EP1957220A1

EP1957220A1 - Method of operating a twin-roll casting machine for casting molten metal into cast strip

Info

Publication number: EP1957220A1
Application number: EP06819633A
Authority: EP
Inventors: André D'HONE; René LEENEN
Original assignee: ThyssenKrupp Nirosta GmbH
Current assignee: Outokumpu Nirosta GmbH
Priority date: 2005-11-18
Filing date: 2006-11-20
Publication date: 2008-08-20
Anticipated expiration: 2026-11-20
Also published as: EP1957220B1; KR101323335B1; ATE431209T1; KR20080065310A; DE502006003743D1; US7938167B2; WO2007057469A1; DE102005054996A1; US20090090484A1

Abstract

A method for operating a two-roll casting machine for casting molten metal into cast strip, which machine has two casting rolls which are each rotatively driven in opposite directions about an axis of rotation and between them delimit a casting gap on its longitudinal sides, with side plates that can be placed on the casting rolls, which side plates seal the casting gap on its narrow sides in the casting operation, bridging the casting gap with a refractory material, wherein the side plates are moved during the casting operation in a direction which is aligned parallel to the direction of conveying in which the cast strip leaves the casting gap. With such a method the formation of grooves distorting the casting result and the casting operation can be suppressed with increased certainty and the service life of the side plates can be increased, compared to the prior art.

Description

Method for operating a two-roll casting machine for casting molten metal into cast strip

The invention relates to a method for operating a Zweiwalzengießmaschine for casting molten metal, in particular molten steel, to cast strip, the two casting rolls, which are driven in opposite directions about an axis of rotation and define between them a casting gap on the longitudinal sides, and with deliveries to the casting rolls Side plates which seal the casting gap on its narrow sides in the casting operation by bridging the casting gap with a refractory material on the end faces of the casting rolls, wherein the side plates are moved during the casting operation in the vertical direction, which is aligned parallel to the conveying direction, in which cast strip leaves the casting gap, wherein arise in the refractory material of the side plates with increasing operating time gap-like recesses.

In strip casting by the two-roll method, the melting space at the end faces of the casting rolls is usually sealed with side plates which carry a ceramic insert forming the actual seal. This ceramic is moved axially during the casting process in the direction of the casting rolls, on the one hand to compensate for the resulting by the abrasion of the casting rolls removal of the ceramic material and on the other the wear of the insert, due to abrasion by the in the casting gap forming band edge and chemical reaction is caused.

In practice, it has been found that the wear caused by the strip edges on the inserts of the side plates has a decisive influence on their service life and thus the overall economy of a strip casting plant of the type in question. If the wear caused by the band edges is not counteracted in good time, slit-like recesses, so-called "grooves", originate in the inserts from the surface of the respective insert assigned to the casting gap and extend along the contact region on which the circumference of the insert extends Casting rolls bears against the peripheral surfaces of the insert. In accordance with the increasing strength of the potted melt in the direction of the exit of the casting gap, the width and depth of the grooves increase in the direction of the narrowest region of the casting gap, against which the metal shells solidified on the casting rolls are pressed against one another. As a result of extrusion effects, the gap-like recesses can work into the areas of the insert ceramic lying between the end faces of the casting rolls and their associated contact surfaces of the inserts. The thus enlarged gap-like recesses then have a T-shaped cross-section. In the T-slot-like recesses urgent melt can solidify at the end faces of the casting rolls. The solidified pieces of melts, in technical language also called "T-edges" are entrained by the casting rolls and can cause such significant disturbances of the casting operation or the casting result.

If such recesses form on the insert, molten metal can penetrate into these grooves and solidify there. At the exiting from the casting gap such solidification manifest themselves in that the edge of the strip in cross-section has a "T-shape" formed at the edges in the region of the transition of the narrow sides to the longitudinal sides of the strip cross-section thin webs of solidified metal which can lead to considerable problems in the casting operation and have a decisive negative influence on the quality of the resulting cast strip. Thus, in the casting gap between the insert and the respective casting roller reaching metal is cooled much more than the metal volume, which is located centrally in front of the insert. Whereas the metal which has solidified between the respective casting roll and insert is entrained across the width after its exit from the casting gap due to shrinkage of the strip, the middle part of the strip subsequently falls out of the casting gap

Gravity influence. As a result, arise in the not yet solidified strip material tensions, which bring the risk of the formation of longitudinal cracks in the finished cast strip with it. In extreme cases, the thus separated, with the casting rollers encircling T-edges can result in a casting demolition.

From WO 2004/000487 it is known that the emergence of grooves can be counteracted by the fact that the side plates with their inserts after reaching a stationary casting process in a first time interval in the axial direction of the casting rolls against the end faces of the casting rolls and then moved in a second time interval in a direction which is parallel to the conveying direction, with which the cast strip leaves the casting gap. In this case, the first time interval of the axial movement may overlap the second time interval of the movement taking place parallel to the conveying direction of the belt at least temporarily. However, the first time interval should be used before the second one begins, to ensure proper grinding of the side plate inserts. The feed, with which the side plates are moved with their inserts in the conveying direction of the cast strip, is in the range of 50 mm per hour, preferably in the range of 1 mm to 30 mm per hour casting time.

Practical experiments have shown that the procedure known from WO 2004/000487 can reduce the risk of the formation of grooves and the associated problems. However, it was found that this reduction was not yet sufficient to avoid the emergence of larger grooves under conditions of practical use with adequate safety for continuous operation.

In addition to the above-described prior art, another method for casting a metal strip on a twin-roller casting machine is known from DE 100 56 916 A1, whose casting gap formed between the two casting rolls at its short Pages also by an insert

Refractory bearing side plates is sealed. To optimize the seal while the side plates are moved along an approximately vertical plane in the casting direction, wherein the displacement is superimposed on an oscillating vibration. The aim of this measure is the avoidance of early solidifying, adhering to the side plates particle accumulations, so-called "crusts". These crusts can locally lead to accretions on the side plates, which already solidified in the existing in the casting gap in the areas adjacent to the casting rolls, but between them still liquid metal rich. Problems of this kind occur in particular in those two-roll casting plants, in which in particular the temperature and flow conditions in the side plate area are unfavorable. As the particles adhering to the side panels become loose, the tape will increase locally in the periphery. As a result of the tape casting in the two-roll process in this state still possible massive spreading of the tape, this can lead to an extreme pressure increase on the laterally limiting inserts of the side plates, which provokes a fraction just such, especially in the lower region. Such and also caused by other effects fractures of the lower insert sections usually cause the newly created by the break, upwardly displaced insert lower edge below the process-related band shell touch point, the so-called "kissing point" is. A correspondingly insufficient positioning of the lower edge of the insert can be further caused by unfavorable process parameters, which trigger a shift of the kissing point under the lower edge of the insert. These relationships result in an undesirable, so-called

"Drip edge formation", where the still liquid material pushed out of the casting gap due to banding force or runs out directly in extreme cases.

In practice, this risk is countered by the fact that with corresponding insufficient positioning of the lower edge, the side plates are lowered for a short time. The problem of the formation of gap-like recesses, which occurs in particular in sophisticated and particularly powerful two-roller casters, is not dealt with in DE 100 56 916 A1.

Based on the above-described prior art, therefore, the object of the invention to provide a method in which suppresses the emergence of the casting result and the casting disturbing grooves with increased safety and the life of the side plates and the security, with a stable Casting can be maintained, compared to the prior art is increased.

This object has been achieved by the subject matter of claim 1. Advantageous embodiments of this solution are specified in the dependent claims on claim 1.

According to the invention, the side plates are adjusted with their insert from Gießbeginn taking into account each cast length of the cast strip. The inventive reference to the each cast, directly dependent on the respective casting speed strip length is taken into account in the adjustment of the side plates each produced strip thickness and thus the conditions prevailing in the casting gap conditions. In this way, irrespective of the thickness of the respective band is achieved that at the critical narrowest point of the insert even at long service Grooves at best to an extent that they represent no danger to the casting operation and also in terms of quality of the obtained tape are not critical.

In this context, the invention is based on the empirical assumption that at non-moving side plates due to the emergence of the grooves in the worst case, the point at which melt penetrates into the recess between insert and casting roll recess, at about 3/4 of Height of the bath level measured over the outlet of the casting gap and the width of the groove formed in each case corresponds to a maximum of half the strip thickness to a good approximation. Based on this assumption can then be estimated taking into account the progress of the formation of the grooves in the axial direction of the casting rolls in accordance with the invention, the Absenkrate by which the side plates must be lowered in the vertical direction to prevent the formation of grooves.

Of crucial importance here is that the movement of the side plates according to the invention proceeds continuously and a method of inserts with increased movement rate due to unfavorable process parameters or a fraction of the insert lower edges only along the Casting done. This is the only way to ensure that in the area of the critical sections of the casting gap, in which the already solidified strip edges move along the insert, so much unwrapped insert material is tracked that no deep recesses occur in the contact area between insert and casting roll circumference.

In addition, it is particularly important for the success of the invention that the movement carried out according to the invention starts immediately with the casting operation. In this way, the area in which it could come to the penetration of molten metal between the insert and the casting roll, from the outset in the conveying direction of the belt, ie in vertical orientation of the conveying direction down, relocated. Due to continuous displacement of the side plate in the conveying direction of the cast strip, the cast molten metal has less time in the critical area for solidification, so that the abrasive wear caused by it is low.

The adjustment according to the invention leads to a significant extension of the service life of the side plates used in a two-roll casting machine. Thus, it has been shown that the Anstellrate with which the side plates in addition to the invention in the conveying direction of the tape movement in the axial direction, ie along the rotational or longitudinal axes of the casting rolls, must be moved in a machine operated according to the invention to about one Can be reduced by a third of the setting rates set in conventional two-roll casting machines. Of the Accordingly, the thickness loss of the side plates occurring over the casting time is correspondingly low, so that a lifetime of the inserts of the side plates which is significantly longer than the prior art is achieved.

Practical experiments have shown that good casting results can be achieved with the inventive method and typical casting speeds of 10-150 m / min if the side plates move at a rate of at least 0.1 mm per km of cast strip to at most 50 mm per km of cast strip , preferably 0.5-1.5 mm / km of cast strip, are moved in the conveying direction of the cast strip.

The respectively specifically selected rate of movement of the adjustment according to the invention can be adjusted within the abovementioned limits, taking into account the properties of the respectively processed molten metal and the remaining boundary conditions of the casting. For example, when casting high temperature steel grades and inserting lower hardness inserts, higher rates of movement can be set to compensate for the wear of the insert than is the case when less temperature resistant molten steel is cast using a harder insert. Also, for example, when required by high cooling capacity or a corresponding composition of the process gas used in each case to support the formation of the band shells rapid solidification of the metal and accordingly high band forming forces In order to avoid the formation of grooves, higher movement rates are set than under process conditions in which the respectively cast metal in the casting gap becomes less fast and correspondingly less wear of the inserts occurs.

If drip edges are detected on the cast strip despite the procedure according to the invention, this event can be reacted by moving the insert at an increased rate of movement in the casting direction until the strip emerging from the casting gap again has properly shaped edges. The limitation of the movement rate to a maximum of 50 mm per km cast strip in this context, taking into account today required Gießwalzenoberflachenrauheit and the currently available and common Insertharten to limit the vertical and horizontal forces to the extent that no insert break occurs due to excessive load.

Even with a change in the process parameters, it may be useful to increase the rate at which the side plates are moved in accordance with the invention in order to prevent the formation of dripping edges. If, for example, the thickness of the strip to be produced is changed or the solidification rate of the melt is increased by increased cooling power or use of a specially composed process gas, it may be expedient, for example, to increase the rate of movement of the movement of the side plates according to the invention, in order to accommodate the changes associated with these changes the wear conditions. Depending on how strongly the material of the insert of the side plates is attacked by the melt in contact with it, the determination of the lowering rate should also take into account the time over which contact between the melt and the insert has taken place. For this purpose, it is therefore provided according to a preferred embodiment of the invention that the Absenkrate is increased with increasing contact time. In the formula context provided for determining the most favorable lowering rate, this dependence can then be incorporated as follows:

_ (a _G - a _v ) ^■ Ah _s ^■ t _c

K ^■ t _cref

wherein with t _c the respective current and with t _Cre f an empirically determined for each casting machine, material-specific reference _{contact time} t _Cre f are designated. The reference contact time t _Cre f can thereby be determined by a casting test in which the respectively castable melt material is cast in combination with the insert ceramic to be used in each case with this melt material in the respective two-roll casting machine without the side plates provided with the ceramic inserts to be moved. The reference contact time t _Cre f is then the average time at which a corresponding Groov education has formed over the cast length.

A further advantageous embodiment of the invention is characterized in that the side plates are additionally moved in the axial direction of the casting rolls against their respectively associated end faces with a force superimposed, for example axial Bewegunsgsrate. at This combined adjustment of the side plates in the axial direction of the casting rolls and conveying direction of the cast strip is ensured at all times that the side plates bear with the force required for the sealing of the casting gap on the end faces of the casting rolls. The movement of the side plates in the axial direction of the casting rolls is preferably carried out continuously parallel to their movement according to the invention in the conveying direction and depending, for example, on the high temperature properties of the refractory used for the insert of the side plates, the casting properties of the cast steel, the strip thickness, in the conveying direction of the cast strip effective lowering rate and / or the progress of the training of the insert.

The invention will be explained in more detail with reference to an exemplary embodiment drawing. Each show schematically:

Figure 1 is a Zweiwalzengießmaschine in a side view.

FIG. 2 shows the two-roll casting machine in a section along the section line A-A shown in FIG. 1; FIG.

Fig. 3 is a side plate in frontal view.

The two-roll casting machine 1 comprises, in addition to a large number of other units not shown here, two casting rolls 2, 3 which are parallel to one another in their longitudinal direction are aligned and define a casting gap 4 on its longitudinal sides between them. The casting rolls 2, 3 rotate in opposite directions about their horizontally oriented axes of rotation D 2, D 3, which coincide with their longitudinal axes, so that their circumferential surfaces, coming from above, dip into the casting gap 4. The cast strip B formed in the casting gap 4 leaves the casting gap 4 at its lower narrow exit opening 4a in a vertically oriented direction F.

Both narrow sides of the Gießspalts 4 are sealed by a respective side plate 5, 6, which is formed by a support plate 5a and a supported by the support plate 5a, to the respective side plate 5,6 associated end faces of the casting rolls 2.3 fitting insert 5b is made of a ceramic refractory material. The inserts 5b of the side plates 5,6, which are wider than the respective narrow side of the pouring gap 4 by a specific excess, have a triangular basic shape, corresponding to the shape of the narrow sides of the pouring gap 4, in their opposite, the circumferential surfaces of the casting rolls 2 and 3 respectively Pages 5c, 5d when retracting the side plates 5,6 are formed in a conventional manner by a targeted abrasive wear by means of the casting rolls 2.3 each arcuate recesses whose radius is adapted to the circumferential radius r _G w of the casting rolls 2.3. In this way, in the prepared state for the casting operation, the inserts 5b of the side plates 5, 6 both on the end faces and on the peripheral surfaces of the casting rolls 2, 3 close to. In the axial direction of the casting rolls 2, 3, the side plates 5, 6 are actuated by adjusting means 7 a, 7 b, 8 a, 8 b acting in horizontally along the axes of rotation D 2, D 3 of the casting rolls 2, 3, with the contact force required for a permanent seal and / or with a force superimposed, for example, axial movement rate, pressed against their respective associated end faces of the casting rolls 2.3. The adjusting devices 7a, 7b, 8a, 8b act via an intermediate plate on the side plates 5, 6, on which the respective side plate 5.6 in a vertical, parallel to the conveying direction F of the cast strip B aligned direction Z is slidably mounted. In addition, the intermediate plate carries in each case a pair of 9.10 of vertical, to the direction of action F parallel effective direction Z acting adjusting 9a, 9b, of which one adjustment 9a on the one casting roll 2 and the other adjusting device 9b on the other casting roll associated side of the side plates 5, 6 is arranged.

For producing the strip B cast, for example, from a molten steel melt, the molten steel is poured into the casting gap 4 with casting rolls 2, 3 rotating at a certain peripheral speed, so that a melt pool P forms over the outlet opening 4a, the bath level of which rises at a height hi the outlet opening 4a of the casting gap 4 is located. The melt impinging on the circumferential surface of the casting rolls 2, 3 solidifies there, so that in each case a band shell forms on the casting rolls 2, 3. These band shells are taken from the casting rolls 2.3 in the direction of the outlet opening 4a of the casting gap 4 until they are in the so-called Nip-point NP meet and there are pressed together to the cast strip B to be produced.

With the beginning of the casting operation, the side plates 5, 6 are continuously lowered by their respectively associated adjusting 9.10 in the vertical direction of action Z parallel to the conveying direction F of the cast strip B with a certain Absenkrate. In this way, by constantly feeding in insert material, the height and thus the extent of the grooves G1, G2 is minimized, which is between the circumferential surfaces of the casting rolls 2, 3 and the side faces 5 c, 5 d of the inserts 5 adjoining them , 6 due to the abrasive, unavoidable by the contact of the respective insert 5.6 with the solidifying in the casting gap 4 molten steel melt.

If a strip B with a strip thickness s of 3 mm is to be cast from a molten steel, the result is a height hi of the melt pool bath level above the outlet opening 4a of the pouring gap 4 of 400 mm, a height difference h _{G of} the respectively considered groove G1, G2 to the nip point NP of the casting gap 4 of 300 mm, a circumferential radius r _G w of the casting rolls 2, 3 of 750 mm, an axial groove formation rate a _G of 1.5 mm / km, an axial feed rate a _v of 1.0 mm / km, a critical groove depth k _G of 1.0 mm and an exponent n of 0.5 empirically determined from the so-called root-T-law, using the formulaic relationships given above

^G ' Δh = h "- J f (s _G + V ^r GW ² - ^h L ² I - ^r G, * -1

and

(a _R - a. Ah v "=

K

for the lowering of the side plates 5, 6 in the direction of action Z a Absenkrate of 0.86 mm per km cast strip.

If the axial feed rate a _{v is} to be reduced to 0.75 mm / km cast strip in order to extend the service life of the respective insert 5, 6, for example, the vertical lowering rate increases to 1.29 mm / km cast strip, in accordance with the above formula , With decreasing axial feed rate, the necessary vertical lowering rate for groove avoidance is thus to be increased.

If the influence of the chemical decomposition of the insert, which occurs as a result of the contact between the insert and the melt, should also be compensated, the respective contact time t _c between melt and insert can be taken into account when calculating the vertical settling rate v _s according to the following proviso:

_ (a _G - a _v ) ^■ Ah _s ^■ t _c

K ^■ t _cref

The reference _{contact time} t _Cre f is determined empirically for each of the steel materials to be cast and for each casting machine. Basically, it is true that ever smaller is the contact time t _c between ceramic insert and melt, the lower is the respectively required movement rate of the ceramic.

As a typical example of this dependence, in the table below, the groove formation rates a _G and axial feed rates a _v determined for a SiO ₂ ceramic having an Si content of more than 50% and a Mn-containing melt are set for different contact times t _c and Mn contents% Mn indicated

% Mn [% 0.3 1.2 0, 3 1, 2 tc [S] 0.5 0.5 0, 4 0, 4 aG [mm / km cast tape] 1.5 3, 0 1, 2 2 , 4 aV [mm / km cast band] 1,0 2,5 0, 7 1, 9

REFERENCE NUMBERS

1 two-roll casting machine

2, 3 casting rolls

4 casting gap

4a outlet opening of the casting gap 4

5 side plate

5a supporting plate of the side plates 5

5b insert of the side plates 5

5c, 5d sides of the insert 5b associated with the peripheral surfaces of the casting rolls 2, 3

6 side plate

7a, 7b, 8a 3b acting in the direction of action Y adjustment

9 pairs of adjusting devices 9a, 9b

9a, 9b acting in the direction of action Z adjustment

10 pairs of adjusting devices 9a, 9b

B cast tape

D2, D3 axes of rotation of the casting rolls 2,3

F conveying direction of the cast strip

Gl, G2 recesses ("grooves") hi height of the melt pool bath level above the

Outlet opening 4a of the pouring gap 4 h _G height difference of each considered groove

G1, G2 to the nip-point NP of the casting gap 4 NP Nip-Point

P melt pool r _G w circumference radius of the casting rolls 2.3 s thickness of the strip B

Y horizontal direction of action

Z vertically aligned effective direction

Claims

November 17, 2006P ATENTANSPR Ü CHE

1. A method for operating a Zweiwalzengießmaschine for casting molten metal, in particular molten steel, to cast strip (B), the two casting rolls (2,3), which are driven in opposite directions about a rotational axis (D2, D3) and between them a casting gap (4) limit its longitudinal sides, and with the casting rollers (2,3) deliverable side plates (5, 6), the casting gap (4) seal in the casting on its narrow sides by bridging the casting gap (4) with a refractory material abutting the end faces of the casting rolls (2, 3), the side plates (5, 6) being moved in the vertical direction (Z) during the casting operation, which is aligned parallel to the conveying direction (F), in which the cast strip (B ) Leaves the casting gap (4), wherein in the refractory material of the side plates with increasing operating time gap-like recesses (G1, G2) arise, characterized in that the lowering rate v _s to the Seitenpla (5, 6) in the vertical direction (Y) depending on the respective cast length of the cast strip (B) can be lowered, is set as follows: (a _G - a _v ) ^• Δh k "

v _s : Lowering rate in mm per km of cast strip (B), a _G : Rate at which the slit-like

Form recesses (G1, G2) in the axial direction, in mm per kilometer cast strip (B), a _v : feed rate of an optional one

Movement of the side plates (5, 6) in the axial direction (Y) in mm per km of cast strip (B), k _G : critical depth of the gap-like recess (G1, G2) in mm,

Δh _s : Amount by which the side plates (5, 6) are at least displaced in the vertical direction during the casting operation in order to avoid the band (B) cast per kilometer that is produced with the formation of a gap-like recess (G1, G2) following formula context (in mm):

(h _x - h _G ) ⁿ s with s _G : s _r = - h _χ ^{n ■} 2 hi: height of the bath level; h _G : height difference of the respective considered gap-like formation to the point (NP) of the casting gap (4), in which meet on the casting rolls (2,3) solidified band shells; n: 0.5-0.65; r _G w: circumferential radius of the casting rolls (2, 3); s: thickness of the cast strip (B).

2. Method according to claim 1, wherein the side plates (5, 6) are moved in the vertical direction (Z) by 0.1 mm to 50 mm per kilometer of cast strip (B).

3. The method according to claim 2, wherein the side plates (5, 6) are moved in the vertical direction by 0.5 mm to 1.5 mm per kilometer of cast strip (B).

4. Method according to one of the preceding claims, characterized in that the side plates (5, 6) are additionally moved in the axial direction (Y) of the casting rolls (2, 3) against their respectively associated end faces.

5. The method according to claim 4, characterized in that the side plates (5, 6) in the axial direction of the casting rolls (2,3) in dependence on the respective cast length of the cast strip (B) are moved continuously. Method according to one of the preceding claims, characterized in that the side plates (5, 6) are accelerated lowered at the outlet of melt at their lower in the conveying direction of the belt edges until the release of melt is completed again.