EP2349612B2 - Method and continuous casting plant for manufacturing thick slabs - Google Patents

Description

Technical area

The invention relates to a method for producing thick steel slabs with a casting thickness exceeding 360 mm and a casting width exceeding 1000 mm in a continuous caster.

The casting of steel strands in continuous casters, in which the cast steel strand is first bent in a downstream strand guide and then straightened again after its exit from the continuous casting mold, becomes increasingly difficult as the strand thickness increases. The tensile and compressive stresses that occur in the strand shell during the deformation process lead to crack formation in the strand edge and surface area of the steel strand. So far, there are only a few continuous casting plants with which steel strands with strand thicknesses over 360 mm can be cast and slabs in this thickness range can be produced.

There is currently an increasing demand from the processing industry for slabs in a thickness range from 360 to 450 mm for the subsequent production of correspondingly thick heavy plates.

State of the art

A continuous caster of the "vertical system or vertical bending system" type, which has a long vertical strand guide part with a subsequent bending and straightening zone, is known from the publication of Dr.-Ing. Klaus Harste et al; "Construction of a new vertical caster at Dillinger Hüttenwerke"; MPT International 4/1998; Pp. 112-122 already known. This casting plant, the layout of which is shown in FIG. 8, enables the casting of steel strands with a casting width of 1400 to 2200 mm and a casting thickness between 230 and 400 mm. It has a very long vertical strand guide with cooling devices for intensive strand cooling in this section in order to be able to carry out the subsequent bending and straightening of the steel strand when the strand is completely solidified. This system concept leads to a high construction height of the continuous caster of around 45 m and thus to high investment costs, especially in the necessary infrastructure and to difficult maintenance conditions. At casting thicknesses of 400 mm, the achievable casting speed is around 0.3 m / min, which means that the productivity per strand is relative is low. The low casting speed also means that the cast steel strand cannot be kept hot enough for straightening and therefore has to be kept below a critical temperature with intensive cooling in order to avoid ductility problems that typically occur in temperature ranges from 600 to 850 ° C.

From the DE 31 12 947 A1 an arc continuous caster is already known for casting a steel strand with a strand thickness in a range of 200 to 300 mm, with which slabs of good quality can be produced. In this continuous arc caster, the metal strand is formed in an arched mold with a radius of curvature which lies in a range from 2.0 to 4.9 m and which corresponds to the radius of curvature in a first zone of the subsequent strand guide. In a subsequent very long straightening zone, the steel strand is straightened again, which inevitably results in the formation of a trapezoidal cross-section of the steel strand. This cross-sectional distortion becomes greater the greater the strand thickness and the smaller the radius of curvature in the curved mold and leads to quality problems in the subsequent rolling in the heavy-plate mill.

From the publication
Siemens: "First Ultra-Thick-Slab Caster in China - Siemens to Supply Shougang Qinhuangdao Works with Caster Capable of Casting 400-mm-Thick Slabs" August 13, 2008 (2008-08-13), pages 1-2 , XP002565642,
the casting of a steel strand with a thickness of 400 mm by a continuous caster with a straight mold is known. How an ultra-thick slab can be cast through an arched mold does not emerge from this document.

Presentation of the invention

The object of the present invention is therefore to avoid the disadvantages of the known prior art and to propose a process for the production of thick steel slabs, the production of high quality steel strands and slabs with a casting thickness exceeding 360 mm with good internal quality, low susceptibility to cracking and extensive format accuracy is guaranteed.

Another aim of the invention is to keep the investment and operating costs low while the productivity of the casting plant is high.

• Gießen eines Stahlstranges mit noch flüssigem Kern in einer Bogenkokille mit einem zumindest über einen Teilbereich seiner Längserstreckung ausgangsseitig gekrümmt ausgerichteten Kokillenformhohlraum, wobei der gegossene Stahlstrang die Bogenkokille gekrümmt mit einem aufgeprägten Kokillen-Bogenradius verlässt,
• Umlenken des gegossenen Stahlstranges von einer durch den Kokillen-Bogenradius bestimmten Gießrichtung in eine horizontale Transportrichtung und Stützen und Führen des Stahlstranges in einer Strangführung, die sich vom Austritt des Stahlstranges aus der Bogenkokille bis zum Eintritt in eine Zerteileinrichtung erstreckt,
• Führen des Stahlstranges in einer Kreisbogenführung der Strangführung auf einem Strangführungs-Bogenradius von 9,0 bis 15,0 m,
• Rückbiegen des gegossenen Stahlstranges von dem Strangführungs-Bogenradius auf einen geraden Stahlstrang bei noch flüssigem bzw. teilflüssigem Kern in einer Richtzone innerhalb der Strangführung,
• kontinuierliches Kühlen des gegossenen Stahlstranges in der Strangführung, wobei das Kühlen des gegossenen Stahlstranges geregelt durch Aufbringen von Kühlmittel auf die Breitseiten des gegossenen Stahlstranges mit einer Kühleinrichtung in der Strangführung erfolgt,
• Halten der Oberflächentemperatur des Stahlstranges in die Richtzone der Strangführung über dem Duktilitätstief der jeweiligen Stahlsorte,
• Halten des Anteils der festen Strangschale des gegossenen Stahlstranges bei maximal 95% der halben Strangdicke während der Phase des Rückbiegens in der Richtzone,
• Zerteilen des Stahlstranges auf Brammen vorbestimmter Länge in einer Zerteileinrichtung.

The object on which the invention is based is achieved in a method for manufacturing thick steel slabs with a casting thickness exceeding 360 mm and a casting width exceeding 1000 mm by combining the following features:

• Casting a steel strand with a still liquid core in an arched mold with a mold cavity that is curved at least over a portion of its longitudinal extent on the output side, the cast steel strand leaving the curved mold in a curved manner with an impressed mold arc radius,
• Diverting the cast steel strand from a casting direction determined by the mold arc radius into a horizontal transport direction and supporting and guiding the steel strand in a strand guide that extends from the exit of the steel strand from the arched mold to the entry into a dividing device,
• Guiding the steel strand in a circular arc guide of the strand guide on a strand guide curve radius of 9.0 to 15.0 m,
• Bending back the cast steel strand from the strand guide curve radius to a straight steel strand with the core still liquid or partially liquid in a straightening zone within the strand guide,
• continuous cooling of the cast steel strand in the strand guide, the cooling of the cast steel strand taking place in a controlled manner by applying coolant to the broad sides of the cast steel strand with a cooling device in the strand guide,
• Maintaining the surface temperature of the steel strand in the straightening zone of the strand guide above the ductility depth of the respective steel grade,
• Keeping the proportion of the solid strand shell of the cast steel strand at a maximum of 95% of half the strand thickness during the re-bending phase in the straightening zone,
• Cutting the steel strand into slabs of a predetermined length in a cutting device.

Because of its curved shape and the resulting reduced vertical length, an arched mold does not have the good conditions known from a straight mold for the introduction of the steel melt into the mold and for a uniform strand shell formation. However, in contrast to a straight mold, the need for a strand bend in a bending zone immediately after or at a short distance from the mold can largely or entirely be avoided. This reduces the risk of cracking in the edge or surface area of the strand.

Furthermore, a system with a curved mold has a lower overall height compared to a system with a straight mold with the same strand guide curved radius.

The curved mold used in the proposed method can be designed with a straight inlet section and a curved outlet section, the outlet section having a curvature corresponding to a predetermined mold bending radius. As an alternative to this, the mold cavity of the arched mold can also be continuously curved with a constant mold arc radius. Variations of these embodiments are also possible.

If the mold arc radius of the cast steel strand when exiting the arc mold corresponds to the strand guide arc radius of the arc guide in the strand guide, the cast steel strand is transferred from the arc mold into the arc guide of the strand guide free of bending forces.

Favorable production conditions for a cast steel strand or for slabs with a casting thickness exceeding 360 mm, in particular with a casting thickness in a thickness range of 360 to 450 mm, are achieved if the cast steel strand in the arched mold has a mold arc radius between 9.0 m and 15.0 m is generated, in a subsequent circular arc guide the strand guide is held on this arc radius without further deformation and is straightened again in a subsequent straightening zone within the strand guide from an arc radius (R) between 9.0 m and 15.0 m.

According to an alternative embodiment, the mold arc radius of the cast steel strand when exiting the arched mold can be larger or smaller than the strand guide arc radius in the strand guide and the cast steel strand in a bending zone within the strand guide from the mold arc radius of the cast steel strand when exiting from the Curved mold to be bent to the strand guide curve radius in the strand guide. This embodiment opens up the possibility of keeping the cast steel strand constant over a certain distance after its exit from the curved mold with the curvature impressed on it, which corresponds to the mold arc radius on the mold exit side, and then to make a strand bend to the strand guide arc radius within a bending zone or to carry out this bending process immediately after the exit of the steel strand from the arched mold. In any case, the bending process is kept low when the strand shell thickness is still small.

Bending the cast steel strand down to a predetermined radius of curvature in a bending zone and bending back the cast steel strand in a straightening zone corresponds to the concept of known continuous slab casting plants and has proven itself as such. It is essential for the casting of thick slabs that both processes take place at times when the steel strand still has a liquid or partially liquid core, or it is necessary to regulate the cooling of the steel strand in the strand guide accordingly. With increasing strand thickness, the requirements for the most uniform possible cooling increase, which must necessarily be reflected in a temperature distribution that is as uniform as possible over the strand length and the strand width at a high temperature level, in order to ensure uniform elasticity of the strand and to avoid cracking as a result of temperature differences.

The desired temperature profile specified for the system control is determined by an entry surface temperature of the steel strand in the straightening zone of the strand guide. The aim is to keep the steel strand, including the surface area, in a temperature range above the low ductility, which also minimizes the tendency to form surface cracks.

A system with a curved mold has a shorter strand length from the mold level to the straightening zone compared to a system with a straight mold with the same strand guide curved radius. Therefore, the time that the strand needs for this distance at the same casting speed is shorter than with a comparable system with a straight mold. Due to the shorter time available for cooling, it is possible to keep the surface temperature at a comparatively higher level. This minimizes the tendency for surface cracks to develop.

Expediently, the proportion of the solid strand shell of the cast steel strand is a maximum of 95% of half the strand thickness during the re-bending phase in the straightening zone.

According to an expedient further development, the aim is to mix the preferably partially solidified core zone close to the solidification point of the rod by reducing the thickness of the steel strand using a soft reduction or a dynamic soft reduction, thus achieving an improved microstructure in the core area of the slab and starting line segregation and porosity avoided. Accordingly, a soft reduction, in particular a dynamic soft reduction, is used on the cast steel strand in an area with a still liquid or partially liquid core of the steel strand with an adjusting device for strand guide rollers.

According to an expedient development of the invention, the cast steel strand is bent in a bending zone within the strand guide to an arc radius between 9.0 m and 15.0 m, held in a subsequent arc of the strand guide on this arc radius without further deformation and in a subsequent straightening zone within The strand guide is straightened again starting from a curve radius between 9.0 m and 15.0 m. Especially for casting thicknesses between 360 and 450 mm, this curve radius provides the best surface quality and minimization of cracks in the cast steel strand with simultaneous, precisely regulated cooling in these areas of the strand guidance.

According to an expedient development of the invention, the impact position of the coolant jets on the cooling of the cast steel strand Steel strand regulated at least in a partial area of the strand guide on the basis of a continuous determination of the temperature profile along the transport path of the steel strand and in normal planes thereto.

The amount of coolant applied to the steel strand in the strand guide up to the entry into the straightening zone is expediently regulated as a function of a predetermined temperature profile along the transport path of the steel strand and in a normal plane to it. This is intended to achieve a further increase in the accuracy of the temperature distribution over the surface of the steel strand. The specified temperature profile takes into account the ductility properties of the steel grade to be cast.

A further stabilization of the cooling conditions is achieved if the controlled cooling of the cast steel strand takes place after the dry mode within the strand guide with the integration of peripheral-cooled strand guide rollers. This avoids the need to take into account the cooling of the strand guide rollers as an essential element in dimensioning the intensity of the coolant application, or to align the intensity of the coolant application in individual areas with the needs of the strand guide rollers. This means that the strand surface temperature can be kept at a very high level, at least in the area up to the point where the steel strand is bent back. The strand guide rollers are cooled almost exclusively by internal cooling of the strand guide rollers, the coolant being expediently guided through coolant channels in the roller jacket area as close as possible to the roller jacket surface.

To optimize the deposition rate of non-metallic elements, for example casting powder particles, in the area of the mold and just below it, it is advantageous if the flow movement of the steel melt of the liquid core of the steel strand in the mold or in the area of the strand guide is influenced by an electromagnetic device. In addition to an increased rise of the non-metallic accompanying substances to the bath level surface in the mold, there is a targeted mixing of the steel melt and a reduction in segregation tendencies.

The described method for producing thick steel strands is preferably used when the steel strand is cast with a casting thickness lying in a thickness range of 360 mm to 450 mm.

What is essential is the combination of an arched mold with a downstream strand guide, with a circular arc guide and with a straightening zone, which, in conjunction with a cooling device for controlled cooling of the steel strand, is responsible for shaping, transporting and straightening the cast steel strand with a liquid core and high quality requirements for the cast Steel strand or the slabs in the claimed thickness range ensures.

Bending stresses on the strand shell of the steel strand leaving the arched mold are avoided when the exit-side mold arc radius of the arc mold corresponds to the strand guide arc radius of the circular arc guide within the strand guide.

An optimization of metallurgical requirements and investment and operating costs of the casting plant for the production of a cast steel strand or for slabs with a casting thickness exceeding 360 mm, in particular with a casting thickness in a thickness range of 360 to 450 mm, are achieved if the strand guide arc radius is Circular arc guidance within the strand guidance is 9.0 to 15.0 m. Accordingly, the selected value of the mold arc radius corresponds to the selected value of the strand guide arc radius within this preferred range.

The metallurgical requirements and investment and operating costs of the casting plant are optimized if the exit-side mold arc radius of the arc mold is 8.0 to 20.0 m, excluding the specified value for the strand guide arc radius and the strand guide arc radius within the arc guide the strand guide is 9.0 to 15.0 m.

In both cases, the overall height of the casting plant remains low and the quality specifications that are standard for cast steel strands with, for example, 200 - 250 mm casting thickness, are almost achieved.

The cooling device in the strand guide is equipped with several independently controllable cooling zones across the casting width and / or with height-adjustable spray nozzles with controllable adjustment devices. This enables the strand edge temperature to be influenced in a targeted manner by regulating the amount of cooling water as a function of the width and / or by changing the distance between the spray nozzles and the steel strand surface and thus changing the lateral distance between the coolant jet and the steel strand edge.

One or more electromagnetic devices, such as a stirring coil, for influencing the flow movement of the molten steel of the liquid core of the steel strand are arranged in the mold or in the area of the curved strand guide.

Peripherally cooled strand guide rollers are expediently arranged in the strand guide for supporting and guiding the steel strand. The use of these peripheral-cooled strand guide rollers results in a significant decoupling of the different cooling requirements of the cast metal strand and the strand guide rollers, which are in direct line contact with the hot steel strand and are exposed to its radiant heat.

Brief description of the drawings

Fig. 1

einen Längsschnitt durch eine Stranggießanlage zur Durchführung des erfindungsgemäßen Verfahrens,

Fig. 2

einen Längsschnitt durch eine Stranggießanlage zur Durchführung des erfindungsgemäßen Verfahrens,

Fig. 3

die Anordnung von Spritzdüsen einer Kühleinrichtung in einer Strangführung,

Fig. 4

eine weitere Ausführungsform der Kühleinrichtung mit unabhängig regelbaren Kühlzonen.

Further advantages and features of the present invention emerge from the following description of non-limiting exemplary embodiments, reference being made to the following figures, which show the following:

Fig. 1

a longitudinal section through a continuous caster for performing the method according to the invention,

Fig. 2

a longitudinal section through a continuous caster for performing the method according to the invention,

Fig. 3

the arrangement of spray nozzles of a cooling device in a strand guide,

Fig. 4

a further embodiment of the cooling device with independently controllable cooling zones.

Implementation of the invention

In Figure 1 a schematic longitudinal section shows the structural design of a continuous caster for producing slabs from liquid steel for a casting thickness of 400 mm.

The continuous caster has an arched mold 1 with a curved mold cavity 1a. It is designed as an oscillating, internally cooled adjustable mold with wide side walls and narrow side walls and enables the casting of steel strands with different strand widths and possibly also different strand thicknesses. The mold 1 is equipped with an electromagnetic device 2, such as a stirring coil or an electromagnetic brake, for influencing the flow movement of the steel melt in the liquid core of the cast steel strand.

A strand guide 3 adjoins the mold 1 and extends as far as a cutting device 4 designed as a flame cutting machine for cutting the steel strand into slabs. In the strand guide, the cast steel strand is supported and guided on its broad side walls in a narrow corset by driven and non-driven strand guide rollers 5 and diverted from a casting direction G determined by the mold arc radius RK into a horizontal transport direction T. Groups of strand guide rollers 5 arranged on both sides of the steel strand are combined in strand guide segments 6.

The strand guide 3 comprises a series of successive sections with specific functions, the structure of which is essentially known. The steel strand emerging from the mold 1 is transported along a circular arc with the strand guide arc radius RSt in a circular arc guide 9 and while maintaining this arc radius without applying any bending stresses. The strand guide arc radius RSt here corresponds to the mold arc radius RK, which ensures transport without bending stress. In a first section of the strand guide 3, immediately following the arched mold 1, there is additional strand support with strand guide rollers 5 also on the narrow sides of the steel strand. In a straightening zone 10 following the circular arc guide 9, the steel strand is bent back and straightened. The steel strand is then conveyed in a horizontal strand guide 11 to the dividing device 4.

This structural design can be supplemented by various additional devices, not shown or described, between and within the described sections of the strand guide.

Another possible embodiment of the continuous caster for performing the method according to the invention is shown in FIG Figure 2 shown. Here, too, the strand guide 3 comprises a series of successive sections with specific functions. In a curved strand support device 7, the steel strand emerging from the arched mold 1 is guided and supported in accordance with the mold arc radius RK without the application of bending stresses. In a first area of the strand support device 7 there is also a strand support with strand guide rollers 5 also on the narrow sides of the steel strand. In a subsequent bending zone 8 there is a progressive bending of the steel strand from the mold arc radius RK to the strand guide arc radius RSt of the subsequent arc guide 9. In the arc guide 9, the steel strand is transported further while maintaining the strand guide arc radius. The further transport of the steel strand takes place analogously to the embodiment in Figure 1 .

In the strand guide 3, the steel strand with its liquid core indicated by dashed lines is subjected to regulated cooling. The cooling device 12 comprises, as in FIGS Figures 3 and 4 shown, between the strand guide rollers 5 positionable spray nozzles 13, which are independently adjustable in a normal plane N to the transport direction T at least in partial areas. In each cooling zone Z over the casting width B are corresponding to Figure 3 height-adjustable spray nozzles 13 with associated adjustment devices 14, or as in Figure 4 shown, spray nozzles 13 provided with control valves 18 for regulating the amount of water. The Adjusting devices 14 or the control valves 18 are controlled by a computing unit 15.

One or more of the strand guide segments 6, which are arranged between the straightening zone 10 and the dividing device 4, are assigned special adjustment devices 17 for strand guide rollers 5. These segments form a soft reduction zone 16. The strand guide rollers in these segments can be positioned in a wedge shape on the steel strand and thus enable a slight reduction in the thickness of the metal strand and an improvement in the metallurgical properties in the core zone of the steel strand.

List of reference symbols:

1

Bow mold

1a

Mold cavity

2

electromagnetic device

3

Strand guide

4th

Cutting device

5

Strand guide rollers

6th

Strand guide segment

7th

Strand support device

8th

Bending zone

9

Circular arc guide

10

Straightening zone

11

Horizontal strand guide

12

Cooling device

13

Spray nozzles

14th

Adjustment device for spray nozzles

15th

Arithmetic unit

16

Soft Reduction Zone

17th

Adjusting devices for strand guide rollers

18th

Control valves

RK

Mold arc radius

RSt

Strand guide curve radius

G

Pouring direction

T

Transport direction

Z

Cooling zone

B.

Casting width

Claims (12)

1. Method for manufacturing thick slabs of steel with a cast thickness exceeding 360 mm and a cast width exceeding 1000 mm in a continuous casting plant, characterized by the combination of the following features:

   - casting a steel strand having a core which is still liquid in a bow-type mold (1) with a mold cavity (1a) which is oriented so as to be curved on the output side at least over a partial region of its longitudinal extent, the cast steel strand leaving the bow-type mold in curved form with an impressed mold arc radius (RK),

   - deflecting the cast steel strand from a casting direction determined by the mold arc radius (RK) to a horizontal direction of transportation and supporting and guiding the steel strand in a strand guide (3) which extends from the point at which the steel strand issues from the bow-type mold to the point at which it enters a dividing means (4),

   - guiding the steel strand in a circular arc guide (9) of the strand guide (3) on a strand guide arc radius (RSt) of 9.0 to 15.0 m,

   - bending the cast steel strand back from the strand guide arc radius (RSt) to a straight steel strand while the core is still liquid or partially liquid in a straightening zone (10) within the strand guide (3),

   - continuously cooling the cast steel strand in the strand guide (3), the cast steel strand being cooled in a regulated manner by applying coolant to the wide sides of the cast steel strand using a cooling means (12) in the strand guide (3),

   - maintaining the surface temperature of the steel strand in the straightening zone (10) of the strand guide (3) above the ductility trough of the respective grade of steel,

   - maintaining the proportion of the solid strand shell of the cast steel strand at at most 95% of half the strand thickness during the bending-back phase in the straightening zone (10),

   - dividing the steel strand into slabs of predetermined length in a dividing means (4).
2. Method according to Claim 1, characterized in that the cast steel strand is passed from the bow-type mold to the strand guide without bending forces, the mold arc radius (RK) of the cast steel strand corresponding, at the point at which the steel strand issues from the bow-type mold, to the strand guide arc radius (RSt) in the strand guide (3).
3. Method according to Claim 2, characterized in that the cast steel strand is produced in the bow-type mold with a mold arc radius (RK) between 9.0 m and 15.0 m, is maintained in a subsequent circular arc guide (9) of the strand guide (3) at this arc radius without further deformation and is restraightened in a subsequent straightening zone (10) within the strand guide (3) from an arc radius (R) between 9.0 m and 15.0 m.
4. Method according to Claim 1, characterized in that the mold arc radius (RK) of the cast steel strand is greater or less, at the point at which the steel strand issues from the bow-type mold, than the strand guide arc radius (RSt) in the strand guide (3) and the cast steel strand is bent, in a bending zone (8) within the strand guide (3), from the mold arc radius (RK) of the cast steel strand, at the point at which the steel strand issues from the bow-type mold, to the strand guide arc radius (RSt) in the strand guide (3).
5. Method according to Claim 4, characterized in that the cast steel strand is bent, in the bending zone (8) within the strand guide (3), to an arc radius (R) between 9.0 m and 15.0 m, is maintained in a subsequent circular arc guide (9) of the strand guide (3) at this arc radius without further deformation and is restraightened in a subsequent straightening zone (10) within the strand guide (3) from an arc radius (R) between 9.0 m and 15.0 m.
6. Method according to one of the preceding claims, characterized in that, during the cooling of the cast steel strand, the position in which the jets of coolant strike the steel strand is regulated on the basis of a continuous determination of the temperature profile along the path of transportation of the steel strand and in planes normal thereto.
7. Method according to one of the preceding claims, characterized in that the amount of coolant that is applied to the steel strand in the strand guide (3) until the steel strand leaves the straightening zone (10) is regulated as a function of a predefined temperature profile in consideration of the ductility properties of the respective grade of steel along the path of transportation of the steel strand and in a plane normal thereto.
8. Method according to one of the preceding claims, characterized in that the cast steel strand is cooled in a regulated manner after dry operation involving peripherally cooled strand guide rolls (5).
9. Method according to one of the preceding claims, characterized in that a soft reduction, in particular a dynamic soft reduction, is applied to the cast steel strand in a region with a steel strand core which is still liquid or partially liquid using a device for attaching strand guide rolls (17).
10. Method according to one of the preceding claims, characterized in that the flow movement of the steel melt of the liquid core of the steel strand is influenced by an electromagnetic means (2) in the mold (1) or in the region of the strand guide.
11. Method according to one of the preceding claims, characterized in that the steel strand is cast with a cast thickness of up to 450 mm.
12. Method according to one of the preceding claims, characterized in that the steel strand is cast at a casting speed between 0.5 and 1.0 m/min.

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