WO1996035532A1

WO1996035532A1 - Mould

Info

Publication number: WO1996035532A1
Application number: PCT/SE1996/000597
Authority: WO
Inventors: Staffan Hertzman; Bengt Brolund
Original assignee: Institutet För Metallforskning
Priority date: 1995-05-09
Filing date: 1996-05-07
Publication date: 1996-11-14
Also published as: AU6142496A; SE9501708D0; SE9501708L

Abstract

A mould for the continuous casting of metal, preferably steel, has been provided, into strands of substantially quadrangular cross section. Between an inlet side (2) and an outlet side (3), it has a mould cavity (13) which is open at both ends and which at the inlet end (2), along a circumference line of the mould cavity cross section between corners ot the mould cavity, has at least two circumference sections. Each one of these delimits a cross-sectional enlargement of the mould cavity (13) in relation with the same circumferential cross section of the mould cavity cross section at the outlet end (3), the cross-sectional enlargements diminishing in the feeding direction of the strand. All corner angles (Ζ) change from a higher value at the inlet side (2) of the mould, to a lower value at the outler side (3) of the mould, up to half of the angle alteration taking place in the two uppermost twelfths of the total length of the mould. This design results in a better contact and support for the strand shell, the risk of corner cracks being substantially reduced.

Description

MOULD

The invention relates to a mould for continuous castin of metals, preferably steel, according to claim 1. Since the beginning of continuous casting with through flow moulds, professional circles have dealt with problems relating to the creation of air gaps between the strand shell and the mould wall below the surface of the molten metal. The air gap is created when the strand shell gradually undergoes a chemical shrinkage during the passage through the mould. The generally applied attempt to solve this problem is to make the mould conical. Of course, this gap formation deteriorates the heat transfer between the mould and the strand shell considerably and causes an uneven cooling of the strand, which results in deficiencies in the billet, such as rhombic shape, cracks, structural defects, etc. In order to generally shape as good a contact as possible between strand and mould wall along the whole length of the mould, and thereby the best possible conditions for the deviation of heat, a number of suggestions have be.en made, such as pressing in a coolant into the airgap and mould cavities of different conicities, etc.

US-A-4 207 941 describes a mould for the continuous casting of steel strands with polygonal, in particular square cross-sections. The cross-section of the mould cavity, that is open at both sides, is at the inlet side a square with corner fillets and at the outlet side an irregular dodecagon. However, when moulding with this mould a jamming of the strand may easil arise, which may lead to a tearing up of the strand and a break through. Further, a dodecagon is moulded instead of a square. I is particularly difficult to dimension such moulds for differen

CONFIRMATION COPY moulding speeds during a continuous moulding, which is necessary at long sequential mouldings with many changes of ladles.

In DE-A-3 907 351 a mould for continuous casting of a bloom is described. The two longitudinal sides are provided with bulges at the inlet side of the mould, which are continuously retrograded along a part height of the mould. At the outlet side of the mould, the cross-section of the mould cavity is right- angled and provided with the desired bloom cross-section. The only purpose of the two bulges facing each other is to provide sufficient place for a moulding pipe. There are no bulges on the two narrow sides and also no shaping of the strand shell by means of the mould walls.

AT-B-379 093 describes a mould for the continuous casting of a bloom. The circumference line of the cross-section of the mould cavity at the inlet side can be divided into four circumference sections. At two circumference sections at the inlet side, which simultaneously form the longitudinal sides of the bloom section, are provided cross-sectional enlargements with the shape of protruding bulges in relation with the same cross-section at the outlet side of the strand. The measure of the bulge, which in this case corresponds to the arch height, diminishes continuously in the moving direction of the strand and is nil at the exit of the mould. At the two other circumference sections, i.e., the short sides of the bloom, run the narrow-side walls, which contrary to the two longitudinal or broad sides, diverge in the direction of the strand. These short- or narrow-sides diverging in the direction of the strand, are necessary for solving the posed problem, namely to prevent a jamming and a formation of creases on the broad sides. An improvement and an even distribution, respectively, of the cooling over the whole circumference of the mould, and thereby an improvement of the strand quality relative to outer surface and structure, is not possible to obtain with this mould, since the narrow and broad sides are cooled differently strongly. An increase of the moulding speed is limited by the undefined cooling conditions on the narrow sides. The strong cooling of the broad sides and the poor cooling of the narrow sides further increase the risk for a break-through both at strongly varying, and in particular at high moulding speeds. Further, with this mould shape, there is a great risk of an increased mould wear. In EP-A-498 296 a mould for continuous casting is disclosed according to which the circumference line of the inlet side has circumference sections between the corners with cross- sectional enlargements of the mould cavity. These enlargements have the shape of bulgings, the chordal heights of of the bulgings decreasing in all circumference sections in the moving direction of the strand along at least a partial length of the mould cavity. According to this publication, a measurable cooling of the strand shell should thereby be obtained along the whole circumference in order to on one hand improve the strand quality, and on the other hand increase the moulding rate.

Moreover, the construction shall also make possible differences in the moulding rate during the process of casting.

The construction according to EP-A-498 296 has only partly managed to fulfil the posed problems. The reduction of the cross-sectional area has sometimes turned out to be too big, which has caused problems of jamming of the cast strand and of an increased mould wear. In existing plants, the chosen casting rate has turned out to be too high for this mould. It has been constructed in first hand for high rates, not for freedom from cracks and a generally faultless casting. However, it does not fully compensate for corner shrinkage, wherefore the corner design leaves more to be desired. Furthermore, the strand binds or "pinches" at lower casting rates.

A primary object of the present invention is to compensate for the corner shrinkage of the strand, both at higher and lower casting rates.

A further object of the present invention is to simplify the design of the mould as much as possible.

Still another object of the present invention is to obtain an optimal contact between the strand and the mould wall along the whole length of the mould.

A further object of the present invention is to compensate for the thermal shrinkage that takes place between the inlet and outlet of the mould.

These and further objects have been solved in a surprising way by shaping the mould in the way as defined in the characterizing clause of claim 1.

Thus, in accordance with the invention, by conferring to the mould a corner configuration that corresponds to the solidification behaviour of the material, this leads to a better contact. and support for the strand shell and thereby an almost stress-free shell. Thereby, the casting of crack-sensitive qualities is simplified, and the risk diminishes of corner cracks in the corner regions, such as cross cracks, corner cracks and impressions. Furthermore, the risk of arising rhombic shapes of the shell decreases, a problem that preferentially occurs at the casting of blooms and billets.

For illustrative but non-limiting purposes, the invention will now be further described with reference to the appended drawings. These are herewith briefly presented: Figure 1 shows an outline drawing of how a mould according to the invention shall be designed. Figure 2 shows a top view of an embodiment of the mou according to the invention.

Figure 3 shows a top view of another embodiment according to the invention. Figure 4 shows a top view of still another embodiment according to the invention.

Figure 5 shows an outline diagram of how a solidification course may be monitored.

In fig 1, a mould 1 is generally shown for the continuous casting of metals, preferably steel, into substantially rectangular or square cross-sections. The mould usually made of copper or a copper-based alloy. Reference numeral 2 designates the inlet side of the mould while 3 designates its outlet side. Between inlet and outlet side is t mould cavity 13. The mould is suitably made of one sole piece which suitably is straight, although bowed moulds and block or plate moulds are also feasible.

An essential feature of the present invention is that the corner angles Φ decrease downwards in a determined way, in order to obtain the advantages that are characteristic for the invention relative to a minimized formation of cracks, an increased casting rate and an optimized heat transfer between strand and mould. Along the whole length of the mould, an alteration of Φ shall take place from between 90-98° at the to to between 90-92° at the bottom. If the total length of the th mould is subdivided into twelve equally long segments accordin to fig 1, then upto half of the angle alteration shall occur i segments 0 and 1". Another 25% of the angle variation should ta place in segments 2 to 4, inclusive. The remaining angle chang should occur down to the mould bottom or outlet side 3, i.e. i the segments 5 to 11, inclusive. As to its amount, the angle al¬ teration should be between 2 and 8°, preferably between 2 and 4°.

The corner angle diminution according to the invention may take place in a number of ways according to fig 2, 3 and 4. In fig 2, the outermost line is the inlet opening 2 and the innermost line the outlet opening 3. Lines 4, 5 and 6 illustrate the inner wall of the mould at different levels in the mould. Thus, line 4 relates to the cross-sectional contour at a height corresponding about to the limit between segments 0 and 1, line 5 to the cross-section somewhere within segment 2 and line 6 the cross-section somewhere within the segment 6. The walls may be arched like in EP-A-498 296, but they may also be shaped according to a simplified embodiment as in fig 2 in straight portions, which simplifies their production quite considerably. The main issue is that all four corner angles Φ decrease gradually according to the invention.

Moreover, it has surprisingly been established that not all four inner mould walls have to be varied along the whole length of the mould. Thus, it is sufficient that three or even only two mould walls are shaped so that the necessary angle variation is attained, while the remaining side or two sides, respectively, are made planar and substantially vertical. This may be seen in fig 3 and 4, respectively.

In fig 3, the two angles Φ^Λ adjacent the planar mould wall should be substantially equally large and vary according to the same sequence as the two corner angles Φ. This is achieved by letting the two bulging walls 7,7' bulge unsymmetrically, i.e. the chordal height is basically twice as large at a certain distance above the middle line m than at the same distance below said middle line. At the two corner angles Φ, basically half the angle variation is obtained by the bulging of the wall 8. Fig 4 shows a mould with two opposed, substantially planar and nearly vertical walls 9,9', depending upon the choic of conicity, and two intermediate, opposed, bulging walls 10,10'. Hence, in this case, the angle variation of angle Φ in all four corners according to the invention is achieved by only varying the bulgings of two walls.

Thus, contrary to EP-A-498 296, the bulging of the walls vertically according to the invention is not uniform with a substantially constant conicity, but differentiated , with a varying conicity. In this way, the mould has been "tailor-made" relative to the shrinkage gradient of the solidifying shell.

Since the contraction of the corner angle of the stran does not only depend on the heat shrinkage, but also on the phase transformation that takes place within the temperature range in question, an experimental evaluation can be performed for individual types of material, in order to investigate their solidification and shrinkage courses. In accordance with fig 5, by introducing molten metal (preferably a certain steel alloy) 11 into a V-shaped groove between two steel plates 12, 12' , the erection of the plate 12' can be monitored as a function of tim or temperature. As a measure of the erection, the angle Φ relative to the horizontal plane is used, which corresponds to the corner angle Φ in fig 1 to 4, or the erection height Δ at the end of the plate 12' . The angle increase course is then transformed into the variation gradient of the corner angle Φ the mould; thereby the angle should be about 90° at low temperatures and the angle at high temperatures shall correspon to the largest mould corner angle, at the mould inlet.

However, all materials fulfil the above defined margin conditions. All the investigated solidification courses had a corner angle variation of less than 8°.

Claims

1. Mould for the continuous casting of metal, preferably steel, into strands with a substantially quadrangular cross-section, comprising a mould cavity (13) which is open in both ends and located between an inlet side (2) and an outlet side (3), which cavity has at least two circumference sections between corners of the mould cavity along a circumferential line of a cross-section of the mould cavity adjacent to the inlet end (2), each of said circumference sections delimiting a cross- sectional enlargement of the mould cavity (13) in relation to the same circumferential cross-section of the mould cavity cross-section adjacent to the outlet end^' (3), the cross- se^'ctional enlargements decreasing in the feeding direction of the strand, c h a r a c t e r i z e d in that all corner angles (Φ) are altered from a higher value at the inlet side (2) of the mould to a lower value at the outlet end (3) of the mould (3) , upto half of the angular change taking place in the uppermost two twelfths of the total length of the mould.

2. Mould according to claim 1, c h a r a c t e r i z e d in that upto a further 25% of the whole corner angle alteration takes place in the third and fourth twelfths of the total length of the mould, the remaining angle alteration taking place in the fifth, sixth, seventh, eighth, ninth, tenth, eleventh and twelfth twelfths.

3. Mould according to claim 1 or 2, c h a r a c t e r i z e d in that the corner angles (Φ) are altered from a highest value at the inlet side (2) of the mould of between 90 and 98° to a lowest value at the outlet side (3) the mould between 90 and 92°.

4. Process for the continuous casting of metal, preferably steel, into strands of substantially quadrangular cross-section, thereby allowing the molten bath to flow into t inlet side (2) of a mould, to pass a mould cavity (13) in the mould, whereby a strand shell is formed, and to come out at an outlet side (3) of the mould, the mould having at least two circumference sections between corners of the mould cavity (13) along a circumferential line of the cross-section of the mould cavity adjacent to the inlet end (2), each of said circumferen sections delimiting a cross-sectional enlargement of the mould cavity in relation to the corresponding circumferential cross- section of the mould cavity cross-section adjacent to the outl end (3) , the cross-sectional enlargements decreasing in the feeding direction of the strand, c h a r a c t e r i z e d i that the four corner angles (Φ) of the mould are altered from higher value at the inlet side (2) of the mould to a lower val at the outlet end (3) of the mould (3), upto half of the angul change taking place in the uppermost two twelfths of the total length of the mould.

5. Process according to claim 4, c h a r a c t e r i z e d in that upto a further 25% of the whole corner angle alteration takes place in the third and fourth twelfths of the total length of the mould, the remainin angle alteration taking place in the fifth, sixth, seventh, eighth, ninth, tenth, eleventh and twelfth twelfths.

6. Process according to claim 4 or 5, c h a r a c t e r i z e d in that the corner angles (Φ) are altered from a highest value at the inlet side (2) of the mould of between 90 and 98° to a lowest value at the outlet side (3) of the mould between 90 and 92°.