WO2007039485A2

WO2007039485A2 - Ingot mold for casting slabs

Info

Publication number: WO2007039485A2
Application number: PCT/EP2006/066617
Authority: WO
Inventors: Gianpietro Benedetti; Nuredin Kapaj; Alfredo Poloni
Original assignee: Danieli & C. Officine Meccaniche S.P.A.
Priority date: 2005-09-22
Filing date: 2006-09-22
Publication date: 2007-04-12
Also published as: ITMI20051762A1; EP1934004A2; WO2007039485A3; EP1934004B1

Abstract

An ingot mold for casting slabs Abstract An ingot mold has two concave faces (4, 5) with a concavity in the shape of a semi- circular arc in which the faces (4, 5), located near the connecting corner of the narrow walls (6 and 7), form an angle (gamma), with a plane parallel to the longitudinal plane of symmetry X, the value of which is between 1° and 19°.

Description

Ingot mold for casting slabs

DESCRIPTION Field of the invention

The present invention relates to an ingot mold for casting ultra thin slabs to be used, in particular, in a manufacturing plant for the manufacture of metal strips. State of the art

Numerous types of ingot molds used for casting metal strips, in particular of steel, are known in the state of the art. These ingot molds can produce a slab which is subjected to a number of thickness reductions, sometimes associated with surface treatments and machining, depending on the process used.

Since the thickness of the slab coming out of the ingot mold is considerable, in comparison to the final thickness to be achieved at the end of the production line, numerous rolling cages are required. Since the passage through every cage involves a cooling of the strip, after the passage through some rolling cages, an induction furnace or heating furnace for heating the strip is required for bringing the temperature of the strip back to the value necessary for rolling the material. Yet, by doing so, the metal strip machinery will be particularly long because the slabs produced by ingot molds of the known type, also called crystal lizers, have consider- able thicknesses when coming out of the ingot mold, hence they will require numerous intermediate devices which will increase the total length of the machinery to a great extent.

Great efforts have been made to achieve ingot molds suitable to produce thinner slabs so as to use, along the production line of the strip, less rolling cages to reduce the thickness, thus with the advantage of reducing the total length of the line and to reduce the energy costs for producing the strip.

Currently, with strip manufacturing machinery of the known type, the minimum producible thickness value of a slab coming out of the ingot mold, when using very performing devices, is between 50 and 60 mm. As a result, lower thickness values in- volve a series of problems which are hardly possible to solve; the main problems are described as follows:

- there is insufficient space for the submerged unloading device, thus its small distance from the wide plates of the crystallizer produces solidification bridges; further- more, it is necessary to reduce the thickness of refractory material of the unloading device, hence negatively affecting its duration;

- there is too small a surface at the meniscus to effectively melt the lubrication powders, hence resulting in evident critical conditions, especially at high casting speed; - it is difficult to achieve a slab coming out of the ingot mold which still has a certain length of the liquid core and be able to carry out a liquid core pre-rolling - also known as" soft-reduction" - on the slab, which is necessary to improve its internal quality;

- the fluid mechanics inside the crystallizer is not optimal inasmuch as a reduced volume of molten steel, resulting from the reduced thickness, does not allow good tur- bulence damping which can cause an unstable meniscus, hence with unwanted incorporation of the lubrication powders, which could also cause remelting phenomena of the solidified skin.

Document US5460220 describes an ingot mold for the production of slabs of such cross section suitable particularly for a "soft reduction" treatment which helps improve the structural qualities of the strip. Such ingot mold, however, is suitable to produce slabs with a thickness that still requires considerably large machinery in the production line.

Thus, it is necessary to provide an ingot mold suitable to cast particularly thin slabs to overcome the aforesaid drawbacks. Summary of the invention

One main purpose of the present invention is to achieve an ingot mold for casting ultra thin slabs to be used, for instance, in a manufacturing plant for the manufacture of metal strips, in particular steel strips. Another purpose of the ingot mold, according to the invention, is to produce thin thicknesses, about 20mm, at high casting speeds, up to 16 m/min, thereby ensuring good quality inside the slab.

Therefore, the present invention intends to solve the foregoing problems by achieving an ingot mold for casting ultra thin (20 - 50 mm) steel slabs, with casting speed between 6 and 16m/min, thus defining a longitudinal plane of symmetry and a cross- sectional plane of symmetry, reciprocally orthogonal, comprising a passing cavity with an entrance section and an exit section of the steel, the cavity surface consisting of two first walls - arranged reciprocally facing two opposite sides of the longitudinal plane of symmetry and with respective concavities facing the longitudinal plane of symmetry - and two second narrow rectangular walls bcated parallel to the cross- sectional plane of symmetry, wherein the first and second walls form four longitudinal corners in the reciprocal connecting areas and where - in a section located on a cross-sectional plane perpendicular to the longitudinal and cross-sectional planes of symmetry - the respective concavities are in the shape of a semi-circular arc of a predetermined radius whereby the straight line tangent to the semi-circular arc intersecting the relative corner forms an angle ?, the value of which is between 1 °and 19° with a plane parallel to the longitudinal plane of symmetry. According to the invention, by using the ingot mold it is possible to cast a slab with a thickness much smaller than the thickness that can be achieved with known ingot molds, between 20÷50mm, yet helping maintain a certain length of liquid core downstream of the exit section of the ingot mold, and wherein the core remains liquid even in the zones nearest the furthermost side parts. Hence, when the slab is subjected to the liquid core pre-rolling, it will be possible to obtain a refinement of the internal structure by reducing central porosity as well as the segregation phenomenon.

Thus, the possibility to cast ultra thin slabs of the above-mentioned dimensions makes it possible to achieve metal strip machinery with a reduced height and length compared to the machinery known in the art. Moreover, it is necessary to point out an additional advantage: the strip obtained at the end of the passage throughout the whole hot-rolling and casting line is already at its final thickness and ready to be used, without a need to carry out an additional treatment in a cold-rolling line.

The dependent claims describe preferred embodiments of the invention. Brief description of the Figures Further features and advantages of the invention will become apparent by means of the detailed description of preferred embodiments, given by way of not limitative examples of an ingot mold, illustrated with the aid of the following figures wherein: Fig. 1 represents a three-dimensional view of an ingot mold of the invention; Fig. 2a represents a section along the plane y-z of the ingot mold of Fig. 1 ; Fig. 2b represents a plan view of the ingot mold of Fig. 1 ;

Fig. 3a represents a section along the plane y-z of another embodiment of the ingot mold of Fig. 1 ;

Fig. 3b represents a plan view of the embodiment of Fig. 3a; Fig. 4 represents a semi-section view along the plane x-y of the ingot mold of Fig. 1 . Detailed description of preferred embodiments of the invention With reference to the Figures, where an orthogonal reference axis system is defined, x, y, z, there is a representation of an ingot mold - with a longitudinal plane of symime- try X parallel to the axes of reference x-z, and a cross-sectional plane of symmetry Y parallel to the reference axes y-z - comprising two wide plates 1 , 2 which, when coupled, define a through cavity 3 or basin or casting chamber.

The surface of said cavity is defined by two wide walls 4, 5, reciprocally arranged and facing out on opposite sides relative to the plane of symmetry X, and by two narrow walls 6, 7 generally flat, rectangular and parallel, arranged at a right angle relative to the plane of symmetry X. The wide and narrow walls form four longitudinal corners in their connecting zones.

The wide walls 4, 5 are concave. The concavity faces the longitudinal plane of symmetry X. The concavity of the walls 4, 5 is such as to define a section of the lenticular- shaped basin.

Advantageously, by taking into consideration a section which is transversal and perpendicular to plane X, as described in Figure 4, the tangent to the semi-circular arc of the wide walls 4, 5 intersecting the related corner will form an angle ?, the value of which is between 1 °and 19°, with a plane parallel to the plane of symmetry X. The range of the angle ? ensures the presence of low stresses and limited deformations on the skin in the narrow walls zone and therefore the possibilities of tearing the skin are minimal, resulting in spillage of the molten steel, known as the "break-out" phenomenon. An elliptical section, rather than a lenticular section, would involve radii of curvature, hence greater angles.

The ingot mold, according to the invention - wherein "I" is the distance between the narrow walls and "f" is the camber of the concave walls 4, 5 of the basin - advantageously has the following values in the entrance section: - "I" is between 750 and 2000 mm; - "f" is between 15 and 100 mm.

Advantageously, the ingot mold or lenticular crystallizer of the invention prevents the edges from excessively pivoting during pre-rolling, thereby avoiding cracks or other flaws in these zones. Of particular relevance is the fact that the internal structure of such ingot mold ensures a suitable volume of molten steel, thus when the slab comes out, there will still be a certain length of the liquid core. The lenticular shape maintains the liquid core even when very close to the furthermost sides of the slab, thereby making a subsequent liquid core pre-rolling possible, the so-called "soft reduction", for the cast slab.

Furthermore, the volume of molten steel inside the ingot mold is enough to dampen the fluid mechanical turbulence generated by the unloading device. A first advantageous variation of the ingot mold, according to the present invention, includes an entrance section for the steel section larger than the exit section located near the bottom end of the ingot mold. In particular, the radius of curvature of the semi-circular arc on the cross-sectional plane has a linearly increasing value between the entrance section and a predetermined intermediate section, which corresponds to the terminal section of the casting chamber before the exit section of the steel. This will ensure a suitable distance of the submerged unloading device from the wide walls, thereby preventing unwanted solidification bridges.

In this case, such cavity or casting chamber 3 will longitudinally pass through generating lines converging towards the exit. As a result, in the section described in Fig. 2a, there is a decrease in width with a conical trend. Advantageously, the ingot mold of this invention includes an inclination of the walls of said casting chamber, converging towards said intermediate section, determined by an angle a (0° - 7°) wherein said angle a is measured according to the plane of symmetry z-y.

The bottom end of the ingot mold advantageously includes a portion 10 with a parallel-generating line, the length of which has been predetermined and shaped as the basin to allow the insertion and the removal of the dummy bar inside of it, thus used for starting the continuous casting process.

A second variation of the ingot mold according to the invention includes entrance and exit sections of equal dimensions, as described in the section in Fig. 3a. In this case, the radius of curvature of the semi-circular arc on the cross-sectional plane has the same value in each point of the longitudinal extension of the cavity between the entrance section and the exit section of the steel.

In an advantageous embodiment, the narrow walls 6, 7 can change their tapering shape. More precisely, they can be moved inward towards the exit section of the steel so as to reduce the width of the exit section, thus avoiding shrinkage problems during the solidification in the ingot mold.

The casting of very thin slabs, through the ingot mold of the invention, allows for the possibility to create a starting product, i.e. the slab thereof, with a thickness which is already very close to that of the finished product, namely the strip. Advantageously, the thickness of these thin slabs, when exiting, is between 20 and 50 mm and their casting speed is between 6 and 16 m/min.

The typical convex shape of the slab, in other words the central swelling at the exit of the ingot mold, is cancelled, that is flattened, through the use of rollers placed at the foot of the ingot mold and appropriately shaped, thus performing the liquid core pre- rolling.

This allows for a considerable reduction of the number of machines used in the system/process, which means remarkable saving possibilities in terms of first investment and energy.

Claims

1. Ingot mold for casting ultra thin (20 - 50 mm) steel slabs, with casting speed between 6 and 16 m/min, thus defining a longitudinal plane of symmetry (X) and a cross- sectional plane of symmetry (Y), reciprocally orthogonal, comprising a through cavity with an entrance section and an exit section of the steel, the cavity surface consisting of two first walls (4, 5) - arranged reciprocally facing two opposite sides of the longitudinal plane of symmetry (X) and with relative concavities facing the longitudinal plane of symmetry (X) - and two second narrow, flat, rectangular walls (6, 7) located parallel to the cross-sectional plane of symmetry (Y) , wherein the first and second walls form four longitudinal corners in the reciprocal connecting zones and where - in a section located on a cross-sectional plane perpendicular to the longitudinal (X) and cross-sectional (Y) planes of symmetry - the respective concavities are in the shape of a semi-circular arc of a predetermined radius whereby the straight line tangent to the semi-circular arc intersecting the relative corner forms an angle ?, the value of which is between 1 °and 19° with a plane parallel to the longitudinal plane of symmetry (X).

2. Ingot mold according to claim 1 wherein the radius of curvature of the semi-circular arc on the cross-sectional plane has the same value in each point of the longitudinal extension of the cavity between the entrance section and the exit section of the steel.

3. Ingot mold according to claim 1 wherein the radius of curvature of the semi-circular arc on the cross-sectional plane has a linearly increasing value between the entrance section and a predetermined intermediate section located before the exit section of the steel so as to form a truncated-cone-shaped surface.

4. Ingot mold according to claim 3, wherein a portion between said intermediate sec- tion and exit section of the steel has parallel generating lines.

5. Ingot mold according to one of the previous claims wherein "I" has a value between 750 and 2000 mm and "f" is between 15 and 100 mm at the entrance section, wherein "I" represents the distance between the narrow walls and

"f" represents the camber of the concave walls 4, 5 of the cavity on the viewing plane parallel to axes x, y.