EP0498296B1 - Mould for continuous casting of metals, especially of steel - Google Patents

Abstract

In moulds for continuous casting of polygonal strand cross-sections, especially with quadrangular or hexagonal cross-section, the mould cavity (6) can have different geometrical shapes on the pouring-in side (4) and the strand outlet side (5). A better cooling of the crust of the strand is to be achieved by a targeted deformation of the strand cross section within the mould, in order to improve the strand quality and in addition to increase the casting speed. It is proposed for this purpose to provide, in each peripheral section of the mould cavity (6), a cross-sectional enlargement (7) in the form of a bulge (9), the extent (10) of the bulge (9) decreasing in the running direction (11) of the strand, at least along a partial length (12) of the mould cavity (6), such that the cross-sectional shape of the strand is deformed during passage through the partial length (12) of the mould cavity (6). <IMAGE>

Description

The invention relates to a mold for the continuous casting of metals, preferably steel, according to the preamble of claim 1 or claim 2.

Since the beginning of continuous casting with continuous molds, experts have dealt with the problem of the formation of air gaps between the strand crust and the mold wall below the bath level. This gap formation significantly reduces the heat transfer between the mold and the strand crust and causes an uneven cooling of the strand crust, which leads to strand defects such as rhomboidality, cracks, structural defects, etc. In order to create the best possible all-round contact between the strand crust and the mold wall and thus the best possible conditions for heat dissipation, many suggestions have been proposed, such as walking beams, coolant injection into the air gap, mold cavity with different conicity, etc. .

From US Pat. No. 4,207,941, a mold for the continuous casting of steel strands with polygonal, in particular with square cross sections, is known. The cross section of the mold cavity, which is open on both sides, is a square with corner fillets on the pouring side and an irregular pentagon on the strand exit side. In the corner areas towards the corner groove the casting cone is continuously enlarged in the direction of the strand, and in the area of the groove on a partial length of the mold it is approximately twice as large as in the central area of the mold wall. When casting with such molds, jamming of the strand inside the mold can occur, which leads to strand breaks and breakthroughs. A twelve-corner is cast instead of a square. In particular, it is difficult to dimension such molds for different casting speeds during an ongoing casting, as they are unavoidable with long sequence castings with many ladle changes.

DE-A-3 907 351 describes a mold with a pouring funnel for a thin slab. The two broad sides are provided with bulges on the pouring side of the mold, which are continuously formed back along a partial height of the mold. On the strand exit side of the mold, the cross section of the mold cavity is rectangular and aligned with the desired thin slab cross section. The sole purpose of the two opposite bulges is to create the space required for a dip tube. No bulges and no deformation of the strand shell through the mold walls are provided on the two narrow sides.

From AT-B-379 093, which forms the preamble of claims 1 and 2, a mold with a mold cavity open on both sides for continuous casting of a thin strand is known. The circumferential line of the mold cavity cross section on the pouring side can be divided into four circumferential sections. Cross-sectional enlargements in the form of bulges compared to the same cross-section on the strand exit side are provided on two sides of the circumference, which simultaneously form the broad sides of the thin strand cross section. The size of the bulge, which corresponds to the arch height in this example, decreases continuously in the direction of the strand and is zero at the mold exit. In contrast to the two broad sides, the narrow side walls diverging in the direction of the strand run on the two other circumferential sections, the narrow sides of this thin slab mold. These narrow sides, which diverge in the direction of the strand, are necessary to solve the task set there, in order to prevent bruises and wrinkles on the broad sides. An improvement or equalization of the cooling over the entire mold circumference and associated improvement of the strand quality with regard to surface and structure cannot be achieved with this mold because the narrow and broad sides have different conicity and therefore cool to different degrees. If the casting speed varies greatly, this different cooling is intensified. An increase in the casting speed is limited by the indefinite cooling conditions on the narrow sides. The strong cooling of the broad sides and the weak cooling of the narrow sides further increase the risk of breakthroughs in the case of widely varying, but especially at high, casting speeds.

The invention has for its object to overcome the disadvantages mentioned. In particular, deformation of the strand cross section within the mold is intended to achieve cooling of the strand crust that can be measured over the entire circumference, in order to improve the strand quality on the one hand and to increase the casting speed on the other hand. However, differences in casting speed during a running casting should also be made possible without the disadvantages mentioned, such as strand break and breakthrough.

According to the invention, this object is achieved by the entirety of the features of claims 1 or 2.

With the mold according to the invention it is possible, in the case of billets and small pre-block cross sections, to force cooling that is uniform in all circumferential sections and that can be measured in terms of its intensity within predetermined limits. This can influence the crystallization of the strand crust and improve the strand quality. Skewed edges, surface and structural defects are avoidable. Through the targeted Ver Forming the cross section can further improve the uniformity of cooling along the strand circumference, even at different casting speeds, in the mold according to the invention. The risk of strand breaks or breakthroughs can be significantly reduced at high casting speeds.

The bulges of the mold cavity in each section of the circumference on the pouring side each represent curved lines in the mold according to the invention, which have a higher dimensional stability of the mold tube, in particular in the highly heat-exposed bath level area, compared to classic tubular molds. With tube and other molds, this higher dimensional stability improves the dimensional stability of the mold cavity during the service life of the mold on the one hand and the strand quality on the other.

The bulge is usually reduced from the pouring side along the mold cavity over a part length or over the entire length of the mold. At the mold exit, for example, a curved line can remain in each circumferential section. According to a further embodiment, it is additionally proposed to provide the cross-section of the mold cavity on the mold exit side in a straight line on all sides between corners. At the mold exit, the mold can also be round or a pre-profile, e.g. in the form of a double-T beam.

When dimensioning the bulge or the bend height, it should be noted that even with short dwell times of the strand crust in the mold, i.e. at high casting speeds, no jamming of the strand in the border areas of two colliding circumferential sections, e.g. in the corners. For this purpose, the difference between the arc length at the bath level and at the mold exit or the chord length at the mold exit is determined and compared with the shrinkage of the strand crust transverse to the direction of the strand. The difference mentioned can be selected by the size of the bulge or the bend height so that it is essentially in agreement with the shrinkage mentioned. According to one exemplary embodiment, the light dimension between opposite circumferential sections of the mold cavity on the pouring side, measured in the region of the largest bulge, can be selected to be about 5-15%, preferably at least 5% or 8%, larger than the light dimension between opposite circumferential sections on the strand exit side.

The dimension of the arc height can decrease degressively or possibly progressively in the casting direction and can approach zero. According to a further embodiment, the dimension of the arc height can advantageously decrease steadily in the cross-sections following the strand running direction. According to a further exemplary embodiment, the change in the bow height in the strand running direction can also be defined as a degree of taper. The shape and size of the bulge are the same in all sections. The conicity of the bulge changes in size along the circumferential section. According to one embodiment, a taper between 0 and 1% / m and at the center of the peripheral section between 10 and 35% / m can be provided at the two ends of each peripheral section.

Another possible variation is the choice of the length or partial length of the mold cavity with bulges. In principle, it is possible that the arc height of the bulge is reduced over the entire length of the mold cavity. However, only partial lengths are conceivable. An advantageous embodiment provides for a partial length of at least 50% of the mold length. In today's molds of 800 mm length, the partial length is at least 400 mm.

In the case of rectangular conical molds according to the prior art, the taper in the corners or in the corner areas is greater by a factor of 2 than on the side walls. In the case of molds with a degree of taper that exceeds the usual level of 0.9-1.2% / m, this fact can lead to jams and strand breaks. Instead of the conically arranged walls of the molds known in the prior art, the strand cross-sectional shape is deformed according to the invention as it passes through the partial length of the mold cavity and the cooling capacity is controlled in the process. In the border area between two abutting circumferential sections or in the corners of the mold cavity, the design of the taper can be freely selected, regardless of the size and degree of taper of the bulge. This allows molds to be built for the first time, whose taper in the corners or corner areas can be selected independently of the taper and the shape of the bulged side surfaces. For example, it is possible to make the taper in the corners positive, neutral or negative depending on the degree of the deformation of the bulge, the shrinkage of the strand crust, etc.

In one exemplary embodiment, it is proposed to provide the conicity measured over the diagonal over the partial length with bulges in the order of magnitude between 0-1% / m, or between 0-0.5% / m.

For various known reasons, the corners of the mold cavity are rounded in polygonal strand cross sections. It has proven to be particularly advantageous if the corners of the mold cavity have fillets with a radius of 3 to 8% of the side length of the cross section.

The circumferential sections with bulges can be composed of circular lines, curves or of th straight lines.

Exemplary embodiments of the invention are explained below with reference to figures.

• Fig. 1: Einen Längsschnitt durch eine Rohrkokille nach der Linie I-I von Fig. 2,
• Fig. 2: eine Draufsicht auf die Kokille gemäss Fig. 1,
• Fig. 3: eine Draufsicht auf ein Beispiel einer Ecke eines ausgebauchten Formhohlraumes mit vier Höhenkurven,
• Fig. 4: eine Draufsicht auf ein weiteres Beispiel einer Ecke eines ausgebauchten Formhohlraumes mit vier Höhenkurven,
• Fig. 5: eine Draufsicht auf ein weiteres Beispiel eines halben Formhohlraumes mit vier Höhenkurven,
• Fig. 6: eine Draufsicht auf eine runde Kokille und
• Fig. 7: eine Draufsicht auf eine Kokille, deren Formhohlraum durch Bogenlinien begrenzt ist.

Show it:

• 1: a longitudinal section through a tubular mold along the line II of Fig. 2,
• 2: a plan view of the mold according to FIG. 1,
• 3: a plan view of an example of a corner of a bulged mold cavity with four height curves,
• 4: a plan view of a further example of a corner of a bulged mold cavity with four height curves,
• 5: a top view of another example of a half mold cavity with four height curves,
• Fig. 6: a plan view of a round mold and
• Fig. 7: a plan view of a mold, the mold cavity is delimited by curved lines.

1 and 2 show a mold 3 for the continuous casting of polygonal strand cross sections, in the present example of square strand cross sections. An arrow 4 points to a pouring side and an arrow 5 to a strand exit side of the mold 3. The cross sections of a mold cavity 6 have different geometrical shapes on the pouring and strand exit side. 2, the cross-section of the mold cavity 6 on the pouring side 4 between the corners 8-8 "'is provided with cross-sectional enlargements in the form of bulges 9 compared to the strand exit side. An arc height 10, which represents the extent of the bulge , decreases steadily in the strand running direction 11 over a partial length 12 of the mold cavity 6. The mold cavity cross sections in the planes 14 and 15 delimit a Kokiiienteii 13 with a square cross section with fillets 16, as is known in the prior art.

A circumferential line 17 shows the mold cavity cross section in the plane 14 and a circumferential line 18 shows the mold cavity cross section in the plane 15. The cross section of the mold cavity 6 is straight on all sides between the corners 8 on the mold exit side. An arrow 2 denotes a peripheral section of the peripheral line of the mold cavity 6. In this mold, 4 peripheral sections with similar cross-sectional enlargements 7 are provided. Instead of the square basic shape of the mold cavity 6, a hexagonal, rectangular etc. cross section could also serve as the basic shape.

A light dimension 20 between opposite sides of the mold cavity 6 on the pouring side 4 in the area of the largest bulge is 5 to 15% larger than a light dimension 21 between the opposite sides on the strand exit side 5. In other words, the light dimension 20 can also be at least 5% or at least 8% larger than the light dimension 22 in the plane 14 at the end of the partial length 12.

berechnet werden, wobei Bo die Kokillenbreite oben oder Lichtmass 20 in mm, Bu die Kokillenbreite unten oder Lichtmass 22 in mm, L die massgebende Länge in m und T die Konizität (oder Taper) in %/m bezeichnet. Nach dieser Formel gerechnet können Konizitäten von 10 - 35 %/m gewählt werden.The arch height 10 of the bulge 9 decreases continuously in the direction of the strand 11 with the following cross sections. The taper of the maximum arc height 10 along a line 24 can be according to the formula

are calculated, where Bo denotes the mold width at the top or light dimension 20 in mm, Bu the mold width at the bottom or light dimension 22 in mm, L the decisive length in m and T the conicity (or taper) in% / m. Conicities of 10 - 35% / m can be selected using this formula.

The part length 12 in this example is 400 mm or about 50% of the mold length, which measures about 800 mm.

3 shows height curves 30-33 a corner of a bulged mold cavity 35. The height curve 30 represents the top edge of the mold cavity 35 of the mold 34. The wall thickness of a mold tube is indicated by 36. 33 shows the height curve at the mold exit. Between the curves 30 and 33, the taper can be read out at two intermediate heights. Curves 31 and 32 show the decreasing arc heights of the bulges, which cause the strand crust to deform during casting. In the area of the fillet 38, the taper of the mold cavity 35 along a diagonal cut along the line 39 is 0-1% / m, preferably 0.1-0.5% / m. A deformation of the strand crust along line 39 is generally not provided.

FIG. 4 shows similar height curves 40-43 as in FIG. 3. The main difference lies in the design of the fillet 48 along the diagonal line 49. The fillet 48 has a negative cone in the direction of the strand. In the corner area in the strand running direction, an expansion of the mold cavity is thus provided. Depending on the format of the strand, the selected arc height of the bulge that has to be deformed back, it may be of interest to provide a negative cone at the corners 48 along the diagonal line 49 in order to prevent any jamming of the strand in the mold. Depending on the geometric design of the corner area, the cooling in the edge area of the strand can also be controlled. A negative taper along the diagonal line 49 may also be desirable to accommodate tendon extensions when reshaping bulges that are not compensated for by the shrinkage the.

In Fig. 5 the bulges are delimited by compound straight lines. Elevation curves 50 - 53 represent a steady decrease in the bulges. In order to ensure that there are no abutting edges in the middle of the bulged sides, a rounding 54 is appropriate. The straight lines run tangentially to a fillet 58. In this example, no taper is provided along the groove 58 in the direction of the strand. In a section along the diagonal 59, the fillet 58 runs essentially parallel to the longitudinal central axis of the mold.

To determine the taper of the fillets 38, 48, 58 in FIGS. 3-5, calculations and / or casting tests are necessary. On the partial length of the mold, the chord associated with each circular arc lengthens on the one hand as the bulge height of the bulge decreases. On the other hand, the shrinkage of the strand crust transversely to the direction of the strand running can be calculated at a specific casting speed and compared with the tendon extension. The taper in the corner area can be determined from the difference between the two values. It should be noted that at high casting speeds, i.e. with short dwell time of the strand crust in the mold, the value for the shrinkage is smaller than at low casting speeds.

FIGS. 6 and 7 show molds whose mold cavities 60 and 70 are delimited by curved and circular surfaces. Circumferential lines 61, 71 of the mold cavity cross section are each divided into three peripheral sections 62, 72. The number of peripheral sections 62, 72 can be chosen freely, with essentially round molds, as shown in the figures, generally being divided into 3-6 peripheral sections 62, 72. Each peripheral section 62, 72 on the pouring side has a cross-sectional enlargement in the form of a bulge 63, 73 in relation to the strand exit side. In these examples, the cross-sectional enlargements are represented by bulges delimited by an arc. The dimension of the bulge 63, 73 is represented by arrows 65, 65 ', 65 "and 75, 75' and by their length. This dimension decreases on the partial length of the mold cavity in such a way that the strand cross-sectional shape deforms as it passes through the partial length. The shape and the size of the bulge 63, 73 are the same in all the circumferential sections 62, 72. The conicity of the bulges 63, 73 measured in the direction of the strand run are different in size along the circumferential sections 62, 72. At the two ends 66, 66 ', 76 , 76 'of each circumferential section 62, 72 is the taper zero to 1% / m and in the middle 67, 77 of the circumferential sections a taper between 10 - 35% / m is generally provided.

In the case of essentially round mold cavity cross sections, it is also possible to deform the strand into two partial lengths which follow one another directly or have an intermediate zone between the partial lengths. With such molds, the peripheral sections of the successive partial lengths can be offset from one another, preferably offset by half a peripheral section.

In order to achieve long service lives of such molds or to improve the strand surface, all measures known in the prior art for reducing friction, such as lubrication, surface treatment, coatings, material selection of the mold, etc., can be used.

All figures show straight tube molds for a better overview. However, the invention can also be used on sheet molds and on block molds, plate molds, etc.

Claims (11)

1. A mould for continuous casting of metals, preferably for steel, comprising a cavity (60, 70) open at both ends and formed with at least two peripheral portions (62, 72) at the pouring-in end of the mould along a peripheral line (61, 71) of the mould cross-section, each portion having a cross-sectional enlargement of the mould cavity in the form of bulges, relative to the same peripheral portions of the mould cavity cross- section at the billet outlet end of the mould, and the arc heights of the bulges (63, 73) in the direction of advance of the billet decrease in such a manner that during casting, a billet shell forming in the mould cavity (60, 70) changes shape along the peripheral portions while in transit through the mould cavity (60, 70), characterised in that at the pouring-in end the peripheral line (61, 71) of an approximately round mould cross-section is divided into at least three substantially equal-sized peripheral portions (62, 72) and each peripheral portion (62, 72) at the pouring-in end has the cross-sectional enlargement of the mould cavity in the form of a bulge, and the arc heights of the bulges (63, 73) in all peripheral portions decrease in the direction of advance of the billet, at least along part of the length of the mould cavity (60, 70).

2. A mould for continuous casting of polygonal, preferably quadrilateral or hexagonal, steel billets, comprising a cavity (6) open at both ends and formed with at least two peripheral portions (2) at the pouring-in end (4) of the mould (3) along a peripheral line of the mould cross-section between corners of the mould cavity (6), each portion having a cross-sectional enlargement (7) of the mould cavity in the form of bulges (9), relative to the same peripheral portions of the mould cavity cross-section at the billet outlet end (5) of the mould, and the arc heights of the bulges (10) in the direction of advance (11) of the billet decrease in such a manner that during casting, a billet shell forming in the mould cavity (6) changes shape along the peripheral portions while in transit through the mould cavity (6), characterised in that at the pouring-in end the peripheral line of the polygonal mould cross-section between all corners (8 - 8"') have peripheral portions (2) with cross-sectional enlargements (7) of the mould cavity (6) in the form of bulges (9) , and the arc heights (10) of the bulges in all peripheral portions decrease in the direction of advance of the billet (11), at least along a part (12) of the length of the mould cavity (6).

3. A mould according to claim 1 or 2, characterised in that the shape and the size of the bulges (9, 63, 73) are identical in all peripheral portions (62, 63).

4. A mould according to any of claims 1 to 3, characterised in that the conicity of the bulges (9, 63, 73) measured along the peripheral portion (2, 62, 63) in the direction (11) of advance of the billetvaries, the conicity preferably being between 0 and 1 %/m at the two ends (66, 66', 76, 76') of each peripheral portion (62, 72) and between 10 and 35 %/m in the middle (67, 77) of the peripheral portion (62, 72).

5. A mould according to claim 2, characterised in that the mould cavity cross-section at the billet outlet end has a preliminary shape, preferably the shape of an H-girder.

6. A mould according to any of claims 1 - 5, characterised in that the cross-sectional enlargement in each peripheral portion (62, 72) is bounded by a circular portion (63, 73).

7. A mould according to any of claims 2 - 6, characterised in that the internal width (20) between opposite peripheral portions at the pouring-in end (4), measured in the region of the maximum bulge (9), is about 5 - 15%, preferably at least 5% or 8% respectively, greater than the internal width (21 between the same peripheral portions at the billet outlet end (5).

8. A mould according to any of claims 1 to 7, characterised in that the partial length (12) is at least 50% of the length of the mould.

9. A mould according to any of claims 2 - 6, characterised in that in the case of a square cross-section, the conicity measured along a diagonal section is 0 - 1 %/m, preferably 0.1 - 0.5 %/m.

10. A mould according to any of claims 2 - 9, characterised in that the corners (8 - 8"') of the mould cavity (6 - 35) have grooves (16, 38, 48, 58) with a radius of 3 - 8% of the side length of the cross-section.

11. A mould according to any of claims 1 to 10, characterised in that the conicity along the part length (12) in the boundary region between two abutting peripheral portions (62, 72) is determined by a geometrical calculation of the peripheral length of the billet and by a calculation of the shrinkage of the crust of the billet transversely to the longitudinal axis thereof.

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