EP1937429A1

EP1937429A1 - Method and apparatus for continuous casting

Info

Publication number: EP1937429A1
Application number: EP06841185A
Authority: EP
Inventors: Uwe Plociennik; Jens Kempken; Peter Jonen; Ingo Schuster; Tilmann BÖCHER
Original assignee: SMS Demag AG
Current assignee: SMS Group GmbH
Priority date: 2006-01-11
Filing date: 2006-12-28
Publication date: 2008-07-02
Anticipated expiration: 2026-12-28
Also published as: PL1937429T3; KR101037078B1; AU2006337470A1; JP2009522110A; ES2321234T3; BRPI0620971B1; CA2635128A1; US8596335B2; WO2007087893A1; ATE425827T1; CA2635128C; KR20080081173A; AU2006337470B2; US8522858B2; TWI382888B; RU2377096C1; TW200732062A; US20090095438A1; DE502006003212D1; US20120111527A1

Abstract

The invention relates to a method for the continuous casting of slab, thin slab, bloom, preliminary section, round section, tubular section or billet strands (1) and the like from liquid metal in a continuous casting plant (2) in which metal discharges perpendicularly downwards from a mould (3), wherein the metal strip (1) is then guided vertically downwards along a perpendicular strand guide (4) and is cooled in the process, wherein the metal strip (1) is then defelcted from the vertical direction (V) into the horiziontal direction (H) and wherein mechanical forming (5) of the metal strip (1) is effected in the final region of the deflection into the horizonal direction (H) or after the deflection into the horizontal direction (H). In order to obtain a surface which has as little scale as possible, provision is made according to the invention for the metal strip (1) to be cooled with a heat transfer coefficient of between 2500 and 20 000 W/(m<SUP>2</SUP> K) in a first section (6, 6A, 6B) in the conveying direction (F) of the metal strip (1) downstream of the mould (3) and upstream of the mechanical forming (5), wherein the surface of the metal strip (1) is heated to a temperature above Ac3 or Ar3 in the conveying direction (F) downstream of the cooling in a second section (7) by heat compensation in the metal strip (1) without or with reduced cooling of the surface of the metal strip (1), after which, in a third section (8), the mechanical forming (5) is effected. The invention also relates to a continuous casting plant, in particular for carrying out this method.

Description

Method and apparatus for continuous casting

The invention relates to a process for the continuous casting of slab, thin slab, ingot, pre-profile, round profile, pipe profile or billet strands and the like of liquid metal in a continuous casting, wherein the metal emerges from a mold vertically downwards, wherein the metal strip is then guided vertically downward along a vertical strand guide and thereby cooled, wherein the metal strip is then bent from the vertical direction in the horizontal direction and wherein in the end region of the bend in the horizontal direction or after the bend in the horizontal direction mechanical deformation of the metal strip takes place. Furthermore, the invention relates to a continuous casting plant, in particular for carrying out this method.

A generic method for continuous casting is known, for example, from EP 1 108 485 A1 or from WO 2004/048016 A2. In this case, liquid metal, in particular steel, is discharged vertically downwards via a mold, wherein it solidifies and forms a metal band which is gradually diverted or bent from the vertical direction into the horizontal direction. Immediately below the mold is a vertical strand guide, which initially leads the still very hot metal strip vertically below. Subsequently, the metal strip is gradually bent by appropriate rollers or rollers in the horizontal. Once this has been done, usually a straightening process follows, ie. H. The metal strip passes through a straightening device, in which a mechanical deformation of the metal strip takes place.

The cooling of the metal strip after its exit from the mold is of great importance. For this purpose, EP 1 108 485 A1 proposes a device for cooling the cast strand in a cooling zone, in which the strand is welded by means of pairs of rolls which are transverse to the strand axis along the strand withdrawal direction. are arranged one above the other, supported, wherein the application of coolant further cools the strand. For efficient cooling of the metal strip, the proposed device comprises a coolant conveying between each two superimposed rolls coolant element extending along the longitudinal axis of the rollers and is designed so that between see the respective cooling element and the roller and the cooling element and the strand gap arise, wherein the respective cooling element is provided with at least one coolant-promoting, opening into a gap channel.

WO 2004/048016 A2 provides for optimal temperature control of the cast metal strip, which is determined by the outlet temperature, which is determined by controlling the surface temperature at the end of the metallurgical strand length of Gießstrangs, a dynamic spray system in the form of water volume distribution and pressure distribution or pulse distribution over the strand width and the strand length is functionally controlled to a temperature course curve calculated for the strand length and the strand width.

A large number of other solutions deal equally with the question of how a cast metal strand can be cooled efficiently and in the right way from the process. Reference is made to JP 61074763 A, JP 9057412, EP 0 650 790 B1, US Pat. No. 6,374,901 B1, US Pat. No. 2002/0129921 A1, EP 0 686 702 B1, WO 01/91943 A1, US Pat. to JP 63112058, JP 2004167521 and JP 2002079356.

It has been found that in addition to the procedurally correct or efficient cooling of the cast metal strip whose scaling plays a significant role. Due to the very high temperature of the metal strip immediately after the exit of the metal from the mold, the strip is subject to a strong scaling effect, which in particular adversely affects the subsequent process steps. It is therefore desirable to keep the degree of scaling as low as possible. The invention is therefore based on the object, a method of the type mentioned and a corresponding device further develop such that it is possible, in addition to optimal cooling of the metal strip also to achieve that the scaling of the strip surface is minimized.

The solution of this problem by the invention according to the method is achieved in that in the conveying direction of the metal strip behind the mold and before the mechanical deformation of the metal strip in a first section, a cooling of the metal strip with a heat transfer coefficient between 2,500 and 20,000 W / (m ² K) , In the conveying direction after cooling in a second section by heat balance in the metal strip with or without reduced cooling of the surface of the metal strip, the surface of the metal strip is heated to a temperature Ac3 or Ar3, after which the mechanical deformation takes place in a third section.

It is preferably provided that in the first section, the cooling of the metal strip with a heat transfer coefficient between 3,000 and 10,000 W / (m ² K) takes place.

If, according to a preferred proposal of the invention, the surfaces of the metal strip are cleaned before being exposed to the cooling medium for cooling, the effect of subsequent cooling can be further improved. The cleaning can be done by descaling, for example, by the opposite in strand or metal strip extraction direction, first reached by the metal strip / strand and thus foremost or topmost coolant (nozzles, nozzle bars od. Like.) The cooling medium Apply under high pressure so that a descaling results.

The mechanical deformation in the third section may be a straightening process of the metal strip or comprise such a process. alternative or additively, it may be provided that the mechanical deformation in the third section is a rolling process of the metal strip or comprises such a process.

The cooling in the first section can - be designed as intensive cooling - limited to the area of the vertical strand guide. In this context, it should be noted that the term of the vertical strand guide should also include that the metal strip is guided largely vertically.

The cooling in the first section can also take place intermittently, wherein the metal strip / strand is alternately intensively and weakly cooled, for example by changing the Kühlmediumbeaufschlagungsdichte [I: min.m ² ] and / or setting different distances of the coolant to the metal strip.

The proposed continuous casting for continuous casting of slabs, Dünnbrammen-, Vorblock-, Vorprofil-, Rundprofil-, Rohrprofil- or billet strands and the like of liquid metal, with a mold, from which the metal exits vertically downwards, a below the mold arranged vertical Strand guide and means for bending the metal strip from the vertical direction in the horizontal direction, wherein mechanical Umformmittel for the metal strip are arranged in the end region of the bend in the horizontal direction or after the bend in the horizontal direction, according to the invention is characterized in that the vertical Strand guide has a number in the conveying direction of the metal strip disposed on both sides of the metal lollen roll, wherein in the region of the rollers first cooling means are arranged, with which a cooling fluid can be applied to the surface of the metal strip, wherein the cooling means in vertical and / or horizontal Direction are arranged displaceably. Alternatively or additionally, the coolant can advantageously be designed to be oscillatable. In addition thereto, second cooling means may be arranged in a fixed position in the region of the vertical strand guide.

The first and / or the second coolant may have a housing, from which the cooling fluid is applied by means of at least one nozzle. The cooling fluid can be applied from the housing by means of two nozzles or nozzle rows.

According to the proposal of the invention, in the area of the secondary cooling of the metal strip, a cooling with a defined intensity, which is chosen so that on the one hand a high-quality metal strip can be produced, which has the desired microstructure and microstructure composition, but on the other hand also the degree of scaling of the strip surface minimal can be held.

The proposal also reduces the accumulation of undesirable side effects on the strip surface.

The proposed procedure results in a sufficient thermal shock such that oxide layers located on the surface of the metal strip are separated off and washed away. This leads to a cleaned strand surface, which is advantageous for uniform cooling of the metal strip and also for possible heating in the tunnel kiln.

The proposed method reduces the risk of precipitation or of so-called "hot shortness", so that advantages are also achieved in this respect .. Due to the necessary for the thermal shock lowering of the surface temperature - this should not fall below the martensite start temperature - there is a transformation of austenite in the metal strip in In the subsequent re-heating due to the large temperature gradient between the strand surface and the core of the metal strip, a return transformation of the fine ferrite into austenite takes place with small grains. In these conversions, the aluminum nitrides (AIN) or other precipitates are overgrown, and on the grain boundaries are percent less aluminum nitrides than the large austenite grain prior to conversion. The finer structure is therefore less susceptible to cracking if excreta should be present.

In the strand guide below the mold, the area for intensive cooling is provided so that the reheating can take place as early as possible. The ferrite transformation and the subsequent transformation into austenite should take place before the mechanical stress of the strand surface, for example in the bending drives. This measure reduces the risk of crack formation due to the temperature drop of the strand caused by the thermal shock. An embodiment of the method provides that said (intensive) cooling comprises about one-quarter to one-third of the (arc) path from the mold to the mechanical forming, followed by about three quarters or two-thirds of this path, on which no more or only reduced cooling is.

The intensive cooling provided according to the invention can be arranged between the strand guide rollers and, depending on the desired cooling effect, extend over a longer region of the strand guide. It may also be advantageous, as mentioned, to apply the intensive cooling intermittently in order not to undercool the surface, particularly in the case of crack-sensitive materials.

Thus, the hot brittleness, ie the cracking of the slab surface, can be reduced, which can be caused in particular by a high copper content in the material. This is particularly relevant for scrap as a starting material, which sometimes has a correspondingly high copper content. In the drawings, embodiments of the invention are shown. Show it:

Fig. 1 shows schematically a continuous casting in the side view with the representation of some of the components of the system;

Figure 2 is an enlarged detail of Figure 1, namely the right branch of the vertical strand guide with first and second cooling means ..;

FIG. 3 shows a further enlarged detail of FIG. 2 with two rollers and a coolant arranged therebetween; FIG. and

4 shows the coolant according to FIG. 3 in detail.

In Fig. 1, a continuous casting plant 2 is shown schematically. Liquid metallic material emerges vertically downwards as a strand or metal strip 1 from a mold 3 in the conveying direction F and is gradually diverted from the vertical V into the horizontal H along a casting arc section. Immediately below the mold 3 is a vertical strand guide 4, which has a number of rollers 10, which lead the metal strip 1 down. A number of rollers 9 act as a means for bending the metal strip 1 from the vertical V into the horizontal H. After the bending has taken place, the metal strip 1 arrives in mechanical deformation means 5. In the present case, this is a straightening driver, which subjects the metal strip 1 to a straightening process by mechanical deformation. It is also possible to provide a rolling process, which usually follows.

The region of the metal strip from the exit from the mold 3 to the mechanical deformation is subdivided into three sections. In a first section 6, intensive cooling of the hot metal strip 1 takes place. In a second section 7, virtually no further cooling takes place Heat in the metal strip 1 heats the cooled surface of the metal strip 1 again. Primarily in the third section 8, but also already in the second section 7, then finally the mechanical deformation takes place. The exemplary embodiment shows that the first section 6 is again subdivided into sections 6A and 6B. This allows in a simple manner an intermittent cooling in the first section 6, namely an intensive cooling in the section 6A and a weaker or reduced or even no cooling in the at least one further follower section 6B, which in turn can subsequently be followed by an intensive cooling section.

The cooling of the metal strip 1 takes place with first coolants 11 and second coolants 12, as can best be seen in FIG. The first coolant 11 work so intensively that a large cooling capacity is present. The second coolant 12 is conventional and per se known coolant, which are used in previously known continuous casting. The design of the coolant 11 is carried out so that the cooling of the metal strip 1 in the first section 6, in particular in the mold 3 immediately adjoining section 6A, which in the extension direction F uppermost or foremost coolant for descaling and thus cleaning the surfaces of the metal strip. 1 can be switched to high pressure, with a heat transfer coefficient between 2,500 and 20,000 W / (m ² K). In this case, the predominant part of the cooling goes back to the first coolant 11.

As regards the heat transfer coefficient mentioned above, the heat transfer coefficient (symbol α), also called heat transfer coefficient or heat transfer coefficient, is a proportionality factor which determines the intensity of the heat transfer at a surface. The heat transfer coefficient here describes the ability of a gas or a liquid to dissipate energy from the surface of a substance or to deliver it to the surface. It depends, among other things, on the specific heat, the density and the thermal conductivity coefficient of the heat-dissipating and heat-dissipating medium. The calculation of the coefficient for heat conduction takes place mostly about the temperature difference of the involved media. The factors mentioned immediately show that the design of the intensity of the cooling has direct effects on the heat transfer coefficient. The cooling performance can be influenced for example by changing the horizontal distance between the cooling means 11 and 12 and the metal strip 1; it becomes lower, the greater the distance.

After cooling in sections 6 and 6A, 6B takes place in the second section 7 by heat balance in the metal strip 1 without further cooling of the surface of the metal strip 1, a heating of the surface of the metal strip 1 by heat compensation to a temperature over Ac3 or Ar3. Only then does the mechanical deformation 5 take place in sections 7 (by bending) and 8, in particular by the straightening in section 8.

The mentioned coolant 11 are not needed for every application. Therefore, they are - as is apparent from Fig. 2 - arranged displaceably in the vertical direction, with corresponding movement means are not shown. Shown are the coolant 11 in solid lines in its active position, wherein the ejected jet cooling water takes the outlined course.

If the intensive cooling is not required, the coolant 11 can be moved vertically in the position shown in dashed lines, so that a classic, lower, d. H. less intensive cooling by the coolant 12 is accomplished.

Other measures for influencing (reducing or increasing) the cooling capacity are to change the distance between the cooling means 11, 12 and the metal strip 1 by horizontal displacement and / or to adjust the coolant 11, 12 in an oscillating manner. Not shown are appropriate piping systems with valves, so that the current required cooling water can be set or switched.

FIGS. 3 and 4 show a variant of the embodiment of the first coolant 11 in greater detail. The cooling means 11 have a housing 13, on whose side facing the metal strip 1, two nozzles 14 and 15 or rows of nozzles extending perpendicularly to the plane of the drawing over the metal strip 1 are arranged. The housing 13 has in its interior according to two chambers 16, 17 which are each fluidly connected to a water supply line. The nozzles 14 and 15 are designed differently, so that different degrees of water flow can be directed to the metal strip 1 - depending on the technological need to achieve a scale-free as possible and thus cleaned surface of the Metallban-.

The nozzles may also be designed as nozzle bars, d. H. as a beam which extends across the width of the metal strip 1 and passes cooling water from a number of nozzle openings on the strip surface.

The proposed device for intensive cooling thus has a housing which can be pushed with a small distance between the continuous casting guide rollers 10 and thus forms a cooling channel. The housing 13 can be protected from destruction by a fender (not shown) in the event of a breakthrough, so that it can be reused in this case. By changing the distance between the strand surface and the housing 13, the cooling effect can be influenced. Further influence on the cooling effect can be achieved by the construction of the housing and the nozzles 14, 15.

So it is possible to divide the nozzles into several groups and to provide the individual nozzle groups with its own water supply. By switching individual nozzle groups on and off and / or by changing them the flow or the fluid pressure can then be varied, the cooling effect. In the case of standard cooling, ie if steels are processed in which intensive cooling does not make sense, a smaller number of nozzles can be switched on. Another possibility is to move away or drive away the intensive cooling device from the spray area of the standard cooling.

A subcooling of the edge region of the metal strip can also be avoided by switching on and off of nozzle groups.

For intensive cooling, spray nozzles can also be used. These should be distributed close to each other across the width of the metal strip to achieve the necessary cooling and cooling necessary and the grain refining and descaling effect associated therewith. By switching these groups on and off, undercooling of the edges can also be avoided. For the casting operation, where intensive cooling is not advantageous, the nozzles can be deactivated, swung away, moved away or the flow of cooling medium (water) can be reduced to ensure standard cooling.

It can also be provided that to the existing secondary cooling additional cooling, consisting of several provided with spray nozzles spray bar are used with a separate water supply. The additional spray bars are only switched on when needed. It is also possible here to avoid subcutaneous cooling of the edges by switching on and off nozzle groups.

In the prior art, special descaling nozzles are known for descaling, which achieve heat transfer coefficients of more than 20,000 W / (m ² K).

Such nozzles come for the present invention because of their excessive cooling effect and the associated low surface temperature the surface of the metal strip is not used or they are not useful here.

The core idea according to the invention can thus be seen in the fact that intensive cooling takes place in the area of secondary cooling, in particular in thin-slab plants, in order to achieve a cleaning of the surface of the slab in which the intensive cooling begins shortly after the mold, viewed in the conveying direction. However, it is further provided that the cooling ends so early that a rewarming above the temperature Ac3 or Ar3 can take place before mechanical stresses occur, as is the case, for example, with the bending driver. The aim is to have no or only a small excretion on the grain boundaries.

The proposed device for intensive cooling has a significantly higher cooling effect than is otherwise the case with the secondary cooling of a continuous casting plant. In prior art systems, the usual heat transfer rates between 500 W / (m ² K) and 2,500 W / (m ² K). On the other hand, desiccation systems are known in which a cooling device is used which realizes heat transfer coefficients of more than 20,000 W / (m ² K).

The heat transfer rates required here are - as already indicated above - material-dependent and also dependent on the casting speed. They result from the maximum cooling rate at which no martensite or interstitial structure is yet produced. For low carbon steels, the cooling rate is about 2,500 ° C / min, which corresponds to a heat transfer coefficient of about 5,500 W / (m ² K) at a casting speed of 5.0 m / min.

By a fast switching between standard and intensive cooling, the proposed continuous casting is very individual and flexible usable. If the proposed systems are used with the described cooling nozzles, as a result of the forming high turbulence of the water between the housing of the coolant and the metal strip with relatively small amount of water higher heat transfer coefficients than in conventional spray cooling can be achieved.

The intensity of the cooling can be varied by the number of nozzles arranged side by side. Furthermore, it is also possible to use additional nozzle bars to conventional spray cooling devices.

The length of the intensive cooling - viewed in the conveying direction F - is determined by the solidification structure to 2 mm below the surface of the metal strip. In the case of dendritic solidification, the intensive cooling length is lengthened by about a factor of 2 to 3 compared with the length in the case of globulitic solidification.

The heat transfer coefficient also results from the design of the coolant, in this case in particular the first coolant 11. The number is selected specifically in the claimed range, since the conditions for intensive cooling of the finished metal strip 1 are optimal and at the same time a largely scaling belt surface can be achieved.

LIST OF REFERENCE NUMBERS

1 metal band

2 continuous casting plant

3 mold

4 vertical strand guide

5 mechanical deformation

6 first section

6A subsection

6B following section

7 second section

8 third section

9 means for bending the metal strip

10 rolls

11 first coolant

12 second coolant

13 housing

14 nozzle

15 nozzle

16 chamber

17 chamber

V vertical direction

H horizontal direction

F conveying or extension direction

Claims

claims:

A process for the continuous casting of slab, Dünnbrammen-, Vorblock-, Vorprofil-, Rundprofil-, Rohrprofil- or billet strands (1) and the like of liquid metal in a continuous casting plant (2), wherein the metal from a mold (3) perpendicular downwards, wherein the metal strip (1) is then guided vertically downwards along a vertical strand guide (4) and thereby cooled, wherein the metal strip (1) then bent from the vertical direction (V) in the horizontal direction (H) and wherein in the end region of the bend in the horizontal direction (H) or after the bend in the horizontal direction (H), a mechanical deformation (5) of the metal strip (1), characterized in that in the conveying direction (F) of the metal strip ( 1) behind the mold (3) and before the mechanical deformation (5) of the metal strip (1) in a first section (6, 6A, 6B) a cooling of the metal strip (1) with a heat transfer coefficient between 2.500 and 20,000 W / (m ² K) takes place, in the conveying direction (F) after cooling in a second section (7) by heat balance in the metal strip (1) with no or reduced cooling of the surface of the metal strip (1) heating of the surface of the metal strip (1) to a temperature about Ac3 or Ar3 takes place, after which the mechanical deformation (5) takes place in a third section (8).

2. The method according to claim 1, characterized in that in the first section (6), the cooling of the metal strip (1) with a heat transfer coefficient between 3,000 and 10,000 W / (m ² K) takes place.

3. The method according to claim 1 or 2, characterized in that the surfaces of the metal strip (1) are cleaned immediately before cooling.

4. The method according to any one of claims 1 to 3, characterized in that the first portion (6) is divided, wherein the metal strip (1) is cooled intermittently and in one of the mold (3) immediately downstream section (6A) and intensive in at least one following partial section (6B) is weaker and then subsequently cooled again more intensively.

5. The method according to any one of claims 1 to 4, characterized in that the mechanical deformation (5) in the third section (8) is a straightening process of the metal strip (1) or comprises such a process.

6. The method according to any one of claims 1 to 4, characterized in that the mechanical deformation (5) in the third section (8) a

Rolling process of the metal strip (1) is or includes such a process.

7. The method according to any one of claims 1 to 6, characterized in that the cooling in the first section (6, 6A, 6B) is limited to the region of the vertical strand guide (4).

8. Continuous casting plant (2) for continuous casting of slabs, Dünnbrammen-, Vorblock-, Vorprofil-, Rundprofil-, tubular profile or billet strands (1) and the like of liquid metal, with a mold (3), from the Metal vertically downwards exits, a below the mold (3) arranged vertical strand guide (4) and means (9) for bending the metal strip (1) from the vertical direction (V) in the horizontal direction (H), wherein in the end of the Bending in the horizontal direction (H) or after the bend in the horizontal direction (H) mechanical forming means (5) for the metal strip (1) are arranged, in particular for carrying out the method according to one of claims 1 to 6, characterized in that the vertical strand guide (4) has a number of rollers (10) arranged in the conveying direction (F) of the metal strip (1) on both sides of the metal strip (1), first cooling means (11) being arranged in the region of the rollers (10) a cooling fluid can be applied to the surface of the metal strip (1), wherein the cooling means (11) in the vertical and / or horizontal direction (V, H) are arranged displaceably.

9. Continuous casting plant according to claim 8, characterized in that the cooling means (11) are designed to be oscillatable.

10. Continuous casting plant according to claim 8 or 9, characterized in that additional second coolant (12) in the region of the vertical strand guide (4) are arranged stationary.

11. Continuous casting plant according to one of claims 8 to 10, characterized in that the first and / or the second coolant (11, 12) have a housing (13) from which the cooling fluid by means of at least one nozzle (14, 15) is applied ,

12. Continuous casting plant according to claim 11, characterized in that cooling fluid from the housing (13) by means of two nozzles (14, 15) or

Dϋsenreihen is applied.