WO2002030594A1

WO2002030594A1 - Casting steel strip

Info

Publication number: WO2002030594A1
Application number: PCT/AU2001/001269
Authority: WO
Inventors: Hisahiko Fukase; Atsushi Hirata; Akihiro Nomura; Hiroyuki Otsuka; Shiro Osada
Original assignee: Ishikawajima-Harima Heavy Industries Company Limited; Bhp Steel (Jla) Pty Ltd; Castrip, Llc
Priority date: 2000-10-10
Filing date: 2001-10-10
Publication date: 2002-04-18
Also published as: AU2001293517A1; JP2002113556A; TW562713B

Abstract

A casting roll for use in twin roll casting of steel strip. The roll has a casting surface (1a) textured by discrete protrusions (9) tapering upwardly and inwardly to peaks of uniform height (H) disposed in a regular triangular, quadrilateral or polygonal array in which each peak is spaced from an adjacent peak by a distance (S) in the range 70 µm to 500 µm. Adjacent protrusions (9) are separated by generally flat regions of the casting surface through a distance (P) of not less than one half of the base width of the protrusions. The protrusions taper toward the peaks at an included angle of not more than 120° and preferably not more than 90°. The peaks may be rounded or alternatively may be truncated so as to have flat tops.

Description

CASTING STEEL STRIP

TECHNICAL FIELD

This invention relates to the continuous casting of steel strip.

PRIOR ART

It is known to cast metal strip by continuous casting in a twin roll caster. In this technique molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. There term ^wnip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.

Figure 7 shows an example of the twin roll type of continuous casting device that is commonly employed, in which a pair of water-cooled casting rolls 1 is disposed horizontally and parallel to each other, and in which side plates 3 are disposed at the two ends of the nip 2 between the sad casting rolls 1, such that an enclosed casting mold is formed from the said casting rolls 1 and the said side plates 2, and in which molten metal 6 is supplied from a tundish 4 that is disposed above such casting mold through a pouring nozzle 5 that is formed of refractories into the nip 2 between the casting rolls 1 forming a pool of molten steel 7. The molten steel 6 of the pool of molten steel 7 is cooled and solidified by the roll surfaces la of the casting rolls while the said casting rolls 1 revolve, such solidified shells 8 being continuously drawn out downwards from the nip 2 between the casting rolls 1 such as to form a thin steel strip 20 delivered downwardly from nip 2. In such twin roll type continuous casting device, each of the said casting rolls 1 under-goes machining by means of a lathe or the like in order to render the surface of the casting roll smooth. When the molten metal 6 adheres to such smooth surfaces of the casting rolls 1 during the casting process, the initial rate of heat propagation between the molten metal 6 (solidified shell 8) and the casting roll 1 is high. The extraction of the heat from the molten metal 6 rises rapidly and the thermal strain gradient in the molten metal 6 between the surface that is contact with the roll surface la of the casting roll and the interior of the molten metal is very large, producing a combination of tensile and compressive stresses in the solidifying shells 8 which leads to cracking in the cast strip. In recent years, it has been proposed that protrusions and depressions be applied by means of shot blasting and the like to the roll surfaces la of the casting rolls 1, with the molten metal 6 being supported by surface tension on the protrusions only on the roll surfaces la, so that the rapid extraction of heat during the solidification of the molten metal 6 is moderated, and it has already been confirmed that this type of constitution is effective in preventing the cracking of the cast slab (see, for example, JP 2-179343).

PROBLEMS ADDRESSED BY THE PRESENT INVENTION

When protrusions and depressions are simply applied by means of shot blasting and the like to the roll surfaces la of the casting rolls 1, it is difficult to prevent the development of the phenomenon of thermal fatigue cracking in a short period of time in the roll surfaces la of the casting rolls 1, and in fact no solution has yet been found to the problem of the greatly increased costs caused by such thermal fatigue cracking.

United States Patent No. 4250950 reveals regular arrays of protrusions and depressions in the roll surfaces la of the casting rolls 1, but these are intended to vent gases, and the mere provision of such regular array of protrusions and depressions alone does not address the problem of the thermal fatigue cracking of the casting rolls 1. It has hitherto been assumed that thermal propagation between the molten metal 6 and the casting rolls 1 is constant, but the inventors of the present invention have developed a mold temperature measurement method which enables more accurate measurement of heat flux during the casting process than previously possible and this has shown that very high peak fluxes are generated in the initial phase of metal solidification. The new method of measuring the mold temperature involves welding a copper constantan wire directly to the interior of the casting mould near the surface of the casting mould, rather than the method of the prior art in which a sheathed thermocouple which had great resistance to heat propagation was clamped to the casting mold. The results of such heat flux measurements are illustrated in Figure 8. As can be seen from the unbroken curve in Figure 8, during the initial contact between the molten metal 6 and the casting rolls 1, a high thermal load (heat flux) that reaches not less than six times the mean value of 20 MW/m2 is imposed for a very short period of time, and it is considered that such high thermal load applied over a very short period of time during the initial contact between the molten metal 6 and the roll surfaces la is the cause of thermal fatigue cracking.

In the case represented by the unbroken curve in Figure 8 (surface of the prior art), when the protrusions and depressions are formed in the roll surfaces la of the casting rolls 1 by means of shot blasting or the like, the heights of the protrusions are approximately 30μm and the sloping sides of the protrusions form narrow valleys, such valleys possessing no width. The case represented by the broken line curve relates to the example of the mode of the present invention which will be described below.

Moreover, as a result of extensive research by the inventors of the present invention, it has been determined that, when the protrusions and depressions are formed in the roll surfaces la of the casting rolls 1 by means of shot blasting or the like, the protrusions are formed with a high density, and that there is a tendency to produce protrusions with a high degree of irregularity and with poor repeatability. Hence, when the molten metal 6 is supported on the crowns of these protrusions, there is considerable contact between the molten metal and regions of the casting surface between the protrusions. Even where contract is not made there are many very thin gaps across which extensive thermal propagation can occur. These gaps may contain trapped pockets of air or where the casting pool is shrouded with a non-oxidising gas with pockets of the shrouding gas such as nitrogen or argon. Due to the extensive metal contact and thermal propagation across the thin gaps, a very high thermal flux is generated when the molten metal 6 first comes into contact with the protrusions on the roll surfaces la.

Accordingly, it is believed in regard to the development of thermal fatigue cracking in the roll surfaces la of the casting rolls 1, that the high level of irregularity in the protrusions causes high localized thermal loads which have a negative effect.

It is believed that, after the initial contact between the molten metal 6 and the casting rolls 1, solidification commences and the surfaces become distended, the adhesion of the molten metal 6 to the crown parts of the protrusions is lost, and hence within a short period of time the thermal load falls rapidly. The present invention takes account of such problems, and it is an objective of the present invention to moderate the thermal load at the initial contact between the molten metal and the casting rolls, and to prevent the development of the thermal fatigue cracking that arises within a short period of time in the roll surfaces of the casting rolls, and at the same time, to lead to uniform lower rates of heat extraction without adversely affecting productivity.

DISCLOSURE OF THE INVENTION

The present invention provides a casting roll for use in twin roll casting of steel strip, wherein the casting surface of the roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

The protrusions may taper inwardly toward the peaks at an included angle of not more than 120°.

Said included angle may be less than 90°. The tapered protrusions may be truncated such that the peaks have flat tops.

Alternatively, the peaks of the protrusions may be generally hemispherically rounded.

The protrusions may be of pyramidal form with flat sloping side faces. The invention further provides apparatus for continuously casting steel strip, comprising a pair of casting rolls forming a nip between them, a molten steel delivery nozzle for delivery of molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on casting roll surfaces immediately above the nip, and roll drive means to drive the casting rolls in counter-rotational directions to produce a solidified strip delivered downwardly from the nip, wherein the casting surface of each roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

The invention further extends to a method of continuously casting steel strip, comprising supporting a casting pool of molten steel on chilled casting surfaces of a pair of parallel casting rolls above a nip formed between the casting rolls and rotating the rolls to produce a solidified steel strip delivered downwardly from the nip wherein the casting surface of each roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

The casting roll of the present invention may be used in twin roll type continuous casting of ferrous materials including carbon steel and stainless steel. If the distance between the crowns of adjacent protrusions of the casting roll surface is made not less than 70μm, and if flat portions of not less than one half of the base width of the protrusions is provided between each of such protrusions, and provided that the crown part of each protrusion that forms the contact part when contact occurs between the molten metal and the casting rolls and the air gap portion that is formed in the periphery thereof are reduced, the thermal propagation in the initial period of contact between the molten metal and the casting rolls is greatly reduced, and the maximum thermal load that is imposed on the roll surfaces of the casting rolls is greatly moderated. If the distance between the crowns of the adjacent protrusions exceeds 500μm, it becomes difficult for surface tension to support the molten metal on the protrusions and the molten metal readily invades the valleys between the protrusions, and hence an upper limit 500μm on the distance between the protrusions is required. Moreover, if the distance between the crowns of the adjacent protrusions is not more than 70μm, very small irregularities of the order of several μm in the height of the protrusions have an effect on whether or not contact is made with the molten metal, and it has been confirmed that this gives rise to the phenomenon of skipping of points of contact.

Moreover, in order to reduce the small gaps that are formed at the peripheries of the crowns of the protrusions, it is desirable in the present invention that the protrusions be so formed that the sloping sides that form the protrusions be formed with angles of not more than 120°. Moreover, it is also desirable that continuous hemispherical surfaces or flat surfaces be formed in the gradually sloping sloping sides of the crowns of the protrusions.

By the method of the present invention it is possible to perform continuous casting while effectively controlling the thermal propagation during the period of initial contact between the molten metal and the casting rolls, thus making it possible to prevent the phenomenon of thermal fatigue cracking developing within a short period of time in the roll surfaces of the casting rolls and making it possible to extend the service lives of the casting rolls, and also having the desirable effect of preventing the development of surface cracking in the cast slab.

BRIEF DESCRIPTION OF THE DRAWINGS

The mode of implementation of the present invention will be described with reference to the drawings, in which:

Figures 1 to 6 illustrate an example of one form of casting roll surface as a mode of implementation of the present invention;

Figure 7 illustrates the construction and operation of a twin roll caster; and

Figure 8 gives the results of heat flux measurements during casting on casting roll surfaces of the present invention contrasted with measurements during casting onto prior art casting roll surfaces. Figures 1 and 2 illustrate one form of casting roll surface in accordance with the invention. In this mode of implementation of the present invention, a plurality of protrusions 9 of uniform height and on which are formed sloping sides is formed by machining on the casting rolls 1 that are employed by the twin roll type continuous casting device illustrated in Figure 7. The protrusions 9 are so arrayed as to possess a distance S of between 70μm and 500μm between the crowns of adjacent protrusions 9. Moreover, flat portions of distance P of not less than one half of the width L of the sloping sides of the protrusions 9 (ie. the base width of the protrusions) are formed between the said protrusions 9.

In this case, if for example, the width L of the sloping sides that are formed on the protrusions 9 (ie. the base width of the protrusions) is approximately 60μm, a distance S of not less than approximately 90μm will be required in order to form flat portions of distance P of not less than one half of the width L between the crowns of the adjacent protrusions 9. In practice, a distance S selected from the range of approximately from lOOμm to 400μm is preferable, and in the present mode of implementation a distance S of approximately 300μm is employed.

Moreover, in the example illustrated in Figure 2, the crowns of the protrusions 9 on the roll surfaces la are disposed in a regular array in a pattern such as to form a regular grid of squares similar to the cells of a checkerboard, but apart from such squares, the crowns may be arrayed in repeated similar patterns of quadrilateral shapes (rectangles, diamonds, parallelograms, and trapezoids) , and the crowns may be arranged in repeated similar patterns of equilateral triangles or other triangular shapes (rightangle triangles, equilateral triangles, and triangular shapes possessing unequal sides) . Moreover, the crowns may be arranged in regular arrays of repeated patterns that form polygonal shapes through combinations of such triangular and rectangular shapes .

Furthermore, pyramid shapes that form quadrilateral cone shaped sloping sides are shown in the drawings, but the protrusions 9 may also be formed of polygonal cone shapes possessing four or more sides or may possess sloping side of rounded cone shapes, and the crowns of the protrusions 9 may be connected to the sloping sides by gently sloping hemispherically shaped surfaces 9a as shown in Figure 1, or alternatively, by flat surfaces 9b as shown in Figure 3.

Moreover, flat portions that are not in contact with the molten metal 6 are formed between the protrusions 9 as shown in Figure 4, but such flat portions may also be formed or irregular surfaces (not specifically shown) that are sufficiently lower than the protrusions 9, and the height H of the protrusions 9 in relation to the flat portions may be within the range of from 5μ to lOOμm. The protrusions 9 may be formed in such a manner that the included angle θ of the sloping sides that are formed on the protrusions 9 is not more than 120°, and preferably not more than 90°, as shown in Figure 5. The angle θ of the sloping sides that are formed on the protrusions 9 as specified in the present invention indicates the angle formed by extending upwards the gradient line that follows the inclined surface of the sloping sides that form the principal gradient over the interval from the crown portions of the protrusions 9 to the sloping sides, and the point at which such extended gradient line intersects above, and such angle θ applies regardless of whether hemispherically shaped surfaces 9a or flat surfaces 9b are employed with the crowns of the protrusions 9.

Moreover, the width L of the sloping sides indicates the lateral dimension of the sloping sides that are formed on the protrusions 9 with the base point for measurement being the point at which a similar gradient line extended to the base of a valley between the protrusions 9 intersects a line along the surface of the said valley.

When continuous casting is performed by means of the twin roll type continuous casting device employing such casting rolls 1, the molten metal 6 is supported by surface tension on the crown parts of the protrusions 9 and does not come into contact with the flat portions and irregular parts between the protrusions 9, with spaces or gaps 10 being formed in such intervals, as illustrated in Figure 4. The distances S between the crown parts of the adjacent protrusions 9 is not less than 70μm and the widths of the flat portions between each protrusion and the adjacent protrusions 9 are not less than one half of the width L of the sloping sides that are formed on the protrusions 9 in the present mode of implementation, and because the crown parts of the protrusions 9 support the molten metal 6 and the area over which the metal comes into close proximity to the casting surface between the crown parts of the protrusions 9 is minimized, the heat propagation that occurs at the initial contact between the molten metal 6 and the casting rolls 1 is greatly reduced, and the thermal load that is imposed on the roll surfaces la of the casting rolls 1 is greatly moderated.

In Figure 8, the measurements represented by the dashed line relate to the present invention, in which the distance S between the adjacent crowns of the protrusions 9 is set at 30μm, with pyramidally shaped protrusions of 30μm in height and 120μm along each side and with a distance of 180μm between the sloping sides of each such pyramid, and thus the width of the flat portions of the valley bases is sufficiently great. It will be evident from this drawing that the high thermal load during the initial period of contact under the method of the prior art is greatly reduced to approximately one half.

The graph in Figure 6 shows the results of theoretical predictions made by the inventors of the present invention, and shows that, if the angles of the pyramid were made more acute, the thermal load during the initial contact would be more greatly reduced than in the example shown in Figure 8. In the graph, the horizontal axis represents the distance S between the crown parts of the protrusions 9, and the vertical axis represents the heat flux (amount of heat that traverses the surface per unit area per unit time) at the time of the initial contact between the molten metal 6 and the casting rolls 1; in this case, the experiments were conducted on protrusions 9 with a pyramidal shape, the length L of whose long sides was 60μm and whose height H was 30μm and whose crowns formed flat surfaces 9a.

Thus, from the results represented by the curve in Figure 6, when the distance S between the protrusions 9 was approximately 60μm and hence the protrusions were too close to each other, the high density of the protrusions 9 per unit area increased the total surface area of the contact between the molten metal 6 and the crown parts of the protrusions 9, and moreover, because the sloping sides of the adjacent protrusions 9 overlapped each other and virtually no air gaps were formed at the sloping sides of the protrusions 9, the heat flux exhibited a very high value at the initial contact between the molten metal 6 and the casting rolls 1, whereas, by increasing the distance S by only lOμm to approximately 70μm, a great reduction in the heat flux was achieved. More particularly, when the distance S was increased 300μm, the density of the protrusions 9 per unit area was very greatly reduced and a sufficiently large air gap 10 was formed in the periphery of each of the protrusions 10 and the heat flux fell to an average value of 14 MW/cm² at the initial contact between the molten metal 6 and the casting rolls 1 (heat was extracted at a virtually constant heat flux) .

An increase of not less than 300μm in the distance S only yielded a slow reduction in the heat flux, and more particularly, when the distance S was increased to in excess of 500μm, it became difficult for surface tension to support the molten metal on the protrusions 9 and the molten metal 6 readily invaded the valleys between the protrusions 9, and hence the upper limit for the distance S between the crown parts of the adjacent protrusions 9 is set at 500μm.

Moreover, in the foregoing description of the mode of implementation of the present invention, hemispherical surfaces 9a (see Figure 1, Figure 4 and Figure 5) and flat surfaces 9b (see Figure 3) are formed at the crown parts of the protrusions 9 because it is considered that this provides good support for the molten metal 6 at the crowns of the protrusions 9, and if the crowns of the protrusions 9 are too sharply tapered, the tapered ends of the protrusions 9 would pierce and project through the molten metal 6 and fail to provide good support, and hence an appropriate shape to avoid this is required .

On the other hand, the angle θ of the slopxng sides that are formed on the protrusions 9 is set at not more than 120° in order to restrict the spread of the sloping side of the protrusions 9 into the periphery and in order to readily permit the formation of sufficiently large gaps between the protrusions 9, while to the extent that the angle θ of the sloping sides of the protrusion is made smaller, the significance of the formation of the hemispherical surfaces 9a (see Figure 1, Figure 4 and Figure 5) and flat surfaces 9b (see Figure 3) is increased, and if such hemispherical surfaces 9a and flat surfaces 9b are formed, the angle θ is preferably not more than 90°. Consequently, in the mode of implementation described in the foregoing, the heat flux at the initial contact between the molten metal 6 and the casting rolls 1 is considerably reduced and the thermal load that is applied to the roll surfaces la of the casting rolls 1 is greatly moderated such that it is possible to avoid the development of the phenomenon of the thermal fatigue cracking of the roll surfaces la of the casting rolls 1 within a short period of time, to increase the service lives of the casting rolls 1 and to greatly reduce the roll cost, as well as to reduce the frequency of replacement of the casting rolls 1 and thereby to greatly increase the utilization rate of the continuous casting machine .

The generality of the casting rolls and the method of continuous casting employing such casting rolls envisaged by the present invention is not restricted to the mode of implementation thereof described in the foregoing, and various modifications may be made thereto, provided that they do not violate the spirit of the invention. EFFECTS OF THE INVENTION

The method of continuous casting employing such casting rolls envisaged by the present invention greatly inhibits thermal propagation at the initial contact between the molten metal and the casting rolls and is able to greatly moderate the thermal load that is imposed on the roll surfaces of the casting rolls, thereby being able to prevent the development of the phenomenon of the thermal fatigue cracking of the roll surfaces of the casting rolls within a short period of time, to increase the service lives of the casting rolls and to greatly reduce the roll cost, as well as to reduce the frequency of replacement of the casting rolls and thereby to greatly increase the utilization rate of the continuous casting machine. Moreover, and because the present invention is able to stabilize and moderate the great thermal strain in the surface of the cast slab, the present invention is also able to prevent the cracking of the cast steel strip.

Claims

CLAIMS :

1. A casting roll for use in twin roll casting of steel strip, wherein the casting surface of the roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

2. A casting roll as claimed in claim 1, wherein the protrusions taper inwardly toward the peaks at an included angle of not more than 120°.

3. A casting roll as claimed in claim 2, wherein said included angle is not more than 90°.

4. A castxng roll as claimed in any one of claims 1 to 3, wherein the tapered protrusions are truncated such that the peaks have flat tops.

5. A casting roll as claimed in any one of claims 1 to 3, wherein the peaks of the protrusions are generally hemispherically rounded.

6. A casting roll as claimed in any one of claims 1 to 5, wherein the protrusions are of pyramidal form with flat sloping side faces.

7. Apparatus for continuously casting steel strip, comprising a pair of casting rolls forming a nip between them, a molten steel delivery nozzle for delivery of molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on casting roll surfaces immediately above the nip, and roll drive means to drive the casting rolls in counter-rotational directions to produce a solidified strip delivered downwardly from the nip, wherein the casting surface of each roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

8. Apparatus as claimed in claim 7, wherein the protrusions taper inwardly toward the peaks at an included angle of not more than 120°.

9. Apparatus as claimed in claim 8, wherein said included angle is not more than 90°.

10. Apparatus as claimed in any one of claims 7 to 9, wherein the tapered protrusions are truncated such that the peaks have flat tops.

11. A casting roll as claimed in any one of claims 7 to 9, wherein the peaks of the protrusions are generally hemispherically rounded.

12. Apparatus as claimed in any one of claims 7 to 11, wherein the protrusions are of protrusions are of pyramidal form with flat sloping side faces.

13. A method of continuously casting steel strip, comprising supporting a casting pool of molten steel on chilled casting surfaces of a pair of parallel casting rolls above a nip formed between the casting rolls and rotating the rolls to produce a solidified steel strip delivered downwardly from the nip, wherein the casting surface of each roll is textured by discrete protrusions tapering upwardly and inwardly to peaks of uniform height disposed in a regular triangular, quadrilateral or polygonal grid array in which each peak is spaced from each adjacent peak in the array by a distance in the range 70μm to 500μm, adjacent protrusions being separated by generally flat regions of the casting surface through a distance of not less than one half of the base width of the protrusions.

14. A method as claimed in claim 13, wherein the protrusions taper inwardly toward the peaks at an included angle of not more than 120°.

15. A method as claimed in claim 14, wherein said included angle is not more than 90°.

16. A method as claimed in any one of claims 13 to 15, wherein the tapered protrusions are truncated such that the peaks have flat tops.

17. A method as claimed in any one of claims 13 to 15, wherein the peaks of the protrusions are generally hemispherically rounded.

18. A method as claimed in any one of claims 13 to 17, wherein the protrusions are of pyramidal form with flat sloping side faces.