Title of Invention

MOULD TUBE FOR A CONTINUOUS CASTING BOULD FOR THE CONTINUOUS CASTING OF STEELS IN PARTICULAR PERITECTIC STEELS

Abstract

A mould tube for continuous casting mould for the continuous casting of steels, in particular peritectic steels, comprises a first longitudinal section (1) incorporating a predetermined liquid metal level position (h) and a second longitudinal section (2) adjoining the first longitudinal section (1), the first longitudinal section (1) comprises a thermally insulating layer (16) having a thickness (d) so that a thermal resistance of the mould tube (10) in the first longitudinal section (1) has a greater value than in the second longitudinal section (2). The thermally insulating layer (16) covers up an area between the outer surface (11) of the mould tube(10) and a gap of at most 75% of the mall thickness (d) of the mould tube (10) when measured from the outer surface (11) of the mould tube (10). Due to an appropriate section of the thickness profile of the thermally insulating layer in the strand withdrawal direction (14), during casting a predetermined temperature profile can be set on an inside of the mould tube and the growth of a strand shell be optimized.

Full Text

-1A- The invention relates to a mould tube for a continuous casting mould for the continuous casting of steels, in particular peritectic steels, and a continuous casting mould with the mould tube configured according to the features of the invention The technique of continuous casting, in which there is formed against the walls of a mould cavity of a continuous casting mould by the cooling of a metal melt a casting shell with continuously growing thickness and a strand is drawn continuously from an outlet opening in the continuous casting mould, is known to lead when applied to peritectic steels, for example steels with a carbon content of 0.1 to 1 0.14%, to problems which are expressed in particular in a defective surface quality of the strands produced, Such quality defects are undesirable, particularly as further processing of the strands often leads in turn to unacceptable quality defects in the derived products. It is known that a cause of the above-mentioned problems can be considered to lie in a phase transition to which peritectic steels are subjected at a temperature just below their solidification temperature and which is combined with a considerable contraction in volume. With the continuous casting of peritectic steels said phase transition takes place during the initial solidification of a casting shell under conditions in which the casting shell in course of formation is still thin, exhibits low mechanical stability and because of the phase transition forms an uneven surface resting against the mould cavity wall only at certain points, with the result that fully solidified strands have a porous or else cracky layer at the surface. It is known that with the continuous casting of peritectic steels an improved quality of the strand surfaces can be achieved by the initial solidification of the casting shell 2 being modified in an area of the continuous casting mould that includes the liquid metal level by a reduction in the heat flow out of the steel melt or the casting shell. This reduction of the heat flow in the area of the initial solidification is conventionally carried out with the aid of continuous casting moulds which are provided with a heat barrier at the steel-side surface of a longitudinal- section of the mould cavity wall. The heat barrier is furthermore so dimensioned and the longitudinal section so proportioned that the heat current density is on the one hand reduced in the area' of the initial solidification, but on the other is high enough in the longitudinal sections adjacent to the heat barrier to achieve a sufficient growth of the casting shell over the entire distance covered by the strand in the mould cavity. Many concepts are known for providing the mould cavity walls of a continuous casting mould, in the area of a partial length which includes the liquid metal level position obtained during casting, with a heat barrier at the surface bounding the mould cavity. There is known from JOS 1-224 142 a mould provided for the manufacture of peritectic steels, whose mould cavity wall consists of a tubulcLr body with a cylindrical insert disposed on the casting-in side and made of steel or other materials which possess a higher thermal resistance than the material forming the tubular body. such mould, has the disadvantage that the insert forming the heatr-barrier is susceptible to wear and particular measures adding to the manufacturing costs of the mould are required in order to counteract cracking or deformation of the mould cavity wall caused by the thermal loads experienced during casting. An alternative concept for the formation of a heat barrier is disclosed in JOS 1-170 550, taking as an example a 3 built-up mould intended for the manufacture of slabs of peritectic steel. The mould cavity-side surfaces of the copper side walls of said mould comprise in an area including the liquid metal level position bores which are filled optionally with nickel, stainless steel or a suitable ceramic material. There is associated with said alternative concept the disadvantage that - leaving aside the susceptibility to wear of the filling of the bores - it is not applicable on production engineering grounds to tubular moulds for small strand formats, for example billet formats, since the insides of the mould tubes are accessible for a suitable working to only a limited extent. The invention is based on the object of contributing to the solution of the above-mentioned problems and to this end creating a mould tube which is provided with a heat barrier that can be manufactured with simplified production engineering means, is disposed at the liquid metal level position and possesses an improved resistance to wear, and a corresponding continuous casting mould provided with a mould tube. The mould tube according to the invention comprises a first longitudinal section incorporating a predetermined liquid metal level position and a second longitudinal section adjacent to the first one, the first longitudinal section comprising a thermally insulating layerhaving a thickness profile so that thermal resistance of the mould tube in the first longitudinal section has a greater value than that second longitudinal section. The thermally insulating layer covers up an area between the outer surface of the mould tube and 4 a gap of at most 75% of the wall thickness of the mould tube, measured from the outer surface of the mould tube. The thermally insulating_layer of the mould tube according to the invention is arranged on or close to the outside of the mould tube and does not reach as far as the inner surface of the tube. The mould tube can therefore be manufactured from a tubular body that can be worked on the outside in order to provide it with the thermally insulating layer. The working can be undertaken with conventional methods even in the case of tubular bodies which are suitable for the manufacture of mould tubes with small inner diameter and which because of their geometric dimensions cannot be worked on the inside or only with very great difficulty. During casting the thermally insulating layer in the area of the first longitudinal section ensures a raising of the temperature on the inside of the mould tube. Due to the fact that the distance of the thermally insulating layer from the inner surface of the mould tube comes to at least 25% of the wall thickness of the mould tube, during the casting the wear on the mould tube is, due to the thermal and mechanical material loading in the area of the first longitudinal section, reduced compared with a mould tube which is provided with a thermally insulating layer of identical thickness on the inside of the mould tube. With the mould tube according to the invention it is possible, by suitable dimensioning of the thickness profile of the thermally insulating layer, to set determinately the temperature distribution which is produced at the inner surface of the mould tube during casting, in order to modify selectively the growth of a strand shell in the area of the first longitudinal section. This degree of freedom is used with the mould according to the invention to optimize it with respect to the manufacture of peritectic 5. and a gap of almost 75% of the wall thickness of the mold tube when measured from the outer surface of the mold tube. The temperature at an inner surface of the mold tube remains constant in a stand withdrawal direction. In order to optimize the quality of cast products from peritectic steels, on the one hand during casting the temperature at the inner surface of the mould tube is to be as high as possible in the area of the first longitudinal section.. The initial solidification of the steel melt starts with a delay at as great a distance as possible from the liquid metal level, with the effect that the ferrostatic pressure of the melt, which rises with the growing distance from the liquid metal level, increasingly counteracts a local detachment of the forming strand shell from the inner surface of the mould tube, which detachment is stimulated by the peritectic phase transition, and thus promotes the formation of a smooth strand surface. On the other hand during casting the temperature at the inner surface of the mould tube cannot be of any size, since the material properties of fThe mould tube have a restrictive effect. For example, a mould tube made of copper is known to have after heating to a temperature above a critical temperature of 450 °C, the so-called softening temperature, an unacceptably short service life. In an advantageous embodiment of the mould tube according to the invention the thickness of the thermally insulating layer is therefore so dimensioned that during casting the temperature on the inside of the mould tube does not exceed a predetermined critical temperature TK. In a further embodiment of the mould tube according to the invention the outer surface of the mould tube is configured continuously at the boundary between the longitudinal sections. This embodiment is particularly suitable for a use in moulds with water jacket cooling on the outside of the mould tube. Since with such moulds the water jacket is conventionally only a few mm thick and its thickness has to be precisely controlled along the mould tube, a continuous configuration of the transition between the two 6 longitudinal sections makes a particularly simple construction of the water jacket cooling possible. In a development of the mould tube according to the invention the thermally insulating layer is embedded in a tubular body of metal or a metal alloy. Favourable thermal and mechanical properties of the mould tube are obtained if the tubular body is constructed of copper or a copper alloy and the thermally insulating layer of a metal, for example nickel or chromium. These materials are well attuned to one another in terms of their coefficient of expansion, so that a nickel or chromium layer applied to a copper surface is distinguished by a good adhesion and by a high resistance to wear. Further embodiments of the mould tube according to the invention are with respect to a cooling of the outer surface of the mould tube with a coolant constructed in heat engineering terms in such a way that the temperature of the inner surface in the area of the first longitudinal section reaches at most a predetermined critical temperature and is approximately constant at least in a sub-section of the first longitudinal section. In this way the initial solidification of the strand shell can be delayed up to a particularly long distance from the liquid metal level and a particularly smooth strand surface after passage through the peritectic phase transition can be achieved. In order to achieve as constant a temperature profile as possible in the longitudinal direction, the thickness d of the thermally insulating layer must increase at least in a section between the liquid metal level position and the second longitudinal section in the direction of the second longitudinal section. Various embodiments of the mould tube according to the invention will be explained below by means of the accompanying diagrammatic figures, where: 7 Fig. 1A shows an example of the mould tube according to the invention in side view; Fig. IB a cross-section along the line I-I in Fig. l; Fig. 1C a cross-section along the line II-II in Fig. 1A; Fig. 2A a longitudinal section along the line III-III in Fig. 1C for a particular thickness profile of the thermally insulating layer; Fig. 2B a longitudinal section as in Fig. 2A. but for another thickness profile of the thermally insulating layer; Fig. 3 plots of the thickness d of a thermally insulating layer according to Fig. 2A as a function of the wall thickness dw of the mould tube for a predetermined wall temperature, and Fig. 4 a dimensioning of a thermally insulating layer as a function of the wall thickness dw of the mould tube for a predetermined profile of the wall temperature. Fig. 1A shows an example shown in side view of the mould tube 10 according to the invention with a mould cavity 20, a pouring opening 12 and a withdrawal opening 13 for a strand (not shown). The strand withdrawal direction provided during casting is indicated by an arrow 14. The mould tube 10 comprises a first longitudinal section 1 and a second longitudinal section 2, wherein the longitudinal section 1 includes a liquid metal level position h provided during casting and the longitudinal section 2 adjoins the longitudinal section 1 in the strand withdrawal direction 14. The mould tube 10 consists of a tubular body 15 with a 8 thermally insulating layer 16 in the area of the longitudinal section 1. Figs IB and 1C show cross-sections of the mould tube 10: Fig. IB a cross-section in the plane I-I marked in Fig. 1A in the area of the longitudinal section 1, Fig. 1C a cross-section in the plane II-II marked in Fig. 1A in the area of the longitudinal section 1. As can be seen from Figs 1A -C, the thermally insulating layer 16 is disposed on the outside 11 of the tubular body 15. The mould cavity 20 has for example a square cross-section with rounded corners. This selection is arbitrary. The mould tube according to the invention can be provided with any cross-sectional shapes conventional in continuous casting practice. Figs 2A and 2B represent longitudinal sections along the line III-1II in Fig. IB or 1C and characterise two different embodiments of the mould tube 10 according to the invention, which differ in the configuration of the thickness profile of the thermally insulating layer 16 in the longitudinal direction of the mould tube. In both cases the thermally insulating layer 16 is embedded in a depression on the outside of the tubular body 15. In these examples the outer surface 11 of the mould tube 10 is continuous at the edges of the longitudinal section 1. The tubular body consists suitably of copper or a copper alloy. There are considered as materials for the composition of the thermally insulating layer suitably metals such as nickel or chromium, which can be applied to the tubular body 15 by conventional methods, for example plating or electrochemical processes. Other materials, for example ceramic materials, can however also be used for the composition of the thermally insulating layer, provided that they have a lower thermal conductivity than the tubular body 15 and are suitable as regards their adhesion properties and their resistance to wear. 9 The embodiment shown in Fig. 2A of the mould tube 10 according to the invention is characterised in that the thermally insulating layer 16 has in the area between the liquid metal level position h and its edge adjoining the longitudinal section 2 a substantially constant thickness, which is labelled d in Fig. 2A. With this geometry, during casting, assuming uniform cooling of the outer surface 11 of the mould tube 10, the temperature at the inner surface of the mould tube 10 would decrease in the strand withdrawal direction 14 from a point of maximum temperature situated at the liquid metal level position h, particularly since a strand shell forming in the area of the longitudinal section at the inner surface 25 of the mould tube 10 has a thickness increasing in the strand withdrawal direction 14 and ensures that the heat flow between the surfaces 25 and 11 of the mould tube 10 decreases in the strand withdrawal direction 14. By varying correspondingly the thickness of the thermally insulating layer 16 in the strand withdrawal direction 14, the temperature profile obtained at the inner surface 25 of the mould tube 10 can be selectively modified, in order to optimize the strand shell properties. The embodiment shown in Fig. 2B of the mould tube 10 according to the invention is characterised in that the thermally insulating layer 16 increases in a wedge shape from a thickness d to a thickness b in the area between the liquid metal level position and its edge adjoining the longitudinal section 2. The thicknesses d and b can be selected in proportion to the wall thickness dw of the mould tube 10 for example in such a way that the temperature plot on the inside 25 of the mould tube 20 is approximately constant in the strand withdrawal direction 14 and reaches a predetermined value. The detailed temperature plot is here correlated with the strand shell growth at the surface 25. 10 The tubular body 15 is as a rule constructed for a use at a temperature below a maximum, critical temperature TK. The mould tube 10 can be constructed in heat engineering terms as follows for the continuous casting of steel, assuming a cooling of the outer surface by coolant application. In order that the temperature at the inner surface 25 of the mould tube 10 does not exceed a predetermined critical temperature TK, the thickness d of the thermally insulating layer 16 at the liquid metal level position h should be dimensioned according to with ?w: thermal conductivity of the mould tube 10 in the second longitudinal section 2; f: ratio ?w/?i, where ?? signifies the thermal conductivity of the thermally insulating layer 16; TK: critical temperature; Ts: temperature of the steel at the inner surface 25 of the mould tube 10; TL: temperature of the coolant; ?? heat transmission coefficient for the transition between the coolant and the thermally insulating layer 16. In Fig. 3, d = dmax according to equation (1) is plotted,as a function of the wall thickness dw for the two parameters f=4 and f=10, where TK = 450 °C, Ts = 1480 °C and the 11 following values representative of water cooling of ? = 30000 W(m2*K) and TL = 40 °C are assumed. Here TK = 450 °C is a characteristic empirical value for copper. The two parameters f=4 and f=10 are for example representative of a mould tube 10 with a tubular body 15 of copper and a thermally insulating layer 16 of nickel (f=4) or steel (f=10) . As can be seen from Fig. 3, the ratio dmax/dw decreases with growing wall thickness dw of the mould tube 10. The smaller the thickness dw of the mould tube 10 is, the greater must be the share of the thickness of the thermally insulating layer in the total thickness dw of the mould tube 10 in order to raise the temperature on the inside 25 of the mould tube 10 in the longitudinal section 1 to the critical temperature TK, in the example given TK = 450 °C. Furthermore dmax/dw is with predetermined wall thickness dw the greater the smaller f is, i.e. the greater the thermal conductivity' ?i of tine thermally insulating layer is. According to experience the wall thickness dw of the mould tube 10 should be typically about 10% of the side length of a cross-section of the mould cavity 20. If the mould tube 10 is designed for small billets with a side length of the cross-section of about 10 cm, the thickness ratio dmax/dw then becomes for f=4 about 75%. For dmax/dw = 75% and f 12 In Fig. 4 it is shown for the mould tube 10, for cases where the thickness of the thermally insulating layer 16 rises in the strand withdrawal direction 14 from the thickness d at the liquid metal level position to the thickness b, according to a thickness profile which is so determined that during casting the temperature at the inner surface 15 is constant along the thickness profile/ how the ratio b/dw varies as a function of the wall thickness dw. There can be determined from equation (1) and Fig. 4 the ratios b/dw and d/dw for cases where during casting; the critical temperature TK is realised along the thickness profile- A comparison with Fig. 3 supplies the corresponding values for the particular case TK = 450 °C. The curve shape shown in Fig. 4 is not dependent on f. The length of the mould tube 10 comes typically to 80 -100 cm. The length of the longitudinal section 1 lies preferably in the range 10 - 15 cm, wherein the liquid metal level position has preferably settled down in the upper quarter of the longitudinal section 1. In the embodiments given above the thermally insulating layer 16 is always embedded in a depression in the tubular body 15 in such a way that the outer surface 11 of the mould tube 10 is continuously configured. Within the scope of the inventive idea an embedding of the thermally insulating layer 16 in a depression or a continuous configuration of the outer surface 11 could also be dispensed with. The surfaces 11 and 25 of the mould tube according to the invention could also be provided with coatings of suitable materials. 13 WE CLAIM; 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 (I) includes on or close to the outside of the mould tube (10) at least one thermally insulating area C16) and is 20 dimensioned that the thermal resistance of the mould tube (1O) has a greater value in the first longitudinal section (1) than in the second longitudinal section (2) characterized in that the thermally insulating area is a thermally insulating layer (16) which occupies an area between the outer surface (11) of the mould tubs (10) and a distance of not more than 75% of the wall thickness (dw ) of the mould tube (10), measured from the outer surface (11) of the mould tube (10). 2, Mould tube as claimed in claim 1 wherein the outer surface (11) of the mould tube (10) is continuously configured at a boundary between said two longitudinal sections (1,2). 3. Mould tube as claimed in one of claims 1 or 2, wherein the thickness (d) of the thermally insulating layer (16) is so dimensioned that during casting the temperature on an inside (25) of the mould tube (10) does not exceed a predetermined critical temperature Tk. 14 4. Mould tube as claimed in one of claims 1 to 3, wherein the thermally insulating layer (16) is embedded in a tubular body (13) of metal or a metal alloy. 3- Mould tube as claimed in claim 4, wherein the mould tube (10) is constructed in heat engineering terms for continuous casting with cool ing of an outer surface (11) by coolant application and wherein the thickness (d) of the thermally insulating layer (16) is dimensioned at the liquid metal level position (h) according to a relationship, dw: wall thickness of the mould tube (10) in the first longitudinal section (1); ?? : thermal conductivity of the mould tube (10) in the second longitudinal section (2); f: ratio ?w/?? , where ?? signifies the thermal conductivity of the thermally insulating layer (16); TK : critical temperature; Ts : temperature of the steel at the inner surface (25) of the mould tube (10); Tt : temperature of the coolant; ( heat transmission coefficient for the transition between the coolant and the thermally insulating layer (16). 15. 6. Mould tube as claimed in claim 5, wherein f = 4. 7. Mould tube as claimed in one of claims 5 or 6, wherein the thickness (d,b) of the thermally insulating layer increased 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 as claimed in claim 7, wherein the increase in thickness of the thermally insulating layer (l proportioned that the temperature on the inside (25) of the mould tube (10) remains constant during casting in the area of the section. 9. Mould tube as claimed in one of claim 4 to 3, wherein the thermally insulating layer (16) is constructed of a metal, for example nickel or chromium, and the tubular body (15) being of copper or a copper alloy. 10. Continuous casting mould for the continuous casting of steels, in particular peritectic steels, comprising a mould tube (10) as claimed in one of claims 1 to 9, the mould tube (10) being provided with a water jacket cooling device for application of coolant to the outer surface (11) of the mould tube. Dated this 25th day of september 1998 A mould tube for continuous casting mould for the continuous casting of steels, in particular peritectic steels, comprises a first longitudinal section (1) incorporating a predetermined liquid metal level position (h) and a second longitudinal section (2) adjoining the first longitudinal section (1), the first longitudinal section (1) comprises a thermally insulating layer (16) having a thickness (d) so that a thermal resistance of the mould tube (10) in the first longitudinal section (1) has a greater value than in the second longitudinal section (2). The thermally insulating layer (16) covers up an area between the outer surface (11) of the mould tube(10) and a gap of at most 75% of the mall thickness (d) of the mould tube (10) when measured from the outer surface (11) of the mould tube (10). Due to an appropriate section of the thickness profile of the thermally insulating layer in the strand withdrawal direction (14), during casting a predetermined temperature profile can be set on an inside of the mould tube and the growth of a strand shell be optimized.

Documents:

01735-cal-1998-abstract.pdf

01735-cal-1998-claims.pdf

01735-cal-1998-correspondence.pdf

01735-cal-1998-description(complete).pdf

01735-cal-1998-drawings.pdf

01735-cal-1998-form-1.pdf

01735-cal-1998-form-18.pdf

01735-cal-1998-form-2.pdf

01735-cal-1998-form-3.pdf

01735-cal-1998-p.a.pdf

1735-CAL-1998-CORRESPONDENCE-1.1.pdf

1735-CAL-1998-FORM 27-1.1.pdf

1735-CAL-1998-FORM 27.pdf

1735-CAL-1998-FORM-27.pdf

1735-cal-1998-granted-abstract.pdf

1735-cal-1998-granted-claims.pdf

1735-cal-1998-granted-correspondence.pdf

1735-cal-1998-granted-description (complete).pdf

1735-cal-1998-granted-drawings.pdf

1735-cal-1998-granted-examination report.pdf

1735-cal-1998-granted-form 1.pdf

1735-cal-1998-granted-form 18.pdf

1735-cal-1998-granted-form 2.pdf

1735-cal-1998-granted-form 3.pdf

1735-cal-1998-granted-letter patent.pdf

1735-cal-1998-granted-pa.pdf

1735-cal-1998-granted-reply to examination report.pdf

1735-cal-1998-granted-specification.pdf