EP0734295B2 - Process for producing thin slabs - Google Patents

Abstract

A process and a continuous casting installation for the production of thin slabs, preferably of steel with a predetermined solidification thickness of, e.g., 50 mm, in which an optimum surface quality and internal quality of the strand with minimal and predetermined solidification thickness and plant capacity, and accordingly minimal rolling effort, is achieved by a qualitative adjustment of casting and rolling in the region of the strand guide, oscillation of the casting mold by a hydraulically operated lifting platform, feeding of casting powder to the mold, and an immersion nozzle with a specific cross sectional area of flow relating to the process and continuous casting installation, resulting in a satisfactory supply of casting slag and bath movement in the cast surface compared with a standard slab with a thickness of 200 mm. These conditions from the crater end to the cast surface exert a direct influence on the superficial and internal quality of the strand and on the reliability of the casting process.

Description

The invention relates to a continuous caster and a method for producing thin slabs.

It is known from the prior art to use flat immersion spouts, e.g. from DE 37 09 188 A1. Furthermore Hydraulically driven lifting tables are common, which allow lifting height, frequency and shape of the oscillation by deviating from the sinusoidal vibration even during casting and to choose optimally. Bombed molds can be found, for example, in DE 41 31 829 A1 and DE 37 24 628 C1. The casting rolling, in which the casting thickness is reduced so that an improved internal quality of the strand during solidification is obtained is known from DE 38 18 077 A1.

An evaluation of the prior art has shown that the goal of producing thin strands, the solution is more complex Problems required and that all the variables that can be influenced over the entire continuous caster seen so large that the knowledge of the average specialist is far from sufficient and so is it it is unreasonable to find one of the multitude of possible more or less usable solutions, which leads to a satisfactory result with the least possible effort.

From WO-A-8801209 a method and a device for the continuous casting of slabs is known, in which while maintaining the cross-sectional shape of the pouring side of the mold over the entire length of the mold, the strand shell of the central region of the strand emerging from the mold by support and Guide means is deformed, in such a way that after passing through the deformation path in the plane of the slab surface of the edge area.

Finally, reference is made to the DE publication "Stahl und Eisen" (1989, May 16, No. 9/10, pages 453-462). From this, a method and a device for casting and casting rolls of thin slabs are known.

The object of the invention is to provide a method which makes it possible to to achieve the predetermined thickness of the thin strand in such a way that optimal conditions for the supply of slag as well as in the reduction of the strand thickness in the mold and in the guide stand during casting rolling.

The object is achieved by the features of claim 1, advantageous, not completely self-evident Developments contain the subclaims. The solution to the task is independent of mold type; such as. the vertical, vertical bend or circular mold.

The figures serve to illustrate the following exemplary description of the invention.

Fig. 1:

Darstellung der Gießbedingungen in der Kokille

Fig. 2:

Technischer Aufwand für gleichbleibende Oberflächenqualität und Gießleistung in Abhängigkeit der Brammendicke bezogen auf eine 200 mm dicke Bramme x 1.000 mm Breite

Fig. 3.1-3.3:

Technischer Aufwand für gleichbleibende Oberflächenqualität und Brammendicke in Abhängigkeit von der Gießgeschwindigkeit bezogen auf eine 200 mm dicke Bramme x 1.000 mm Breite

Fig. 4:

Hydraulisches Verhalten des Stahles in der Kokille in Abhängigkeit von der Brammendicke bezogen auf eine 200 mm dicke Bramme x 1.000 mm Breite

Fig. 5:

Stranggießanlage

Show it:

Fig. 1:

Representation of the casting conditions in the mold

Fig. 2:

Technical effort for constant surface quality and casting performance depending on the slab thickness in relation to a 200 mm thick slab x 1,000 mm width

Fig. 3.1-3.3:

Technical effort for constant surface quality and slab thickness depending on the casting speed based on a 200 mm thick slab x 1,000 mm width

Fig. 4:

Hydraulic behavior of the steel in the mold depending on the slab thickness in relation to a 200 mm thick slab x 1,000 mm width

Fig. 5:

continuous casting plant

Experiments carried out in the course of working out the invention have shown that the surface quality of a strand essentially depends on the slag guidance. This is due to the meniscus, ie the interplay of the slag height (h _slag ) and the strand shell (h _{strand shell} ) emerging from the bath when the mold rises (Fig. 1).

It has been found that the criterion for optimal lubrication and the avoidance of surface defects (casting powder particles located directly below the strand surface, predominantly in the form of oxides) H slag ≥ h strand shell must be fulfilled.

The slag height h _slag mainly depends on the thickness of the mold inlet cross section and the strand _shell height h _strand shell depends on the lifting height of the oscillating mold.

Vergleicht man für eine vorgegebene Gießleistung von 2.736 t/min die gebräuchliche 200 mm-Bramme mit einer 50 mm-Bramme und setzt sie in Relation (2) für die 200 mm-Bramme gleich 1, so steigt dieser Wert für die 50 mm-Bramme auf 16.62, wie aus der Fig. 2 zu entnehmen ist. D.h., die Relation (2) ist umgekehrt proportional zur abnehmenden Strangdicke, wobei die Abhängigkeit einer Exponentialkurve folgt.

If one considers the size h _slag and its dependence on the thickness of the mold inlet cross section, the relationship shows Handicap = produced strand surface bath surface in m 2 / min x 1 / m 2 . which can also be called the technical effort that needs to be put into the system, unexpectedly the following result:

If you compare the usual 200 mm slab with a 50 mm slab for a given casting rate of 2,736 t / min and set it in relation (2) for the 200 mm slab to 1, this value increases for the 50 mm slab to 16.62, as can be seen from FIG. 2. That is, the relation (2) is inversely proportional to the decreasing strand thickness, the dependence following an exponential curve.

This relationship between the thickness in the mold level (19) and the specific slag production and thus the slag height (4) in the meniscus also leads to the need for the active casting powder thickness to exceed the entire casting work and thus also in the area of the pouring spout must be kept constant.

The constant thickness leads to a constant formation of slag over the width of the mold level and thus to one constant slag supply in the area of the meniscus of the entire continuously forming strand shell (3). This constant. Slag formation from casting powder or granules (5) over the casting width avoids the Danger of insufficient lubrication between the immersion nozzle and the copper broad side plates. This danger exists since the pouring slag has a glassy structure (silicate structure) with a viscous behavior of approx. 0.5-10 poise. Due to their toughness, there can be relative lack of lubrication in the area between the string widths Dipping spout and the broadside of the mold in comparison to the remaining mold area in the mold level come when the The respective distance between the immersion nozzle and the broadside of the mold is less than half the strand thickness at the mold outlet.

If, on the other hand, you look at how the relation (2) increases when the casting speed increases with a defined casting thickness changed, as shown in Fig. 3 for a 75/100 and 125 mm mold, it is found that this only increases linearly - with a slight slope of the straight line.

Of considerable influence on the relation (1) is the turbulence caused by the metal flowing into the mold, which often continues to the bath level and can lead to wave movements, whereby the wave crests can rise above the slag level, causing an interruption leads in the lubrication. This turbulence depends, among other things, on the throughput and the thickness and width of the mold at the cross-section of the dip tube outlet. As a measure of the turbulence, the hydraulic behavior is now defined as the quotient of throughput and thickness and can be represented with the following expression: Hydraulic behavior = Throughput in t / min Thickness in mm

Values for the hydraulic behavior, based on the 200 mm thick slab, can be seen in FIG. 4, for example remove. It can be seen that larger mold thicknesses result in significantly more favorable hydraulic behavior to have.

F_TA = Querschnittsfläche des Tauchausgußaustritts

F_ST = Strangquerschnitt der durcherstarrten Bramme

The relation is also important with regard to turbulence F ST F TA ≤ 50 in which

F _TA = cross-sectional area of the diving spout outlet

F _ST = strand cross-section of the solidified slab

In addition, an electromagnetic brake in the mold area can clearly show the turbulence in the mold area reduce.

From the relations established above and verified by measurements it follows that the reduction in the choice the slab thickness at the mold outlet of e.g. 100 mm to 50 mm and beyond that with a rectangular mold Problems with maintaining the relation (1) increased extraordinarily. That is, apart from the difficulties with Metal supply makes it almost impossible to apply enough casting powder to the small mold inlet cross-section, to lubricate the resulting large strand surface and also to set the relation (4). On the other hand, the casting speed with a strand thickness of e.g. 100 mm in the mold level without considerably increase extra work. This leads to the surprising solution that it is in the range of Thin slab casting is not sensible, it is imperative to reach the slab thickness at the mold outlet, but instead that it is technically much easier to use the slab thickness as it is fed to the rolling mill Using a casting and rolling step to further reduce and ultimately achieve what a multi-roll stand (segment 0), e.g. trained as pliers segment, has proven to be advantageous.

5 shows an example of a continuous caster which contains all the features of the invention.

LIST OF REFERENCE NUMBERS

1

Q _{casting powder}

2

Powder T _li , phase boundary powder / slag

3

h _{strand shell} ,
Height of the strand bowl above the bathroom mirror

4

h _slag ,
slag height

5

h _powder ,
powder height

6

A submerged nozzle

7

deposit

8th

Oxide flow into the slag

9

V _g = casting speed

10

Q _slag = slag consumption

11

air

12

Frost line,
Steel solid / liquid

13

strand shell

14

Oscillation (lifting height, frequency, shape)

15

copperplate

16

distributor

17

A submerged nozzle
External dimensions e.g. 260 x 60 mm
Internal dimensions e.g. 220 x 20 mm

18

optimized casting powder

19

75 + 2 x 12 mm x 800 - 1,600 mm,
Slab format in the mold level (meniscus)

20

20 x 220 mm,
Flow cross section - immersion spout

21

hydraulic mold drive

22

F _ST / F _TA ≤ 50 *)

23

75 + 2 x 0.5 mm or 75 mm,
Slab format at the mold exit

24

Articulated or hydraulic cylinder or the like

25

Segment 0, e.g. designed as pliers

26

hydraulic cylinder or the like

27

50 + 2 x 0.5 mm or 50 mm,
Slab thickness after the casting rolling process

28

Segment 1 ... n with hydraulic adjustment or similar

29

V _gmax 6 m / min

30

50 + 2 x 0.5 mm or 50 mm,
Slab thickness at the end of the strand guide

Claims (3)

1. A method for producing thin slabs, comprising the following steps:

   pouring by means of an immersion nozzle into a convex, oscillating mould, the mould entry cross-section being larger than the mould exit cross-section, whilst observing the condition for the immersion nozzle and the solidification cross-section, FST FTA ≤ 50 wherein

   F_ST = billet cross-section of the fully-solidified slab,

   F_TA = cross-section of the outlet of the immersion nozzle,

   supplying the casting powder on to the metal melt such that the condition hslag ≥ hbillet shell wherein

   h_{billet shell} = height of billet shell above bath level (3),

   h_slag = slag height (4),

   is observed, dependent on the height of oscillation, form and frequency of the mould movement,

   reducing the billet cross-section immediately beneath the mould in a plurality of steps in a multi-roll stand, in order to build up forced convection in parallel to the continuous reduction in billet thickness in the still-molten interior of the billet, which convection corresponds to the action of the electromagnetic stirrer, the final thickness of the billet being achieved when the core is still molten at the end of the multi-roll stand, and

   guiding the solidification such that when the final thickness is reached at the exit from the multi-roll stand there are still two phases in the interior of the billet,

   the casting powder being supplied such that across the entire width of the slab the active casting powder thickness in the bath level, which has been covered with casting powder beforehand and which is relevant for melting the casting slag, is constant.
2. A method according to Claim 1, characterised in that even during the casting the frequency, lifting height and form of oscillation for the mould movement can be freely selected.
3. A method according to Claims 1 and 2, characterised in that the mould is constructed such that a residual convex shaping symmetrically to the centre line of the billet is imparted to the billet at the exit from the mould, which convex shaping in thickness is less than 4% of the final thickness.

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