecting and predicting cracks in the material is in many cases not sufficient to determine the cause of the fault. For example, information about the place of origin of the cracks, stress on the strand and weakening of the material in the continuous casting plant are missing. In order to determine the cause of the errors or the mechanism of origin of errors in a specific case, it may also be helpful to link the process data, information on the system status, material-specific information, and so on.

Presentation of the invention

An object of the invention is to further improve the quality of continuously cast products, in particular steel slabs.

The object is achieved with a method having the features of claim 1. Advantageous developments follow from the dependent claims, the following description of the invention and the description of preferred embodiments.

The method according to the invention serves to determine the causes of casting defects in products cast in a continuous casting plant. These Formulation implies that the method according to the invention at least contributes to or facilitates finding causes that can lead to casting defects. The term "defect" here includes material defects of various types which have been caused during the casting, in particular internal cracks and surface cracks. However, the method may also help to identify causes of other defects, such as contaminants, inhomogeneities in the material, etc., as long as the defects are the result of the casting process in the continuous casting plant.

The molded product is preferably a cast steel slab, but may be another stranded product of a metal, especially a metal alloy.

According to the method of the invention, a defect in the continuously cast product is detected in a first step a). So the focus is not on how errors can be detected manually or automatically, but this is the starting point. As a rule, defects, such as cracks, are found in micrographs of the product. It should be noted that, of course, the method also includes those cases where multiple defects are found in the continuously cast product. If the singular number is chosen here, then this is only for linguistic simplification.

In a second step b), process parameters are determined under which the product having the previously found fault has been cast. The relevant process parameters may include one or more of the following parameters: molded product dimension, molded product material, cooling water distribution, cooling water quantity, casting speed, superheat, casting level. The aim is to determine those process parameters that are required for the recalculation of the casting according to the following step c). Typically, the Process parameters for all cast products stored in a database or file, so that the determination in this case results in a read out of the process parameters from the database or file.

The process parameters determined in this way are now used as input variables with which the casting of the product that has the defect is recalculated by means of a simulation model. This is the above-mentioned step c). For example, the simulation model can theoretically simulate the temperature distribution in the product during casting. Alternative or further output variables may be the solidification profile and / or the position of the sump tip and / or the hot cracking temperature range (brittle temperature range) and / or the ductility of the cast strand.

The theoretical recalculation of the casting is the basis for determining or identifying one or more positions in the continuous casting plant in a further step d), at which the defect is likely to have arisen. The position can be determined particularly reliably if the simulation result is linked to properties of the fault found in step a). For this purpose, the error after step a) is preferably classified and / or characterizing features are determined, preferably the location of the defect in the product, the shape and / or size of the defect, such as the length of the crack. The position of the error can thus be read or calculated. According to a preferred embodiment, after step a), the geometry of the defect is localized in cross section, and then the cross section is located in the product. Preferably, the position of the error in the continuous casting is drawn in a graphical representation of the casting process. However, it may also be sufficient to store the determined position so that it can be assigned to the place of origin in the continuous casting plant, for example the relevant segment.

According to the described method, the cause of faulting during casting can be found and the slab quality in future castings can be improved. So it is now possible in a series casting with the same or similar process parameters, by changing the process values, e.g. the cooling and / or casting speed to reduce the risk of failure at the critical positions.

If the position of fault formation in the continuous casting plant is known, the process parameters can be checked at this point. Preferably, therefore, in a further step e), which follows the step d), the cause of the error is determined. The cause may be, for example, too much or too little cooling at the position in question, too much bulging or stretching of the strand-shaped product, a wrong casting speed or even an alloying error, etc.

Preferably, on the basis of the thus determined cause of the error, a warning is issued for a later casting and / or process parameters are set. For example, the position and the cause of failure determined from it can be fed to an online model, for example. If there is a likelihood of casting defects in later castings with comparable process conditions, a warning can be issued automatically. Alternatively or additionally, an automatic control of process parameters, such as the casting speed, amount of water in the secondary cooling, etc., take place, with the aim of avoiding errors.

Preferably, after step d), a correction of the process parameters takes place. The corrected process parameters can form the basis for one or more additional recalculations with the aim of eliminating the cause of the error and / or to optimize the process parameters. In this or other ways, the process parameters can be improved and output for future pouring as a recommendation or used directly for adaptive control of the continuous casting. The recommended changes to the process parameters may include, for example, casting speed, cooling in the various segments of the plant, soft reduction, hiring, and so on. The proposed changes will take effect at the earliest at the next casting.

Preferably, one or more of the described method steps are carried out computer-based, this applies in particular to method step c).

Further advantages and features of the present invention will be apparent from the following description of preferred embodiments. The features described therein may be implemented alone or in combination with one or more of the features set forth above insofar as the features do not conflict. The following description of the preferred embodiments will be made with reference to the accompanying drawings.

Brief description of the figures

• The FIGS. 1a and 1b schematically show different types of material defects, which in internal cracks ( FIG. 1a ) and surface cracks ( FIG. 1b ).
• The FIG. 2 schematically shows an exemplary continuous casting, which is designed as a "vertical turning plant".
• The FIG. 3 shows an exemplary flowchart for error prevention.
• The FIGS. 4 and 5 show diagrams for the determination of cracking in a continuous casting plant.
• The FIG. 6 shows by way of example the position of the formation of an internal crack, entered into the role scheme of a continuous casting plant.
• The FIG. 7 shows by way of example the position of a crack formation, entered into a diagram of the bulging and stretching.
• The FIG. 8 Figure 12 is a diagram showing an exemplary ductility progression in the center of the strand.

Detailed description of preferred embodiments

In the following, preferred embodiments will be described with reference to the figures. In this case, identical, similar or equivalent elements are provided with identical reference numerals, and a repetitive description of these elements is partially omitted in order to avoid redundancies.

The FIG. 2 shows schematically and simplifies a continuous casting, whose structure is referred to as "vertical turning plant", since the casting strand by means of a strand guide initially guided vertically downwards, then deflected along an arc and transported horizontally.

The structure and operation of the continuous casting of the FIG. 2 In detail: The continuous casting plant 10 is used to produce a metallic product 11 and comprises a mold 12 and an adjoining strand guide 14, along which a strand S of the metallic product 11 emerging from the mold 12 is transported in a conveying direction F. , Below the mold 12 and on both sides of the strand are a plurality of support rollers. 2 arranged, for the cooling of the bar S spray water 4 is discharged or sprayed onto the strand S. At the point where in FIG. 2 the strand S is marked with the reference line 5, the strand still has a liquid sump. The sump tip of the strand S is marked with the reference numeral 6. Downstream of the sump tip 6 is the strand S at the location which is in the FIG. 2 marked with the reference line 7, completely solidified. Downstream of the sump tip 6, a water cooling is also provided along the strand guide 14, which is marked with the reference line 8.

The strand guide 14 of the continuous caster 10 comprises a straightening region I, by means of which the strand S is completely deflected in the horizontal direction. Furthermore, the strand guide 14 comprises a bending region II, through which the strand S, after it has exited the mold 12, is deflected in the direction of the horizontal. The straightening area I and the bending area II are in the representation of FIG. 2 each simplified symbolized by dashed rectangles.

Internal cracks (r1 to r6, cf. FIG. 1a ) in strand S can arise when tensions occur at the solidification front and the strand shell deforms. The solidification front refers to the transition between the liquid core of the strand S and the strand shell, which is already solidified. The front region of the solidification front, viewed in the conveying direction F of the strand S, is referred to as the sump tip 6, as explained above. Since the strand first solidifies on the surface and the temperature increases from outside to inside, the liquid core in the conveying direction has approximately the shape of a wedge, wherein the tip of the wedge is the sump tip 6. The hot crack susceptibility of a material is described by the temperature range BTR (Brittle Temperature Range), which is generally in the transition region between the liquid core of the strand S and the strand shell.

The upper limit of the BTR is called LIT (Liquid Impenetrable Temperature). The lower limit of the BTR is the ZST (Zero Strength Temperature). Various methods and models are known with which the BTR can be determined for the specific material, for example by means of the Scheil-Gulliver model.

In contrast to the internal cracks considered above, surface cracks (r7 to r10, cf. FIG. 1b ) mainly by mechanical loads during casting. The change in curvature in the bending region I of the strand guide 2 and in the adjoining straightening region II lead to strains, which can lead to crack formation if the surface temperatures are too low. A measure of the crack susceptibility on the surface of the strand S is the temperature-dependent Brucheinschnürung (ductility) of the material. Below a material-dependent ductility, the risk of cracking increases. For example, the critical ductility temperature is approximately 860 ° C. for a medium carbon material and approximately 960 ° C. for a tubular steel (API). The drop in ductility depends, among other things, on the size and amount of excretions and phase transformation.

In order to reduce the risk of cracking inside the strand S and on its surface, the aim is to find the location of the crack formation within the continuous casting plant for any cracks found, generally from micrographs. In this way, the cause of the crack formation can be determined and the slab quality can be improved in future castings.

Hereinafter, a method for finding the position of crack generation in the continuous caster according to an embodiment will be described with reference to FIG FIG. 3 where:
The starting points are defects, such as cracks or other defects, which are discovered on cast slabs according to a first step S1. The finding of one or more errors can be done, for example, on a microsection or by a Baumannabzug (sulfur print). However, it does not matter with which concrete method and with which aids errors in the cast material are found.

Subsequently, characteristic properties of the detected error (analogous to several errors) are determined. Thus, according to step S2, the geometry of the defect in the cross-section of the brammer can be determined and, according to step S3, the relevant cross-section can be located in the slab. Found errors can be deposited, for example, in the input mask of a software with length, position and possibly alternative or further characterizing features. This can be done manually or algorithmically. In addition, preferably the casting length, the melt number, the sequence number and / or casting time of the product in which one or more defects have been found, any existing grinding patterns, etc., are stored.

For each casting of a continuous casting plant, usually all process data, such as strand dimensions, analysis, quantities of water, casting speed, superheating, pouring level, etc., are stored in a database or in data files. With the above-recorded data on the error (casting length, melt number, etc.), it is now possible in a next step S4, for example by means of software, to automatically determine the associated process data for the relevant error.

In a subsequent step S5, a re-casting of the casting, which led to the defect, is carried out by means of a casting model. The recalculation is carried out with the process parameters determined from the previous step S4.

By linking the position of the defects in the continuously cast product S and the theoretically determined temperature and solidification curves conclusions can be drawn on the possible origin of the error in the continuous casting. This is preferably carried out algorithmically by means of a computer program. Thus, for example, there is the assignment: number of errors to be evaluated; Description of the error type; Location of the fault; Position and dimension of the fault, such as the length of the crack; Assignment of affected plant position (segment) and process data. Subsequently, the recalculation (replay) of the stored process data mentioned above in step S5 is carried out for the corresponding slabs with a simulation model, such as the DSC (Dynamic Solidification Control) of the SMS group.

According to a further step S6, the ascertained origin of the error can be graphically represented, for example in a diagram. Thus, in the graphical representations, for example, the strand shell growth, the error position can be drawn or otherwise stored, so that the position of the error generation can be read or calculated. In a progressive casting with the same or similar process values, it is now possible, by changing the process values, e.g. the cooling and / or casting speed to reduce the risk of failure at the critical positions.

Prior to this, according to a step S7, a replay can be carried out with corrected process data, with the aim of remedying the cause of the error.

In this or other ways, the process parameters can be optimized and output as recommendations for future castings or used directly for adaptive control of the continuous casting plant. The recommended changes to the process parameters can be, for example, the casting speed, cooling in the various segments of the plant, soft reduction, employment, etc. The proposed changes will take effect at the earliest at the next casting.

In a successful recalculation or simulation with improved or optimized process data, this new process data according to step S8 can be used for the next casting.

The following are examples of analysis and traceability of cracks:
For the detection of internal cracks, the crack position can be drawn or otherwise stored in the representation of the strand shell growth. Since internal cracks usually arise on the solidification front, the position of the crack formation can be found within the continuous casting 100, as in the diagram of FIG. 4 exemplified for a strand of low carbon and with a dimension of 2,600 x 224,5 mm.

If, in addition to the course of the strand shell thickness in the continuous casting plant 100, the temperature range (BTR) critical for hot cracks is calculated, the place of origin of an internal crack can be determined more accurately. In the example of FIG. 5 that like that FIG. 4 is a diagram for determining the position of origin of an internal crack, the crack arises between 5.5 and 9 m below the casting level.

The FIG. 6 shows by way of example the position of the formation of an internal crack, entered into the role scheme of a continuous caster 100. The crack was located between 20 and 30 mm from the slab edge. Because internal cracks in the Normally arising on the solidification front, the bending region of the system, segment 1 can be recognized as the location of the crack formation.

In the FIG. 7 the location of the crack formation is entered into a diagram of bulging and stretching. It can be seen that the cause of the crack in this case can not be due to excessive bulging.

In the FIG. 8 the ductility profile is shown in the middle of the strand. At the end of the straightening range, the ductility drops below 70%. There may be surface cracks.

Through the representation of the time-based process data, such as casting speed, casting temperature and secondary water, with the resulting temperatures, strand shell thicknesses and the sump tip position, it is checked whether stationary or non-stationary casting conditions existed. In non-stationary casting conditions, it is more difficult to determine the positions of cracking than in steady-state casting conditions. Thus, crack prevention measures are preferably taken on samples cast under steady-state casting conditions.

Tracing the formation position of a crack by recalculating the casting by means of the corresponding process parameters can be used in various ways to improve the casting quality. Thus, in a first stage of development, the positions and any resulting causes of the fracture can be fed to an online model. If there is a likelihood of casting defects in later castings with comparable process conditions, a warning can be issued automatically. In a next expansion stage, an automatic control of the casting speed, amounts of water in the secondary cooling and / or other process parameters can be carried out, with the aim of cracking avoid. For this purpose, rules can be created and sent to the online model. With a comparable later casting, the online model can then issue warnings or adaptively regulate the process parameters.

Where applicable, all individual features illustrated in the embodiments may be combined and / or interchanged without departing from the scope of the invention.

LIST OF REFERENCE NUMBERS

2

support rollers

4

splash

5

Liquid swamp

6

crater tip

7

Perforated section of the strand

8th

water cooling

10

continuous casting plant

11

Metallic product

12

mold

14

strand guide

I

leveling range

II

bending area

S

Strand / slab

F

conveying direction

r1 to r10

Types of internal cracks and surface defects

S1 to S8

Process steps of an embodiment

Claims (10)

Method for determining the cause of casting defects in products (S) cast in a continuous casting plant (100), in particular steel slabs, comprising: a) detecting a fault (r1-r10) in a continuously cast product (S); b) determining process parameters under which the product (S) was cast; c) recalculation of the casting according to the process parameters with a simulation model; d) determining one or more positions in the continuous caster (100) where the error (r1-r10) is likely to have occurred. A method according to claim 1, characterized in that after step a) the error (r1 - r10) is classified and / or characterizing features, preferably position of the fault in the product, shape and / or size of the error, are determined. A method according to claim 1 or 2, characterized in that after step a) the geometry of the error (r1 - r10) is located in cross section and then the cross section in the product (S) is located. A method according to claim 1 or 2, characterized in that the process parameters include one or more of the following parameters: molded product dimension, molded product material, cooling water distribution, cooling water quantity, casting speed, superheat, casting level. Method according to one of the preceding claims, characterized in that in step c) the solidification profile and / or the position of the sump tip (6) and / or temperature profiles in the cast strand and / or the hot crack sensitive temperature range (Brittle Temperature Range) and / or the Ductility can be determined. Method according to one of the preceding claims, characterized in that after step d) in a further step e) the cause of the error (r1 - r10) is determined. A method according to claim 6, characterized in that issued on the basis of the cause of the error to a later cast a warning and / or process parameters are set or regulated. Method according to one of the preceding claims, characterized in that after the step d) a correction of the process parameters takes place. A method according to claim 8, characterized in that after the correction of the process parameters, a recalculation with the corrected process parameters. Method according to one of the preceding claims, characterized in that one or more of the positions determined in step d) in one Diagram, preferably a graphic representation of the strand shell growth, are shown.

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