EP1019208B1 - Mould pipe for a continuous casting mould for the continuous casting of steels, especially peritectic steels - Google Patents

Description

The invention relates to a mold tube for a continuous casting mold Continuous casting of steels, especially peritectic steels, according to the Preamble of claim 1 and a continuous casting mold with the mold tube.

The technique of continuous casting, by cooling a molten metal one on the walls of a mold cavity of a continuous casting mold Strand shell with continuously increasing thickness formed and continuous a strand is pulled out of an outlet opening of the continuous casting mold as is known for an application to peritectic steels, for example Steels with a carbon content of 0.1-0.14%, to problems that arise especially in a poor surface quality of the manufactured Express strands. Such quality defects are undesirable, especially one Further processing of the strands in turn often becomes unacceptable Quality defects in the subsequent products leads.

It is known that there is a cause for the problems mentioned in one To see phase transition, the peritectic steels at one temperature are subjected just below their solidification temperature and that with a substantial volume contraction is associated. When continuously casting This phase transition takes place during the peritectic steels Initial solidification of a strand shell takes place under conditions in which the forming strand shell is still thin, low mechanical stability has and due to the phase transition an uneven, only selectively forms the surface adjacent to the mold cavity wall, with the result that solidified strands form a porous or cracked layer on the Have surface.

As is well known, continuous casting of peritectic steels can improve Quality of the strand surfaces can be achieved by the initial solidification of the Continuous shell in an area of the continuous casting mold that encompasses the casting level by reducing the heat dissipation from the molten steel or the strand shell is influenced. This reduction in heat dissipation in the area the initial solidification is usually with the help of continuous casting molds realized with a heat barrier on the steel side surface of a Longitudinal section of the mold cavity wall are equipped. The thermal barrier is dimensioned and the longitudinal section is dimensioned so that the Heat flux density is reduced on the one hand in the area of initial solidification, on the other hand, in the longitudinal sections adjoining the heat barrier is large enough to cover the entire length of the strand in the mold cavity to achieve sufficient growth of the strand shell.

Several concepts are known, the mold cavity walls of a continuous casting mold in the area of a partial length, which corresponds to the position of the foundry level in the foundry comprises, on the surface delimiting the mold cavity with a To provide a thermal barrier.

From the summary of JP 1-224 142 A is one for the production of peritectic steels provided mold, the mold cavity wall from a tubular body with a cylindrical insert arranged on the pouring side Made of steel or other materials that have a higher thermal resistance have as the material forming the tubular body. This mold has the disadvantage that the insert forming the heat barrier is susceptible to wear is and special measures that make the mold more expensive are necessary are due to cracks or deformations of the mold cavity wall counteracting the thermal loads during casting operation.

An alternative concept for the formation of a heat barrier is in the summary JP 1-170 550 A using the example of one for the production of slabs made of peritectic steel certain plate mold disclosed. The surfaces of the mold cavity Side walls of this mold made of copper have the pouring mirror position in one extensive range of holes, which can be optionally with nickel, stainless steel or a suitable ceramic material. With This alternative concept has the disadvantage that - apart from the susceptibility to wear of the borehole fillings - from manufacturing technology Not based on tube molds for small strand formats, for example Billet formats is applicable because the inside of the mold tubes is insufficient are accessible for suitable processing.

From the summary of JP 02-006 038 A is one for the casting of peritectic steels certain mold known, the mold cavity walls are made of copper and on the side facing away from the mold cavity Have cooling water slots. In the cooling water slots are in one Area surrounding the pouring mirror position at periodic intervals of 5 - 20 mm metals or ceramic materials with a lower thermal conductivity embedded as copper. To use a thermal barrier according to this concept To create a predetermined thermal resistance, the embedded Materials into the mold cavity wall over a relatively large depth extend. The realization of such a heat barrier is manufacturing technology complex, especially at a variety of places on the mold cavity wall suitable materials must be embedded relatively deep.

The invention has for its object to solve the problems mentioned to contribute and for this purpose a mold tube that is simplified with a Manufacturing technology means producible, arranged at the casting level position and is equipped with a heat barrier protected against wear, and a corresponding continuous casting mold provided with a mold tube create.

This task is solved by a mold tube, which by the entirety of the Features of claim 1 is characterized, and with a Sfranggiesskokille the features of claim 10.

The mold tube according to the invention has a first, a predetermined one Pouring mirror position including longitudinal section and a second the first adjoining longitudinal section, the first longitudinal section comprises a heat-insulating layer which is dimensioned such that the heat resistance of the mold tube in the first longitudinal section a larger value has than in the second longitudinal section. The mold is characterized by that the heat insulating layer has an area between the outer surface the mold tube and a distance of at most 75% of the wall thickness the mold tube, measured from the outer surface of the mold tube, fills.

The heat-insulating layer of the mold tube according to the invention is on or arranged close to the outside of the mold tube and is not sufficient to the inner surface of the pipe. The mold tube is therefore out a tubular body that can be machined on the outside, to equip it with the heat-insulating layer. The processing is with conventional methods can be carried out even for tubular bodies that are suitable for the Production of mold tubes with a small inner diameter are suitable and because of their geometrical dimensions not or only very much lavishly processed on the inside.

In casting operation, the heat-insulating layer ensures the area of the first Longitudinal section for an increase in temperature on the inside of the Mold tube. The fact that the distance of the heat insulating layer from the inner surface of the mold tube at least 25% of the wall thickness of the mold tube is the wear of the mold tube during Casting operation due to the thermal and mechanical Reduced material stress in the area of the first longitudinal section, compared to a mold tube with a heat insulating layer Equipped with the same thickness on the inside of the mold tube.

In the mold tube according to the invention it is possible to use suitable Dimensioning the thickness profile of the heat insulating layer Temperature distribution, which is found on the inner surface of the Mold tube sets, defined to adjust the growth of a To influence the strand shell in the area of the first longitudinal section in a targeted manner. This degree of freedom is used in the mold according to the invention in order to to optimize them with regard to the production of peritectic steel strands. Around to optimize the quality of cast products made of peritectic steel on the one hand, the temperature on the inside during casting operation Surface of the mold tube in the area of the first longitudinal section be as high as possible. This sets the initial solidification of the molten steel decelerates at the greatest possible distance from the pouring level with which Effect that the ferrostatic pressure of the melt, which with the growing Distance from the pouring level increases, reinforced by a local one, by the peritectic phase transition stimulated detachment of the formed Strand shell counteracts from the inner surface of the mold tube and so favors the formation of a smooth strand surface. on the other hand the temperature on the inner surface during casting operation of the mold tube must not be as large as the material properties of the mold tube have a limiting effect. For example, as is well known Mold tube made of copper after heating to a temperature above a critical temperature of 450 ° C, the so-called Softening temperature, an unacceptably short lifespan.

In an advantageous embodiment of the mold tube according to the invention, the thickness of the heat-insulating layer is therefore dimensioned such that the temperature on the inside of the mold tube in the casting operation does not exceed a predetermined critical temperature T _K.

In a further embodiment of the mold tube according to the invention is the outer surface of the mold tube at the boundary between the Longitudinal sections are stepless. This embodiment is special suitable for use in molds with water jacket cooling on the Outside of the mold tube. Because with such molds the water jacket is usually only a few mm thick and its thickness along the mold tube must be precisely controlled, enables a continuous training of the A particularly simple transition between the two longitudinal sections Construction of the water jacket cooling.

In a further development of the mold tube according to the invention heat-insulating layer in a tubular body made of metal or Metal alloy is embedded. Favorable thermal and mechanical Properties of the mold tube are achieved when the tube body is made Copper or a copper alloy and the heat insulating layer a metal, for example nickel or chrome. This Materials have a good mutual expansion coefficient matched so that a nickel applied to a copper surface or chrome layer due to good adhesion and high Excellent wear resistance.

Further embodiments of the mold tube according to the invention are thermally with regard to cooling the outer surface of the Mold tube designed with a coolant such that the temperature of the inner surface in the area of the first longitudinal section at most one predetermined critical temperature reached and at least in one Part of the first longitudinal section is approximately constant. To this In this way, the initial solidification of the strand shell can become particularly special delayed large distance from the pouring mirror and a particularly smooth Strand surface after passing through the peritectic phase transition be achieved. To keep the temperature profile as constant as possible in the longitudinal direction to achieve, the thickness d of the heat insulating layer must be at least in a section between the mold level and the second Increase longitudinal section in the direction of the second longitudinal section.

Various embodiments of the The mold tube according to the invention is explained using schematic figures.

Fig. 1A:

ein Beispiel des erfindungsgemässen Kokillenrohrs in Seitenansicht;

Fig. 1B:

einen Querschnitt längs der Linie I-I in Fig. 1A;

Fig. 1C:

einen Querschnitt längs der Linie II-II in Fig. 1A;

Fig. 2A:

einen Längsschnitt entlang der Linie III-III in Fig. 1C für ein bestimmtes Dickenprofil der wärmeisolierenden Schicht;

Fig. 2B:

einen Längsschnitt wie in Fig. 2A, jedoch für ein anderes Dickenprofil der wärmeisolierenden Schicht;

Fig. 3:

Verläufe der Dicke d einer wärmeisolierenden Schicht gemäss Fig. 2A als Funktion der Wanddicke d_w des Kokillenrohres für eine vorgegebene Wandtemperatur;

Fig. 4:

eine Dimensionierung einer wärmeisolierenden Schicht als Funktion der Wanddicke d_w des Kokillenrohres für ein vorgegebenes Profil der Wandtemperatur;

Show it:

Fig. 1A:

an example of the mold tube according to the invention in side view;

Fig. 1B:

a cross-section along the line II in Fig. 1A;

1C:

a cross-section along the line II-II in Fig. 1A;

Fig. 2A:

a longitudinal section along the line III-III in Figure 1C for a specific thickness profile of the heat insulating layer.

Fig. 2B:

a longitudinal section as in Figure 2A, but for a different thickness profile of the heat insulating layer.

Fig. 3:

Curves of the thickness d of a heat-insulating layer according to FIG. 2A as a function of the wall thickness d _{w of} the mold tube for a predetermined wall temperature;

Fig. 4:

dimensioning a heat-insulating layer as a function of the wall thickness d _{w of} the mold tube for a given profile of the wall temperature;

1A shows an example of the side view inventive mold tube 10 with a mold cavity 20, one Pouring opening 12 and a pull-out opening 13 for a (not shown) Strand. The strand pull-out direction provided in the casting operation is through an arrow 14 indicated. The mold tube 10 has a first Longitudinal section 1 and a second longitudinal section 2, the Longitudinal section 1 a casting level position h provided in the casting operation comprises and the longitudinal section 2 in the strand extension direction 14 to the Longitudinal section 1 connects. The mold tube 10 consists of a Tubular body 15 with a heat insulating layer 16 in the area of Longitudinal section 1.

1B and 1C show cross sections of the mold tube 10: FIG. 1B a cross section in the plane marked in Fig. 1A I-I in the area of Longitudinal section 1, Fig. 1C is a cross section in the in Fig. 1A marked plane II-II in the region of the longitudinal section 1. As the Fig. 1A-C can be seen, the heat insulating layer 16 on the Outside 11 of the tubular body 15 is arranged. The mold cavity 20 has an example of a square cross-section with rounded corners. This selection is arbitrary. The mold tube according to the invention can with any cross-sectional shapes that are common in continuous casting practice be equipped.

2A and 2B are longitudinal sections along the line III-III in Fig. 1B and 1C and characterize two different embodiments of the inventive mold tube 10, which are in the design of the Thickness profile of the heat insulating layer 16 in the longitudinal direction of the Distinguish mold tube. In both cases it is heat-insulating Layer 16 in a depression on the outside of the tubular body 15 embedded. In these examples, the outer surface 11 of the Mold tube 10 at the edges of the longitudinal section 1 continuously.

The tubular body suitably consists of copper or a copper alloy. Coming as materials to build up the heat insulating layer expedient metals such as nickel or chromium in question with conventional Methods, for example plating or electrochemical processes the tubular body 15 can be applied. But others can too Materials, for example ceramic materials, for the construction of the heat-insulating layer can be used, provided they have a lower thermal conductivity than the tubular body 15 and are in terms of their Adhesion properties and their wear resistance suitable.

The embodiment shown in FIG. 2A of the inventive Mold tube 10 is characterized in that the heat-insulating Layer 16 in the area between the pouring mirror position h and its at the Longitudinal section 2 adjacent edge has a substantially constant thickness, which is denoted by d in FIG. 2A. With this geometry Casting operation with uniform cooling of the outer surface 11 of the Mold tube 10 is the temperature on the inner surface of the mold tube 10 from a point at the pouring mirror position h maximum Decrease temperature in the strand pull-out direction 14, especially since one in the Area of the longitudinal section on the inner surface 25 of the mold tube 10 forming strand shell an increasing in the strand extension direction 14 Has thickness and ensures that the heat flow between the Surfaces 25 and 11 of the mold tube 10 along the Strand pull-out direction 14 decreases.

The temperature profile established on the inner surface 25 of the mold tube 10 can be modified in a targeted manner by a corresponding variation of the thickness of the heat-insulating layer 16 in the strand pull-out direction 14 in order to optimize the strand shell growth. The exemplary embodiment of the mold tube 10 according to the invention shown in FIG. 2B is characterized in that the heat-insulating layer 16 grows in a wedge shape from a thickness d to a thickness b in the region between the pouring mirror position and its edge adjoining the longitudinal section 2. The thicknesses d and b can be selected in relation to the wall thickness d _{W of} the mold tube 10, for example, in such a way that the temperature profile on the inside 25 of the mold tube 10 in the strand pull-out direction 14 is approximately constant and reaches a predetermined value. The detailed temperature profile is correlated with the strand shell growth on the surface 25.

λ_W :

Wärmeleitfähigkeit des Kokillenrohrs 10 im zweiten Längsabschnitt 2;

f:

Verhältnis λ_W/ λ_i, wobei λ_i die Wärmeleitfähigkeit der wärmeisolierenden Schicht 16 bedeutet;

T_K :

kritische Temperatur;

T_S :

Temperatur des Stahls an der inneren Oberfläche 25 des Kokillenrohrs 10;

T_L :

Temperatur des Kühlmittels.

α:

Wärmeübergangszahl für den Übergang zwischen dem Kühlmittel und der wärmeisolierenden Schicht 16.

The tubular body 15 is generally designed for use at a temperature below a maximum, critical temperature T _K. The mold tube 10 can be designed for continuous casting of steel in terms of heat technology, provided that the outer surface is cooled by the application of coolant. So that the temperature on the inner surface 25 of the mold tube 10 does not exceed a predetermined critical temperature T _K , the thickness d of the heat-insulating layer 16 at the pouring mirror position h should be according to d ≤ d W [1-f] + λ W [10 d L + T S - 11T K ] α [T S - T K ] [1-f] = d MAX be dimensioned with

λ _W :

Thermal conductivity of the mold tube 10 in the second longitudinal section 2;

f:

Ratio λ _W / λ _i , where λ _{i means} the thermal conductivity of the heat-insulating layer 16;

T _K :

critical temperature;

T _S :

Temperature of the steel on the inner surface 25 of the mold tube 10;

T _L :

Coolant temperature.

α:

Heat transfer coefficient for the transition between the coolant and the heat-insulating layer 16.

3, d = d _{MAX is} shown graphically according to equation (1) as a function of the wall thickness d _W for the two parameters f = 4 and f = 10, where T _K = 450 ° C., T _S = 1480 ° C. and the following values representative of water cooling are assumed α = 30000 W / (m ² * K) and T _L = 40 ° C. T _K = 450 ° C is a characteristic empirical value for copper. The two parameters f = 4 and f = 10 are, for example, representative of a mold tube 10 with a tube body 15 made of copper and a heat-insulating layer 16 made of nickel (f = 4) or steel (f = 10). 3, the ratio d _MAX / d _W decreases with increasing wall thickness d _{W of} the mold tube 10. The smaller the thickness d _{W of} the mold tube 10, the greater the proportion of the thickness of the heat-insulating layer in the total thickness d _{W of} the mold tube 10 must be to the temperature on the inside 25 of the mold tube 10 in the longitudinal section 1 up to the critical temperature T _K , in the given example T _K = 450 ° C to raise. Furthermore, for a given wall thickness d _W , d _MAX / d _W is the greater the smaller f, ie the greater the thermal conductivity λ _{i of} the heat-insulating layer. Experience has shown that the wall thickness d _{W of} the mold tube 10 should typically be approximately 10% of the side length of a cross section of the mold cavity 20. If the mold tube 10 is designed for small billets with a side length of the cross section of approximately 10 cm, the thickness ratio d _MAX / d _W becomes approximately 75% for f = 4. For d _MAX / d _W ≥ 75% and f <4, the manufacture of the mold tube 10 from a solid tube body 15 becomes problematic, especially since the mechanical stability of the tube body 15 when the recess on the outside 11 intended for receiving the heat-insulating layer 16 is realized of the tubular body 15 is unduly impaired. Furthermore, as the ratio d _MAX / d _{W increases,} the effort involved in realizing the heat-insulating layer 16 increases, in particular in production processes in which the heat-insulating layer 16 is built up by continuously applying thin layers of a suitable material. For the realization of the heat-insulating layer 16, the parameter range f 4 4 is therefore preferred in addition to the condition d _MAX / d _W Schicht 75%.

4 is for the mold tube 10 in the event that the thickness of the heat-insulating layer 16 in the strand pull-out direction 14 increases from the thickness d at the position of the casting level to the thickness b according to a thickness profile which is determined such that in the casting operation along the thickness profile the temperature at the inner surface 25 is constant, stated as the ratio b / d _W as a function of the wall thickness d _W varies. The relationships b / d _W and d / d _W can be determined from equation (1) and FIG. 4 in the event that the critical temperature T _{K is} realized along the thickness profile in the casting operation. A comparison with FIG. 3 provides the corresponding values for the special case T _K = 450 ° C. 4 is not dependent on f.

The length of the mold tube 10 is typically 80-100 cm. The length the length section 1 is preferably in the range 10-15 cm, the Casting level position preferably in the upper quarter of the length section 1 is located.

In the above-mentioned embodiments, the is heat-insulating Layer 16 is always embedded in a recess in the tubular body 15 in such a way that the outer surface 11 of the mold tube 10 is of stepless design. in the The inventive idea could also be based on an embedding of the heat-insulating layer 16 in a recess or on a stepless Formation of the outer surface 11 can be dispensed with. The surfaces 11 and 25 of the mold tube according to the invention could also be provided with coatings suitable materials.

Claims (11)

1. Mould tube for a continuous casting mould for the continuous casting of steels, in particular peritectic steels, with a first longitudinal section (1) incorporating a predetermined liquid metal level position (h) and a second longitudinal section (2) adjoining the first one, wherein the first longitudinal section (1) includes on or close to the outside of the mould tube (10) at least one thermally insulating area (16) and is so dimensioned that the thermal resistance of the mould tube (10) has a greater value in the first longitudinal section (1) than in the second longitudinal section (2), characterised in that the thermally insulating area is a thermally insulating layer (16) which occupies an area between the outer surface (11) of the mould tube (10) and a distance of not more than 75% of the wall thickness (d_w) of the mould tube (10), measured from the outer surface (11) of the mould tube (10).
2. Mould tube according to claim 1, characterised in that the outer surface (11) of the mould tube (10) is continuously configured at the boundary between the longitudinal sections (1, 2).
3. Mould tube according to one of claims 1 or 2, characterised in that the thickness (d) of the thermally insulating layer (16) is so proportioned that during casting the temperature on the inside (25) of the mould tube (10) does not exceed a predetermined critical temperature T_K.
4. Mould tube according to one of claims 1 to 3, characterised in that the thermally insulating layer (16) is embedded in a tubular body (15) of metal or a metal alloy.
5. Mould tube according to claim 4, characterised in that the mould tube (10) is constructed in heat engineering terms for continuous casting with cooling of the outer surface (11) by coolant application, wherein the thickness d of the thermally insulating layer (16) is dimensioned at the liquid metal level position (h) according to d ≤ dW [1-f] + λW[10 TL + TS - 11TK]α[TS - TK] [1-f] with

   d_w:

   wall thickness of the mould tube (10) in the first longitudinal section (1);

   λ_w:

   thermal conductivity of the mould tube (10) in the second longitudinal section (2) ;

   f:

   ratio λ_w/λ_i, where λ_i signifies the thermal conductivity of the thermally insulating layer (16);

   T_K:

   critical temperature;

   T_S:

   temperature of the steel at the inner surface (25) of the mould tube (10);

   T_L:

   temperature of the coolant;

   α:

   heat transmission coefficient for the transition between the coolant and the thermally insulating layer (16).

6. Mould tube according to claim 5, characterised in that f ≥ 4.
7. Mould tube according to one of claims 5 or 6, characterised in that the thickness (d, b) of the thermally insulating layer increases in a section between the liquid metal level position (h) and the second longitudinal section (2) in the direction of the second longitudinal section.
8. Mould tube according to claim 7, characterised in that the increase in thickness of the thermally insulating layer (16) is so proportioned that the temperature on the inside (25) of the mould tube (10) is approximately constant during casting in the area of the section.
9. Mould tube according to one of claims 4 to 8, characterised in that the thermally insulating layer (16) is constructed of a metal, for example nickel or chromium, and the tubular body (15) of copper or a copper alloy.
10. Continuous casting mould for the continuous casting of steels, in particular peritectic steels, with a mould tube (10) according to one of claims 1 to 9.
11. Continuous casting mould according to claim 10, characterised in that a device for applying a coolant to the outer surface (11) of the mould tube (10), for example a water jacket cooling, is provided.

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