EP3488948B1 - Method for analysing causes of errors in continuous casting - Google Patents

Description

Technical field

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

Background of the Invention

In the continuous casting of metals, in particular steel, cracks can occur in the material as the strand cools and solidifies, which leads to a loss of quality or even rejects. The Figures 1a and 1b schematically show different types of material defects that can be roughly differentiated into internal cracks and surface cracks.

In the Figures 1a and 1b the reference symbol S denotes a cast strand, slab, billet, bloom or round. The Figure 1a shows various forms of internal cracks, such as so-called gusset cracks and triple-point cracks 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 Figure 1b shown, for example transverse Edge cracks r7, longitudinal edge cracks r8, cross cracks r9 and longitudinal cracks / depressions r10.

Analyzing the causes of defects that lead to the above and other material defects is difficult and time consuming with continuously cast products. The WO 2009/149680 A1 describes a method for predicting the formation of longitudinal cracks during continuous casting. The EP 2 150 362 B1 describes a method for the detection and classification of surface defects on continuously cast slabs.

In many cases, the detection and prediction of cracks in the material is not sufficient to determine the cause (s) of the error. For example, there is a lack of information about the location of the cracks, the load on the strand and weakening of the material in the continuous caster. In order to determine the cause of the errors or the origin of errors in a specific case, it can also be helpful to link the process data, information on the system status, material-specific information, etc.

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 further developments follow from the subclaims, the following illustration of the invention and the description of preferred exemplary embodiments.

The method according to the invention serves to determine the causes of casting defects in products cast in a continuous casting installation. This

The DE 10 2015 223 788 A1 describes a process for the continuous casting of a metal strand. The US 2015/0343530 A1 describes systems and methods for monitoring casting processes.

Formulation means that the method according to the invention at least contributes to or makes it easier to find causes which can lead to casting errors. The term "defect" here includes material defects of various types that were caused during the casting, in particular internal cracks and surface cracks. However, the method can also help to determine the causes of other defects, such as impurities, inhomogeneities in the material, etc., provided the defects are the result of the casting process in the continuous caster.

The cast product is preferably a cast steel slab, but may also be another strand product made of a metal, especially a metal alloy.

According to the inventive method, a defect in the continuously cast product is determined in a first step a). The focus is therefore not on how errors can be recognized manually or automatically, but this is the starting point. As a rule, defects, such as cracks, are found from micrographs of the product. It should be noted that the method naturally also includes those cases in which several defects are found in the continuously cast product. If the singular is chosen here, then this only serves to simplify the language.

In a second step b), process parameters are determined under which the product that has the previously found defect was cast. The relevant process parameters can include one or more of the following parameters: dimension of the cast product, material of the cast product, cooling water distribution, cooling water quantity, pouring rate, overheating, pouring level. The aim is to determine those process parameters that are required for the recalculation of the casting in accordance with step c) below. Typically they are Process parameters for all cast products are stored in a database or file, so that the determination in this case amounts to reading out the process parameters from the database or file.

The process parameters determined in this way now serve as input variables with which the casting of the product which has the defect is recalculated using a simulation model. This is step c) mentioned above. For example, the simulation model can theoretically simulate the temperature distribution in the product during casting. Alternative or further output variables can be the solidification course and / or the position of the sump tip and / or the temperature range sensitive to hot cracks (Brittle Temperature Range) and / or the ductility of the cast strand.

The theoretical recalculation of the casting is the basis for determining or identifying in a further step d) one or more positions in the continuous caster where the error is likely to have occurred. The position can be determined particularly reliably if the simulation result is linked to properties of the error found in step a). For this purpose, the defect is preferably classified according to step a) and / or characterizing features are determined, preferably the position of the defect in the product, the shape and / or size of the defect, for example the length of the crack. The position of the origin of the error can thus be read off or calculated. According to a preferred exemplary embodiment, the geometry of the defect is localized in cross section after step a) and then the cross section is localized in the product. The position of the occurrence of errors in the continuous casting installation is preferably drawn in a graphic representation of the casting process. However, it may also be sufficient to store the determined position in such a way that it can be assigned to the point of origin in the continuous caster, for example the segment in question.

According to the described method, the cause of the defect formation during casting can be found and the slab quality can be improved in future castings. In a subsequent casting with the same or similar process parameters, it is now possible to change the process values, e.g. cooling and / or casting speed to reduce the risk of errors at the critical positions.

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

On the basis of the cause of the error determined in this way, a warning is preferably issued for a later casting and / or process parameters are set. For example, the position and the cause of the error determined from it can be fed to an online model. If there is now a likelihood of casting errors in later castings with comparable process conditions, a warning can be issued automatically. As an alternative or in addition, automatic control of process parameters, such as the casting speed, amount of water in the secondary cooling, etc., can be carried out with the aim of avoiding errors.

The process parameters are preferably corrected after step d). The corrected process parameters can form the basis for one or more additional calculations with the aim of eliminating the cause of the error and / or optimize the process parameters. In this or another way, the process parameters can be improved and issued as a recommendation for future castings, or used directly for adaptive control of the continuous caster. The recommended changes to the process parameters can include, for example, the casting speed, cooling in the various segments of the system, soft reduction, employment, etc. The proposed changes will not take effect until the next casting.

One or more of the method steps described are preferably carried out on a computer basis, this applies in particular to method step c).

Further advantages and features of the present invention are evident from the following description of preferred exemplary embodiments. The features described there can be implemented on their own or in combination with one or more of the features set out above, insofar as the features do not contradict each other. The following description of the preferred embodiments is made with reference to the accompanying drawings.

Brief description of the figures

• The Figures 1a and 1b schematically show different types of material defects that occur in internal cracks ( Figure 1a ) and surface cracks ( Figure 1b ) can be distinguished.
• The Figure 2 shows schematically an exemplary continuous caster, which is designed as a "vertical bending system".
• The Figure 3 shows an exemplary flow chart for error avoidance.
• The Figures 4 and 5 show diagrams for determining crack formation in a continuous caster.
• The Figure 6 shows an example of the position of the formation of an internal crack, entered in the role diagram of a continuous caster.
• The Figure 7 shows an example of the position of a crack formation, entered in a diagram of the bulge and elongation.
• The Figure 8 is a diagram showing an exemplary ductility curve in the middle of the strand.

Detailed description of preferred embodiments

Preferred exemplary embodiments are described below with reference to the figures. The same, similar or equivalent elements are provided with identical reference numerals, and a repeated description of these elements is partially omitted in order to avoid redundancies.

The Figure 2 shows schematically and simplified a continuous caster, the structure of which is referred to as a "vertical bending system", since the caster is first guided vertically downward by means of a strand guide, then deflected along an arc and transported horizontally.

The structure and functioning of the continuous caster of Figure 2 In detail: The continuous casting plant 10 is used to produce a metallic product 11 and for this purpose comprises a mold 12 and an adjoining strand guide 14, along which a strand S of the metallic product 11, preferably emerging downward from the mold 12, is transported in a conveying direction F. , Below the mold 12 and on both sides of the strand are several support rollers 2 arranged, for cooling the rod S spray water 4 is applied or sprayed onto the strand S. At the point where in Figure 2 the strand S is marked with the reference line 5, the strand still has a liquid sump. The bottom tip of the strand S is identified by the reference number 6. Downstream of the sump tip 6 is the strand S at the point in the Figure 2 is marked with the reference line 7, completely solidified. Downstream of the sump tip 6, water cooling is also provided along the strand guide 14, which is identified by the reference line 8.

The strand guide 14 of the continuous casting plant 10 comprises a straightening area I, through which the strand S is deflected completely in the horizontal direction. The strand guide 14 further comprises a bending region II, through which the strand S, after it has emerged from the mold 12, is deflected in the direction of the horizontal. The straightening range I and the bending range II are shown in the Figure 2 each symbolized simply by dashed rectangles.

Internal cracks (r1 to r6, cf. Figure 1a ) in strand S can arise if tensions occur on the solidification front and the strand shell deforms. The solidification front denotes the transition between the liquid core of strand S and the strand shell, which has already solidified. The front region of the solidification front, as seen in the conveying direction F of the strand S, is referred to as the sump tip 6 as set out above. Since the strand first solidifies on the surface and the temperature increases from the outside inwards, the liquid core has approximately the shape of a wedge in the conveying direction, the tip of the wedge being the sump tip 6. The susceptibility to hot cracking of a material is described by the temperature range BTR (Brittle Temperature Range), which is generally located in the transition area between the liquid core of 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 using the Scheil-Gulliver model.

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

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

In the following, a method for finding the position of the crack formation in the continuous casting installation according to an exemplary embodiment is described with reference to FIG Figure 3 where:
The starting point are defects, such as cracks or other defects, which are discovered on cast slabs in a first step S1. One or more errors can be found, for example, on a micrograph or by means of a Baumann deduction (sulfur impression). However, it does not matter which concrete method and which tools are used to find defects in the cast material.

Then characteristic properties of the fault found (analogously several faults) are determined. Thus, according to step S2, the geometry of the defect in the cross section of the slab can be determined and, in step S3, the relevant cross section can be localized in the slab. Errors found can be stored in the input mask of a software, for example, with length, position and, if necessary, alternative or other characterizing features. This can be done manually or algorithmically. In addition, the casting length, melt number, sequence number and / or casting time of the product in which one or more defects were found, any existing micrographs, etc. are preferably stored.

For each cast of a continuous caster, all process data, such as strand dimensions, analysis, water quantities, casting speed, overheating, casting level, etc., are usually stored in a database or in data files. With the data on the defect (casting length, melt number, etc.) recorded above, it is now possible in a next step S4, for example using software, to automatically determine the associated process data for the defect in question.

In a subsequent step S5, a recalculation (replay) of the casting that led to the error is carried out using a casting model. The recalculation is carried out using 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 profiles, conclusions can be drawn about the possible source of the defect in the continuous casting plant. This is preferably performed algorithmically using a computer program. For example, the assignment is made here: number of errors to be evaluated; Description of the type of error; Location of the fault; Position and dimension of the defect, such as length of the crack; Assignment of the affected plant position (segment) and process data. Then, for the corresponding slabs, the recalculation (replay) of the stored process data mentioned in step S5 is carried out using a simulation model, such as the DSC (Dynamic Solidification Control) from SMS group.

In a further step S6, the location where the error originated can be graphically represented, for example in a diagram. For example, the fault position can be drawn in or otherwise stored in the graphical representations, for example of the strand shell growth, so that the position of the fault origin can be read off or calculated. In a subsequent casting with the same or similar process values, it is now possible to change the process values, e.g. cooling and / or casting speed to reduce the risk of errors at the critical positions.

A re-calculation with corrected process data can be carried out beforehand according to step S7 with the aim of eliminating the cause of the error.

In this or another way, the process parameters can be optimized and given as a recommendation for future castings, or used directly for adaptive control of the continuous caster. The recommended changes to the process parameters can include casting speed, cooling in the various segments of the facility, soft reduction, employment, etc. The proposed changes will not take effect until the next casting.

In the event of successful recalculation or simulation with improved or optimized process data, these new process data can be used for the next casting in accordance with step S8.

The following are examples of crack analysis and tracing:
To detect internal cracks, the crack position can be shown in the representation of the strand shell growth or saved in some other way. Since internal cracks generally occur on the solidification front, the position of the crack formation within the continuous casting installation 100 can be found, as in the diagram in FIG Figure 4 shown as an example of a strand of LowCarbon and with a dimension of 2,600 x 224.5 mm.

If the temperature range (BTR) critical for hot cracks is also calculated in addition to the profile of the continuous shell thickness in the continuous casting installation 100, the location of an internal crack can be determined more precisely. In the example of Figure 5 who like that Figure 4 is a diagram to determine the origin of an internal crack, the crack occurs between 5.5 and 9 m below the pouring level.

The Figure 6 shows an example of the position of the formation of an internal crack, entered in the roll diagram of a continuous caster 100. The crack was between 20 and 30 mm from the slab edge. Because internal cracks in the Usually arise on the solidification front, the bending area of the system, segment 1, can be recognized as the location of the crack formation.

In the Figure 7 the location of the crack formation is entered in a diagram of the bulge and elongation. It can be seen that the cause of the crack in this case cannot be due to an excessive bulge.

In the Figure 8 the ductility curve is shown in the middle of the strand. At the end of the straightening range, the ductility drops below 70%. Surface cracks can occur there.

The display of the time-based process data, such as pouring speed, pouring temperature and secondary water, with the resulting temperatures, strand shell thicknesses and the bottom tip position, controls whether there were stationary or non-stationary casting conditions. In the case of non-stationary casting conditions, the determination of the positions of the crack formation is more difficult than in the case of stationary casting conditions. Accordingly, measures to prevent cracking are preferably carried out on samples that have been cast under stationary casting conditions.

Tracing the origin of a crack by recalculating the casting using the corresponding process parameters can be used in various ways to improve the casting quality. In a first stage of expansion, the positions and any causes of cracks determined from them can be fed to an online model. If there is now a likelihood of casting errors in later castings with comparable process conditions, a warning can be issued automatically. In the next stage of expansion, 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 preventing crack formation 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 control the process parameters.

As far as applicable, all individual features, which are shown in the exemplary embodiments, can be combined with one another and / or exchanged without leaving the scope of the invention as defined in the appended claims.

LIST OF REFERENCE NUMBERS

2

support rollers

4

splash

5

Liquid swamp

6

crater tip

7

Rigid 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

Method steps of an embodiment

Claims (10)

1. Method for determination of causes of casting faults in products (S), particularly slabs of steel, cast in a continuous casting machine (100), comprising:

   a) ascertaining a fault (r1 - r10) in a continuously cast product (S);

   b) determining process parameters under which the product (S) was cast;

   c) checking the casting in accordance with the process parameters by a simulation model; and

   d) determining one or more positions, at which the fault (r1 - r10) had probably arisen, in the continuous casting plant (100) from linking of a simulation result obtained in step c) with characteristics of the fault discovered in step a).
2. Method according to claim 1, characterised in that after the step a) the fault (r1 - r10) is classified and/or characterising features, preferably position of the fault in the product and shape and/or size of the fault, are determined.
3. Method according to claim 1 or 2, characterised in that after the step a) the geometry of the fault (r1 - r10) in cross-section is localised and subsequently the cross-section is localised in the product (S).
4. Method according to claim 1 or 2, characterised in that the process parameters comprise one or more of the following parameters: dimension of the cast product, material of the cast product, cooling water distribution, cooling water quantity, casting speed, overheating, casting surface.
5. Method according to any one of the preceding claims, characterised in that in step c) the course of hardening and/or the position of the end (6) of the liquid phase and/or temperature plots in the cast strip and/or the temperature range sensitive to heat fracture (brittle temperature range) and/or ductility are determined.
6. Method according to any one of the preceding claims, characterised in that after step d) the cause of the fault (r1 - r10) is determined in a further step e).
7. Method according to claim 6, characterised in that on the basis of the cause of the fault a warning is issued and/or process parameters are set or regulated with respect to a later casting.
8. Method according to any one of the preceding claims, characterised in that after the step d) a correction of the process parameters is carried out.
9. Method according to claim 8, characterised in that after the correction of the process parameters a check with the corrected process parameters is undertaken.
10. Method according to any one of the preceding claims, characterised in that one or more of the positions determined in the step d) is or are represented in a diagram, preferably a graphical illustration of the growth of the strip skin.

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