WO1996035533A1

WO1996035533A1 - Mould for continuous casting

Info

Publication number: WO1996035533A1
Application number: PCT/SE1996/000598
Authority: WO
Inventors: Bo Rogberg; Bengt Brolund; Staffan Hertzman
Original assignee: Sandvik Aktiebolag; Institutet För Metallforskning
Priority date: 1995-05-09
Filing date: 1996-05-07
Publication date: 1996-11-14
Also published as: AU5709296A; KR19990008455A; JPH11504864A; CA2219910A1; SE9501709D0; SE516076C2; BR9608283A; SE9501709L

Abstract

A mould is provided for the continuous casting of preferably steel. It has an inlet side (2) and an outlet side (3) and a mould cavity (4) between these. At the top, the four main inner walls in the mould cavity bulge inwards towards the middle of the mould, whereafter this bulging decreases downwards towards the oulet side of the mould. In this way, the mould has been given a shape that corresponds to the solidification behaviour, which results in a better contact and support for the strand shell.

Description

MOULD FOR CONTINUOUS CASTING

The invention relates to a mould for continuous casting 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.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 been 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 easily arise, which may lead to a tearing up of the strand and a break¬ through. Further, a dodecagon is moulded instead of a square. It is particularly difficult to dimension such moulds for different 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

CONFIRMATION PY 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. 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 shrinkage gradient that arises between the inlet and outlet of the mould, thereby minimizing pull and pressure stresses. 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, the mould has been given a shape that corresponds to the solidification behaviour of the material. This results in a better contact and thus a better support for the strand shell, and thereby an almost tension-free shell. Thereby, the casting of crack-sensitive sorts is simplified and the risk of corner cracks in the corner regions, such as cross cracks, corner cracks and impressions is minimized. Further, the risk of a rhombic* strand-shape is decreased, a problem that often arises, e.g., when casting blooms and billets. The construction according to the invention exposes the shell to a minimum of external influence and causes a very small mould wear (contrary to, e.g., AT-A-379 093), by the fact that the mould has a shape that substantially follows the free dimension variations of the strand during casting.

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 a mould according to the invention in a perspective obliquely from above. Figure 2 shows a formalized view straight from above of two moulds according to the invention.

In fig 1 is generally shown a mould 1 for the continuous casting of metals, preferably steel, into substantially rectangular or square cross-sections. It is usually made of copper or any copper-based alloy. Reference numeral 2 designates the inlet side of the mould and 3 designates its outlet side. Between these extends the mould cavity 4. It is suitably made of one sole, straight piece, although bowed moulds and block or plate moulds are also feasible.

In order to achieve the above mentioned objects, the mould is shaped with inner walls 5 which bulge inwards, towards the middle of the mould. At the top, at the inlet side 2 of the mould, they have a bending radius R_ref, which within given ranges increases indefinitely towards the outlet side 3 of the mould. By this shape of the the mould, the maximal value of the mechanical tension elongation is reduced in a surprising manner by at least 25% in the upper parts of the mould and by about 50% in the lower half of the mould.

In order to generalize the desired dimensioning of the mould, it is preferred to describe it by dimension-less numbers:

R' = R - ' = z Rref ^■•-^■

where R' is the dimensionless radius, R is the radius at the height z, R_ref is the radius at the inlet side (see fig 1) , z' is the dimensionless distance from the upper side of the mould, z is the factual distance from the upper side of the mould and L is the total height of the mould. On basis of the obtained results, the radius R of the inwardly bulging walls 5 may decrease in ^"the feed direction of the strand according to the function

R(z) = LR_ref/(L-z) , or

2 2

R(z) = LR_ref/(L-z) ; or dimensionless:

R' = 1/1-z' or

R' = l/(l-z')

It is easily understood that these functions increase indefinitely towards the outlet end of the mould. Referring to the bending radius R_ref at the upper edge of the mould, it should suitably be between 4 and 7 m, preferably between 5 and 6,5 m.

The dimensionless radius shall suitably be within the following ranges along the vertical axis of the mould:

Table 1

z' *■•» — uppe r *•*•^■ — 1 owe r

0.00 1.00 1.00 o.?o 1.56 1.25

0.40 2.78 1.67

0.60 6.25 2.50

0.R0 25.00 5.00

0.90 100.00 10.00

1.00 00 00 Preferably, the dimensionless radius lies within the following ranges:

Table 2

*■•• — uppe r *■•» — 1 owe r

0.00 1.00 1.00

0.20 1.33 1.25

0.40 1.94 1.67

0.60 3.44 2.50

0.70 5.28 3.33

0.80 ^• 10.00 5.00

0.90 . 32.50 10.00

1.00 00 00

Preferably, the mould shape goes from bulging sides and corner angles Φ larger than 90° at the inlet side, to a substantially square or rectangular shape at the bottom side of the mould. According to a preferred embodiment of the present invention, the corner angles Φ decrease downwards in a determined way, in order to obtain further advantages with regard to a minimized formation of cracks, increased casting rate and optimized heat transfer between the strand and the mould. Thus, along the whole length of the mould, there is advantageously an angle alteration of Φ from between 90 to 98° at the top to between 90 to 92° at the bottom. If the total length of the mould is divided into twelve equally wide segments according to fig 1, then up to half the angle alteration should take place in the segments 0 and 1. Up to another 25% of the angle alteration should occur within the segments 2 to 4, inclusive. The remaining angle alteration should take place down to the bottom or outlet side 3 of the mould, i.e., in the segments 5 to 11, inclusive. As to its amount, the angle alteration should suitably be between 2 and 8°, preferably between 2 and 4°.

In a formalized way, fig 2 shows the cross-sectional shape of the mould cavity 4 at the inlet side 2 and the outlet end 3, respectively, of the mould. The cross-section of the inlet side corresponds to line 6 while the cross-section of the outlet end is substantially rectangular and is depicted either by the rectangle 7 or 8. Thus, as exemplified by the rectangle 7, the corner distance of the mould in one direction may be lar,ger at the lower edge of the mould than at the upper edge, i.e. the mould may to some extent obtain an inverted conicity. Hence, the mould should have some conicity in order to compensate for the cooling shrinkage of the arising strand shell, and whose reduction of the corner distance may be added to the extension of the corner distance that occurs when the bulging side surface 5 is gradually straightened out. ^• According to a preferred embodiment of the present invention, the bulginess of the mould walls start 5 to 40 mm from the corners 9, as measured along the substantially planar wall portion 10; preferably 10 to 30 mm.

Claims

1. Mould for the continuous casting of metals, preferably steel, into strands of a substantially rectangular or square cross-section, comprising a mould cavity (4) which is open at both ends and located between an inlet side (2) and an outlet side (3), c h a r a c t e r i z e d in that the four main inner walls of the mould are curved inwardly towards the centre of the mould.

2. Mould according to claim 1, c h a r a c t e r i z e d in that each main inner wall (5) is curved inwardly by a radius of curvature (R_ref) ^at the inlet side (2) of the mould, said radius increasing infinitely downwards towards the outlet end of the mould.

3. Mould according to claim 2, c h a r a c t e r i z e d in that the radius of curvature at the inlet end (2) of the mould is between 4 and 7 , preferably between .5 and 6,5 m.

4. Mould according to any of the preceding claims, c h a r a c t e r i z e d in that the inwardly curved parts of the main inner walls begin at between 15 and 50 mm from the respective mould corner (9) .

5. Mould according to any of the preceding claims, c h a r a c t e r i z e d in that all four corner angles (Φ) are altered from a higher value at the inlet side (2) of the mould to a lower value at the outlet side (3) of the mould.

6. Mould according to claim 5, c h a r a c t e r i z e d in that upto half of the angular change takes place in the uppermost two twelfths of the total length of the mould.

7. Mould according to claim 5 or 6, 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°.

8. Process for the continuous casting of metals, preferably steel, into strands of a substantially rectangular or square cross-section, the molten bath being allowed to flow down into an inlet side (2) of a mould, pass a mould cavity (4) in the mould, whereby a strand shell is formed, and come out through an outlet side (3) of the mould, c h a r a c t e r i z e d in that the four main inner walls of the mould are curved inwardly towards the centre of the mould.

9. Process according to claim 8, c h a r a c t e r i z e d in that each main inner wall (5) is curved inwardly by a radius of curvature (R_rβf) at the inlet side (2) of the mould, said radius increasing infinitely downwards towards the outlet end of the mould.