GB2316639A

GB2316639A - Cooling continuously cast metal strip

Info

Publication number: GB2316639A
Application number: GB9717180A
Authority: GB
Inventors: Harold Roland Kaul
Original assignee: BHP STEEL; IHI Corp
Current assignee: BHP STEEL; IHI Corp
Priority date: 1996-08-27
Filing date: 1997-08-13
Publication date: 1998-03-04
Also published as: JPH1071455A; AUPO188696A0; DE19737117A1; FR2752759A1; GB9717180D0; KR19980019028A

Abstract

Outer peripheral surfaces 11 of casting rolls 1 of a twin roll caster for casting metal strip are chilled by flow of cooling water through longitudinal flow passages 22. Water is supplied to passages 22 of each roll through radial passages 24 in an end wall 16 of that roll and is taken away from those passages through further radial passages 24 in an opposite end wall 16 of that roll. The flow of water to one of the rolls 1 is directed to the radial passages 24 at one end of the twin roll assembly and is taken from the radial passages 24 at the other end of the twin roll assembly whereas the flow of water to the other roll 1 is directed to the radial passages 24 at said other end of the assembly and is taken from said one end of the assembly. The cooling water may flow in all longitudinal passages 22 of one roll in one longitudinal direction and in all of the longitudinal passages of the other roll in the reverse longitudinal direction.

Description

1 2316639

TWIN ROLL CASTING BACKGROUND OF THE INVENTION

This invention relates to twin roll casting of metal strip. It has particular, but not exclusive application to the casting of ferrous metal strip.

in a twin roll caster molten metal is introduced between a pair of contrarotated 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. The term Onipn is used herein the 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 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 it= diately above the nip. This casting pool may be confined between side plates or dam held in sliding engagement with the ends of the rolls.

The casting surfaces of the casting rolls are generally provided by outer circumferential walls provided with longitudinal cooling water passages to and from which water is delivered through generally radial passages in the end walls of the rolls. When casting ferrous metals the rolls must support molten metal at very high temperatures of the order of 16400C and their peripheral surfaces must be maintained at a closely uniform temperature throughout in order to achieve uniform solidification of the metal and to avoid localised overheating of the roll surface.

it has been found that efficient cooling of the outer end corners of the rolls presents a particular problem. it is known to provide circumferential notches at the outer corners of the rolls to receive the side dam plates with a sliding fit. This arrangement enables efficient cooling of the roll ends since the casting pool terminates at a position disposed inwardly from the ends of the circumferential wall of the roll and the cooling water passages pass directly across this region of the wall, but this arrangement can only be used with relatively thick circumferential walls. For more efficient cooling it is desirable to have a thinner circumferential wall and to locate the cooling passages closer to the peripheral surface of that wall. This usually precludes notching of the roll ends and the side dam plates must then engage the outer ends of the circumferential wall so that the casting pool extends through to the roll ends. The water flow passages in the circumferential wall of the roll do not traverse this region and localised heating of the outer corners of the roll becomes a severe problem.

The problem of localised heating of the outer corners of the casting rolls is addressed by the invention disclosed in Australian Patent Application No 33021195 whereby cooling water flowing to and from the longitudinal passages is directed through transition passages into the corner regions of the rolls to provide more effective cooling in these regions. However, it has not been possible to achieve absolutely uniform cooling from one side of the roll to the other and in particular to obtain uniform cooling at both of the end corners of a single roll. This problem is manifested by the production of strip of asymmetrical cross-section ie strip having an asymmetrical thickness variation across the width of the strip.

The most desirable strip cross-section may vary according to the particular use intended for the strip. For example, if the strip is to be subsequently cold rolled it should desirably be produced with a small positive crown in the centre, ie it should be slightly thicker in the centre than at its edges. However, if the strip is to be subsequently used in its as-cast state it way be produced with uniform thickness across its width. The casting rolls must be machined to an initial profile such that when they expand on heating to their operating temperature they adopt a profile which produces a strip of the required shape. In all cases it is desirable that the shape of the strip be symmetrical. However, this has proved very difficult and we have found in particular that the shape of the strip will commonly depart from the desired design shape toward the outer edges of the strip and in particular one edge margin is often significantly thicker than the other. After extensive trials and modelling we have determined that these fluctuations are due to variations in cooling efficiency from one side of the rolls to the other. The present invention addresses this problem by arranging for the flow of water to one of the rolls to be directed oppositely to the flow of water to the other roll. It has been found that this produces a marked improvement in the symmetry of the strip profile. SUMMARY OF THE INVENTION

According to the invention there is provided a method of continuously casting metal strip comprising introducing molten metal into the nip between a pair of parallel chilled casting rolls to form a casting pool of the molten metal supported on casting surfaces of the rolls above the nip, and rotating the rolls to produce a solidified metal strip delivered downwardly from the nip, wherein the outer peripheral surfaces of the casting rolls are chilled by flow of cooling water through water passages extending within the rolls adjacent those surfaces and longitudinally of the rolls, the flow of water for each roll is directed to the longitudinal passages in that roll through radial passages in an end wall of that roll and flows from the longitudinal passages through further radial passages in an appropriate end wall of that roll, the flow of water to one of the rolls is directed to the radial passages in the end wall of that roll at one end of the roll assembly defining the nip and is taken away from the radial passages in the end wall at the other end of the roll assembly, and the flow of water to the other of the rolls is directed to the radial passages in the end wall of that other roll at said other end of the roll assembly and is taken away from the radial passages in the end wall at said one end of the roll assembly, the flows of water to both rolls being at essentially the same temperature.

Preferably, the cooling water for both rolls is delivered from a common water supply.

The invention also provides apparatus for continuously casting metal strip comprising an assembly of a pair of casting rolls forming a nip between them and each provided with water flow passages extending adjacent the outer peripheral surfaces of the rolls longitudinally of the rolls, a metal delivery nozzle for delivery of molten metal into the nip between the casting rolls to form a casting pool of molten metal supported on the cast roll surfaces above the nip, roll drive means to drive the casting rolls in counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip, and cooling water supply means for supply of cooling water to send longitudinal passages in the rolls, wherein each casting roll comprises a central shaft mounting the roll for rotation about a central axis, a circumferential wall disposed about the central axis and provided with said longitudinal water flow passages, and walls extending between the shaft and the ends of the circumferential wall and the cooling water supply means comprises radial passages formed in the end walls and communicating with the respective longitudinal water flow passages of the rolls, water supply ducts to supply cooling water at essentially the same temperature to the radial passages in the end wall of one roll at one end of the roll assembly and to the radial passages in the end wall of the other roll at an opposite end of the roll assembly, and water return ducts to take water away from the radial passages in the end wall of said one roll at said opposite end of the assembly and to take water away from the radial passages in the end wall of said other roll at said one end of the roll assembly. Preferably, the water supply ducts and the water return ducts are formed in the shafts of the rolls. 20 Preferably further, the cooling water supply means further comprises a common source of cooling water for both rolls connected to the water supply ducts to supply cooling water to both rolls at essentially the same temperature. Preferably, said common source of cooling water comprises a cooling water pump connected to the water supply ducts for both rolls. BRIEF DESCRIPTION OF THE DRAWINGS in order that the invention may be more fully explained one particular embodiment may be described in some detail with reference to the accompanying drawings in which:

Figure 1 is a vertical cross-section through a strip caster constructed in accordance with the invention; Figure 2 is a horizontal cross-section through the casting rolls of the caster illustrated in Figure 1; Figure 3 is an enlarged cross-section through one of the rolls; Figure 4 illustrates variation in strip profile of the kind avoided by practise of the present invention; and Figure 5 illustrates the manner in which a water supply is connected to cooling water passages in the casting rolls in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The strip caster illustrated in Figure 1 comprises a pair of twin casting rolls 1 forming a nip 2 between them. Molten metal is supplied during a casting operation from a ladle 3 via a tundish 4 and a delivery nozzle 5 into the nip between rolls 1 so as to produce a casting pool 6 of molten metal above the nip. Ladle 3 is fitted with a stopper rod 7 actuable to allow the molten metal to flow from the ladle through an outlet nozzle 8 and a refractory shroud 9 into tundish 4.

Casting rolls 1 are provided in a manner to be described in detail below with internal water cooling passages supplied with cooling water through the roller ends and they are contra-rotated by drive means (not shown) to produce a continuous strip product 21 which is delivered downwardly from the nip between the casting rolls.

As thus far described the illustrated apparatus is as more fully described in granted united States Patents 5,184,668 and 5,277,243 and Australian Patents 631728 and 637548. Reference may be made to these patents for full constructional and operational details of the apparatus.

The two rolls are of identical construction. More specifically each is constructed in the manner which is fully described in Australian Patent Application 33021195 so an to be provided around its periphery with longitudinal water flow passages to and from which water is supplied through radial passages in end walls of the roll.

As most clearly seen in Figures 2 and 3 each roll 1 is mounted on a central shaft 12 for rotation about a central roll axis 13. The roll further comprises a circumferential wall in the form of a cylindrical copper sleeve 14 which defines the outer peripheral surface 11 of the roll. Sleeve 14 fits over a generally cylindrical mandrel 15 and the ends of the roll are closed by roll end walls 16.

Each end wall 16 is formed by a main annular member 17 and a ring member 18 which is fastened to member 17 by circumferentially spaced fastening screws 19. These composite end walls are fastened to the ends of the mandrel 15 by longitudinally extending axially spaced end clamping studs 21.

The outer periphery of mandrel 15 is formed with longitudinally extending circumferentially spaced channels which are closed by the roll sleeve 14 to form water flow passages 22 spaced circumferentially around the inner periphery of the sleeve. The edges of the some of the water flow channels of the mandrel are extended outwardly to fit into grooves in the inner periphery of sleeve 14 so as to key the sleeve to the mandrel.

Water is passed to and from the roll cooling water flow passages 22 via flow passages 23 formed in central shaft 12, radial passages 24 formed in the end wall components 17 and transition passages 25 which interconnect the radial end wall passages 24 and the longitudinal passages 22. More particularly, the water flows into the roll through one end of the shaft, then outwardly through the passages in the end wall at that end of the shaft to the passages 22. It then flows in one direction along the passages 22 to the other end of the roll where it flows inwardly through the radial passages in the respective end wall to exit through the passage in the other end of the 5 shaft.

Transition passages 25 are formed by the shaping of the end wall components 17, 18 and the interior peripheral surface of sleeve 14. The outer peripheral surface of end wall ring 18 is formed with a series of circumferentially spaced longitudinal channels 30 which register with the channels of flow passages 22 and which therefore serve as continuations of those passages. Some of the rims of the channels 30 may be raised to engage with the slots in the interior surface of sleeve 14 so as to key the end walls to the sleeve. A pair of annular grooves 27 are formed in the inner periphery of sleeve 14 one adjacent each end of the sleeve so as to register with the outer rims 28 of the end wall rings 18. Each transition passage 25 is thus defined by an annular gap between the end wall ring 18 and the end wall annular component 17 and one of the grooves 27 which forms a flow passage around the rim of the end wall ring 18 extending into the divided flow passages 30 and thence to the main water flow passages 22. In this way the transition flow passages extend in a curved bend outwardly toward the outer most corners 31 of the roll and this, together with the constriction of the transition passage in this region to increase the water velocity, enables dramatically improved cooling of the ends of the roll sleeve.

It is important that the transition passages extend outwardly into the sleeve beyond the radially outward extremities of the longitudinal passages 22 whereby to direct the cooling water closer to the end corners 31 of the roll than to the remainder of the outer peripheral surface of the roll. It will also be seen that those parts of the transition passages defined by grooves 27 are spaced longitudinally outwardly beyond the radial passages 24 in the end walls so as to brought into close proximity with the roll corners 31. With this arrangement the transition passages extend in smoothly curved bends into regions of the roll disposed both radially and longitudinally outwardly from the intersecting directions of the longitudinal and radial passages 22 and 24.

The outer parts of the transition passages adjacent the roll end corners 31 are shaped as smooth-walled ducts such as to maintain continuous flow of water throughout those ducts without stagnation. Specifically, the outer curved walls defined by the annular grooves 27 in sleeve 14 and the rims 28 of the end wall rings define ducts in the form of smoothly curved bends which constrain the flow so as to avoid formation of stagnation pockets. It has been found that the inner walls provided by rims 28 of the end wall rings 18 serve an important function in constricting the flow to avoid stagnation and cavitation which can dramatically reduce heat transfer and lead to poor cooling of the roll end corners.

The interfaces between the end walls and the sleeve may be sealed by O-ring seals 34 and the interface between the end walls and the shaft can be sealed by appropriate 0ring seals 35.

Prior to the present invention apparatus as illustrated in Figures 1 to 3 was operated such that the cooling water flowing through the cooling passages 22 flowed in the same longitudinal direction in both rolls. However, with such operation it was found that it was impossible to machine the rolls to an initial profile such that they would deliver a strip product having a desired design profile. In particular, there were unwanted variations in thickness from one side of the strip to the other with particularly significant variations in the edge regions of the strip. Figure 4 illustrates typical variations experienced with the apparatus operated in this marmer.

Figure 4 provides a plot of strip thickness at locations of varying distance from one edge of the strip.

The full line plots the desired design profile of the strip and the dotted line illustrates a typical profile actually achieved on operation of the apparatus. After extensive testing it was determined that the significant increase in thickness of one side of the strip was due to bulging caused by re-melting of liquid at the exit from the nip.

Attempts to overcome this bulging problem by machining negative crowns in the rolls were not successful because the asymmetrical nature of the profile variation could not be accommodated.

After extensive trials and modelling it was determined that the asymmetrical variation was due to differences in heat transfer characteristics at the two ends of the rolls which were dependent on the direction of water flow. The effect is particularly severe at the corner regions of the roll because of the complex tortuous shape of the transition passages 25 and the difference in the directions in which the water enters these passages when entering from the radial passages 24 and exiting from the longitudinal passages 22. Water entering the roll through the respective radial passages 24 flows radially outwards to the outer regions of the transition passages to provide very effective heat transfer whereas at the other end of the roll the water leaving the cooling passages 22 flows along the sides of the transition passages rather than upwardly against them. The differences in water flow direction and the differing boundary layer conditions are sufficient to produce variations in heat transfer effectiveness to result in differential expansion of the end regions of the roll and the resulting asymmetry in the strip profile. However, by reversing the flow of water through one of the rolls compared with the other it was found that the asymmetry of the profile could be virtually eliminated and it became possible to machine the rolls very accurately to produce a strip having a desired syimetrical thickness profile. The reason for this is that variations from the design profile due to variations in cooling efficiency when the roll is operating at its working temperature are produced in mirror image in the other roll so that the gap between the rolls establishing the thickness at any one location is generally true to the design thickness.

Figure 5 illustrates the manner in which cooling water is supplied to the rolls in accordance with the present invention. This figure illustrates a pump 41 which delivers water through supply lines 42 to the water passages in the shafts of rolls 1 at opposite ends of the roll assembly (indicated as 23A in the figure) so that water is delivered to one end of one roll and to the other end of the second roll. Water flows from the passages in the other ends of the shafts (indicated as 23B in the figure)through return lines 43 to a cooling tower 44 and back to the pump through a return line 45. Thus, the passages 23A serve as water supply ducts to supply cooling water to the two rolls and the passages 23B serve as return ducts to take water away from rolls, with the flow of water through the two rolls mutually reversed. Since both of the supply ducts 23A receive cooling water from the common supply pump 41, cooling water is delivered to both rolls at essentially the same temperature which is also important to maintaining a desired symmetrical thickness profile.

The illustrated casting rolls may typically be of the order of 500 rim diameter and have an outer sleeve thickness of the order of 20-35 mm. The longitudinal flow passages may typically be of the order of 4 =m deep x 20 mm wide and the transition passages may be formed so as to provide generally the same flow cross-section as the longitudinal passages. The flow area provided by the radial passages 24 may typically be of the order of 35 mm diameter to provide a water flow rate of 72 llsec.

The illustrated strip caster has been advanced by way of example only and the invention is not limited either to the general caster layout as illustrated or to the particular roll construction seen in Figures 2 and 3. in particular, it is not essential that the rolls have a sleeved construction and the longitudinally extending circumferentially spaced water flow channels 22 could be drilled into solid circumferential walls of the rolls. Moreover, it is not essential that the cooling water pass through all of the longitudinal passages of each roll in a single direction and it would be feasible to provide a multipass grouping of these passages to provide for back and forth flow along both rolls to further even out the effects of temperature fluctuations during casting.

Claims

CLAIMS:

1. A method of continuously casting metal strip comprising introducing molten metal into the nip between a pair of parallel chilled casting rolls to form a casting pool of the molten metal supported on casting surfaces of the rolls above the nip, and rotating the rolls to produce a solidified metal strip delivered downwardly from the nip, wherein the outer peripheral surfaces of the casting rolls are chilled by flow of cooling water through water passages extending within the rolls adjacent those surfaces and longitudinally of the rolls, the flow of water for each roll is directed to the longitudinal passages in that roll through radial passages in an end wall of that roll and flows from the longitudinal passages through further radial passages in an opposite end wall of that.roll, the flow of water to one of the rolls is directed to the radial passages in the end wall of that roll at one end of the roll assembly defining the nip and is taken away from the radial passages in the end wall at the other end of the roll assembly, and the flow of water to the other of the rolls is directed to the radial passages in the end wall of that other roll at said other end of the roll assembly and is taken away from the radial passages in the end wall at said one end of the roll assembly, the flows of water to both rolls being at essentially the same temperature.

2. A method as claimed in claim 1, wherein the cooling water for both rolls is delivered from a common water supply.

3. A method as claimed in claim 2, wherein the cooling water is pumped by a supply pump to opposite ends of the roll assembly.

4. A method as claimed in claim 3, wherein the water taken from the rolls is passed through a cooling tower.

5. A method as claimed in any one of the preceding claims, wherein the cooling water flows in all of the longitudinal passages of one roll in one longitudinal direction and in all of the longitudinal passages of the 5 other roll in the reverse longitudinal direction.

6. Apparatus for continuously casting metal strip comprising an assembly of a pair of casting rolls forming a nip between them and each provided with water flow passages extending adjacent the outer peripheral surfaces of the rolls longitudinally of the rolls, a metal delivery nozzle for delivery of molten metal into the nip between the casting rolls to form a casting pool of molten metal supported on the cast roll surfaces above the nip, roll drive means to drive the casting rolls in counter- rotational directions to produce a solidified strip of metal delivered downwardly from the nip, and cooling water supply means for supply of cooling water to said longitudinal passages in the rolls, wherein each casting roll comprises a central shaft mounting the roll for rotation about a central axis, a circumferential wall disposed about the central axis and provided with said longitudinal water flow passages, and walls extending between the shaft and the ends of the circumferential wall and the cooling water supply means comprises radial passages formed in the end walls and communicating with the respective longitudinal water flow passages of the rolls, water supply ducts to supply cooling water at essentially the same temperature to the radial passages in the end wall of one roll at one end of the roll assembly and to the radial passages in the end wall of the other roll at an opposite end of the roll assembly, and water return ducts to take water away from the radial passages in the end wall of said one roll at said opposite end of the assembly and - is - to take water away from the radial passages in the end wall of said other roll at said one end of the roll assembly.

7. Apparatus as claimed in claim 6, wherein the water supply ducts and the,water return ducts are formed in the shafts of the rolls.

8. Apparatus as claimed in claim 6 or claim 7, wherein the water supply means further comprises a common source of cooling water for both rolls connected to the water supply ducts to supply cooling water to both rolls at essentially the sam temperature.

9. Apparatus as claimed in claim 8, wherein said common source of cooling water comprises a cooling water pump connected to the water supply ducts for both rolls.

10. Apparatus as claimed in claim 9, wherein the water supply means further comprises a water cooling tower to receive water returned through said return ducts for recirculation via said pump.

Ii. Apparatus as claimed in any one of claims 6 to 10, wherein the radial passages and longitudinal passages of each roll are interconnected by transition passages which extend radially beyond the outward extremities of the longitudinal passages and longitudinally outwardly beyond the radial passages so as to extend into the end corner regions of the roll.

12. Apparatus as claimed in claim 11, wherein the outer parts of the transition passages are shaped as smoothly curved bends to constrain the continuous flow of water therethrough such as to avoid formation of stagnation pockets.

13. Apparatus as claimed in claim 11 or claim 12, wherein the radial passages of each roll comprise a series of circumferentially spaced individual passages in each end wall of the roll.

14. Apparatus as claimed in claim 13, wherein the transition passages of each roll comprise a single annular passage at each end of the roll interconnecting the series of radial passages in the end wall at that end of the roll 5 with the longitudinal passages in the circumferential wall

15. Apparatus as claimed in any one of claims 6 to 14, wherein the longitudinal and radial passages of the rolls are interconnected to provide for flow of cooling water in a single pass through all of the longitudinal passages of each roll with the direction of longitudinal flow in the two rolls mutually reversed.