EP0741617B2 - Process for producing rectangular thin slabs - Google Patents

Abstract

The invention relates to a process and a continuous casting facility for the production of thin slabs, preferably of steel with a predetermined congealing thickness of (for example) 50 mm. In the said process, an optimal casting surface and internal quality, with minimal and predetermined congealing thickness and plant capacity, and thus minimal complexity of rolling material, is achieved by the optimal combination of such elements as the following: rolling of cast metal in the area of the casting guide (segment 0), cambered ingot mould with a cross-sectional area which increases from inlet to outlet, hydraulically driven lifting platform, casting powder and supply thereof, immersion discharge with specific flow cross section. Qualitative adjustment of these process and system parameters results in satisfactory supply of casting slag and circulation in the meniscus by comparison with a standard 200 mm thick slab. The conditions from the basin top to the meniscus have a direct effect on the superficial and interior quality of the casting and 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. The Casting rolls, in which the casting thickness is reduced so that an improved internal quality of the Stranges is obtained is known from DE 38 18 077 A1.

From the DE magazine "Stahl und Eisen" (1989, May 16, No.9 / 10, Seieten 453-462, - pouring and casting rolls thinner Slabs at Mannesmannröhren-Werke AG), it is known to use a diving spout in a slab caster, to use a rectangular mold, a mold powder feed and a multi-roll stand.

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.

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. The solution to the task is independent of Mold type, e.g. 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 Brarnmendicke 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} ) on the lifting height of the oscillating mold.

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 for a given casting rate of 2,736 t / min 50 mm slab and sets 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 one Strand thickness, the dependence following an exponential curve.

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, are, for example, that of (FIG. 4) 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 in the mold of e.g. 100 mm to 50 mm the problems in maintaining the relation (1) are extraordinary elevated. That is, apart from the difficulty in supplying metal, it becomes almost impossible to rely on the Apply a small amount of mold powder to the small mold inlet cross-section to cover the enormous strand surface to lubricate and also set the relation (4). On the other hand, the casting speed can be with a strand thickness of e.g. Increase 75 mm in the mold and thus in the bathroom mirror without any extra effort. This leads to the surprising solution that it is not useful in the field of thin slab casting Slab thickness from the mold to the end of solidification (sump tip) to keep constant, but that it is technical is much easier, the slab thickness, as it is fed to the rolling mill, with the help of a casting-rolling step to reduce and achieve what a multi-roll stand (segment 0), e.g. designed as a pliers segment, has proven to be advantageous.

A continuous casting installation which contains all the features of the invention can be seen as an example from (FIG. 5).

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

_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
Outside dimensions e.g. 250 x 45 mm
Internal dimensions e.g. 220 x 15 mm

18

optimized casting powder

19

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

20

15 x 220 mm,
Flow cross section - immersion spout

21

hydraulic mold drive

22

F _ST / F _TA ≤ 50 *)

23

75 x 800 - 1,600 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 mm, slab thickness after the casting and rolling process

28

Segment 1 ... n with hydraulic adjustment or similar

29

V _gmax 6 m / min

30

50 mm, slab thickness at the end of the strand guide

Claims (1)

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

   pouring the metal melt by means of an immersion nozzle into an oscillating rectangular mould whilst observing the conditions 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, wherein even during the casting the frequency, lifting height and form of oscillation for the mould movement can be freely selected.

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