METHOD FOR CONTROLLING THICKNESS PROFILE OF STRIP

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

It is known to cast metal strip by continuous casting in a twin roll caster. 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 . The term ^λλ nip" is used herein to refer to the general region of which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller 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. This casting pool may be confined between end closure side plates or dams held in sliding engagement with the ends of the rolls .

In twin roll casting, it is difficult to maintain a constant strip thickness along the strip and a constant thickness profile across the strip. Conventionally thickness control is addressed by mounting one of the casting rolls for movement laterally toward and away from the other roll , setting a gap between the roll and applying controlled forces to bearings at each end of the moveable roll, for example by means of biasing springs or hydraulic actuators . However, variations can develop during a casting run due to changing solidification conditions at the casting pool, casting roll distortions and other factors which cannot be controlled solely by the application of forces on the rolls .

Japanese specification JP 60-27458A proposes a twin roll caster using chilled rolls that incorporate within them tapered pistons that are actυable to provide roll crown correction during casting, the thickness of the steel strip delivered from the nip of the caster being measured and the roll crown being corrected by movement of the tapered pistons in response to measured thickness values . However crown correction alone is not sufficient to maintain strip thickness within a target range both along the strip and across the strip since it does not address the problem of changes in thickness longitudinally of the strip which may develop progressively through a casting run.

DISCLOSURE OF THE INVENTION

According to the invention there is provided a method of casting metal strip comprising introducing molten metal between a pair of chilled casting rolls forming a nip between them to form a casting pool of molten metal supported on the rolls and confined at the ends of the nip by pool confining end closures , rotating the rolls so as to cast a solidified strip delivered downwardly from the nip, and transporting the strip away from the nip, characterised by the steps of inspecting the strip as it is transported away from the nip to measure the thickness of a central part of the strip between its edges and to further measure strip thickness at a plurality of positions across the strip between its edges , varying the rotational speed of the casting rolls in response to variations in the measured thickness of the central part of the strip along the strip whereby to vary the time for solidification of metal on the rolls in the formation of said strip such as to control those thickness variations along the strip, and

resiliently deforming at least one of the casting rolls in response to variations in the measured strip thickness at said plurality of positions across the strip to control those thickness variations across the strip.

Variations in the thickness of the central part of the strip may be controlled by varying the speed of the casting rolls so as to maintain the thickness of the central part of the strip within a target thickness range.

The speed of the casting rolls may be decreased when the thickness of the central part of the strip is tending to fall below the target range of strip thickness and may be increased when the thickness of the central part of the strip is tending to exceed the target range.

Variation in the measured strip thickness at said plurality of positions may be controlled by the casting roll deformation so as to maintain the strip thickness at each of those positions within the target thickness range.

The resiliently deformable roll may be so deformed when the strip thickness at any one of said locations across the strip is tending to depart from the target range.

The variations in thickness of both the central part of the strip and at said plurality of locations across the strip may be determined as variations in the longitudinal direction of the strip over a strip length generated by a whole number of revolutions of the casting rolls .

More specifically said strip length may be a length generated by a single revolution of the casting rolls .

More specifically a mark may be made on the strip upon each revolution of the casting rolls and successive

marks on the strip may be detected so as to determine said strip length.

The invention further provides apparatus for casting metal strip comprising:

a pair of parallel casting rolls forming a nip between them; a metal delivery system for delivering molten metal into the nip to form a casting pool of molten metal supported above the nip; a pair of pool confining end closures disposed one at each end of the pair of casting rolls ; roll drive means to rotate the rolls in opposite directions to deliver a cast strip downwardly from the nip; and strip transport means to transport the strip away from the nip; characterised by strip inspection means to inspect the strip as it is transported away from the nip to measure the thickness of a central part of the strip between its edges and to further measure strop thickness at a plurality of pistons across the strip between its edges ; a roll speed controller operable to vary the speed of the casting rolls in response to variations in measured thickness of the central part of the strip along the strip whereby to vary the time for solidification of the strip such as to control those thick variations along the strips; and roll deforming means operable to deform at least one of the casting rolls in response to variations in the measured strip thickness at said plurality of positions across the strip to control those thickness variations across the strip.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates diagraitimatically the general construction of a twin roll caster,

Figure 2 illustrates a twin roll caster installation provided with means for performing the present invention,

Figure 3 is a plan view of part of a twin roll caster suitable for use in the installation of Figure 2 ,

Figure 4 is a side elevation of caster components seen in Figure 3 ,

Figure 5 a cross-section or the line 5-5 in Figure 4, and

Figure 6 illustrates a caster with an alternative roll deforming construction for operation in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 shows a twin roll caster comprising a pair of horizontally disposed chilled casting rolls 1 and a pair of side plates or dams 2 which engage the ends of rolls 1. Casting rolls 1 are constructed such that cooling water can be circulated through the rolls and a nip 6 between the rolls can be adjusted depending on the thickness of a steel strip 3 to be produced.

Casting rolls 1 are contra-rotated at constant velocity such that the outer peripheries of the rolls move downwardly to the nip G at constant velocity. A melt- supplying nozzle 4 is disposed immediately above the nip G and between the side plates 2 to receive molten steel from a ladle (not shown) . Nozzle 4 supplies the molten steel

to a casting pool supported on the casting rolls and confined at the ends of the nip G by the side plates 2.

Molten steel in the casting pool 5 solidifies onto the supporting surfaces of the casting rolls 1 to form solidified shells which come together at the nip G to form a solidified steel strip 3 which is delivered downwardly from the nip .

In order to set the gap between the rolls at the nip G at least one of the casting rolls is mounted for transverse movement toward and away from the other casting roll such that its position can be adjusted. One of the casting rolls may be fixed and the other moveable or they may both be mounted for transverse adjustment. One of the casting rolls may be mounted in journal bearings to which biasing forces can be applied through biasing strings or other means to push the casting roll toward the other roll during casting as will be described in more detail later in this specification with reference to Figure 3.

In the operation of a twin roll caster such as shown in Figure 1 , the solidified shells on the outer peripheries of the casting rolls 1 can thicken due to gradual reduction in temperature of the molten steel supplied from the ladle to the nozzle 4, resulting in an increase in thickness of steel strip 3. On the other hand when an exhausted ladle is replaced with a new ladle filled with molten steel solidification of the shells may be delayed with resulting in a decrease in thickness of the steel strip. Moreover, the solidification temperature of the molten steel can vary depending on the carbon silicon and manganese contents so it has been difficult to continuously cast steel with the thickness being kept within a target range.

Figure 2 illustrates the manner in which the twin roll

— 1 —

strip caster of Figure 1 can be adapted and installed for operation in accordance with the present invention. In Figure 2 the metal delivery nozzle 4 and side plates 2 have been omitted for clarity. One of the casting rolls 1 is provided with marking means 8 for making a mark 7 on the steel strip 3 upon each rotation of the chilled rolls 1. The marking means 8 is a convexity or concavity on the outer periphery of one of the chilled rolls 1 adjacent to the roll edge . The convex or concave mark 7 is made on the steel strip 3 delivered from the nip G upon each rotation of the chilled rolls 1 when the marking means 8 is the concavity or convexity, respectively.

The steel strip 3 delivered from the nip G between the chilled rolls 1 is guided sideways by table rolls (not shown) via pinch rolls 11 into a horizontal rolling machine (not shown) .

In order to resiliently deform one of the chilled rolls 1, a pressing force F is adapted to be afforded horizontally to a roll neck of the roll by means of roll profile correction means such as a cylinder as will be described in detail with reference to Figures 3 to 5.

Incidental to the twin roll caster are noncontact type sensor 6 for measuring widthwise thickness distribution of the steel strip 3 , mark sensing means 9 closely adjacent to the sensor 6 and profile control means 10.

The noncontact type sensor 6, which is a combination of plurality of thickness sensors arranged widthwise of the steel strip 3, is positioned upstream of the punch rolls 11 in the direction of movement of the steel strip 3 and detects thicknesses at widthwise centre and plural portions between the widthwise centre and edge of the steel strip 3.

The mark sensing means 9 may be of a type sensing the mark 7 made on the steel strip 3 as variation of distance from reference position to a surface of the steel strip 3 or may be of a type sensing remarkably varied profile portion on the surface of the steel strip 3 through image processing.

The profile control means 10 has a first profile retaining function for prevention of excess/insufficiency in thickness of the steel strip 3 due to temperature in the molten metal pool 5 and a second profile retaining function for prevention of non-uniformity in thickness distribution of the steel strip 3 due to profile of the chilled roll 1.

The first profile retaining function is such that a present target range in thickness of the steel strip 3 is always compared with a widthwise central thickness value of the steel strip noncontact type sensor 6, a rotary motor (not shown) being activated to gradually increase or decrease the peripheral velocity of the chilled rolls 1 when the measured value is about to an upper or lower limit of the target range of strip thickness , respectively.

The peripheral velocity of the chilled rolls 1 , which is the same as the moving velocity of the steel strip 3, may be determined on the ground of the number of times of the marks 7 passing per unit time obtained from mark passing information 13 from the mark sensing means 9 as well as the outer diameter of the chilled rolls 1.

More specifically, provided for example that the target thickness value is 2 mm and upper and lower limits of the target range of thickness are 2.1 and 1.9 mm, respectively, and when the measured widthwise central

thickness value of the steel strip 3 exceeds the target thickness value and is approaching the upper limit of the target range of strip thickness, then the peripheral velocity of the chilled rolls 1 is gradually increased by the profile control means 10 to shorten time of the solidified shells in contact with the outer peripheries of the rolls, thereby suppressing increase in thickness of the overall steel strip 3 delivered from the nip G.

To the contrary, when the measured widthwise central thickness value of the steel strip 3 underruns the target thickness value and is approaching the lower limit of the target range of strip thickness, then the peripheral velocity of the chilled rolls 1 is gradually decreased by the profile control means 10 to prolong the time of the solidified shells in contact with the outer peripheries of the rolls, thereby suppressing decrease in thickness of the overall steel strip 3 delivered from the nip G.

Thus, the phenomenon that thickness of the steel strip 3 increases due to gradual lowering in temperature of the molten steel can be averted; and the phenomenon that thickness of the steel strip 3 decreases due to rising in temperature of the molten metal pool 5 upon replacement of the ladles can be also averted.

The second profile retaining function is such that the present target range in thickness of the steel strip 3 is always compared with the measured thickness values on the plural widthwise portions of the steel strip obtained by the thickness information 12 from the noncontact type sensor 6. When any of the measured values is approaching the upper or lower limit of the target range of strip thickness, the chilled roll or rolls 1 being resiliently deformed so as not to be away from the measured value on the portion from the target value of thickness, thereby making the nip G approaching to optimum.

When, for example, local plastic deformation due to heat develops on the chilled rolls 1 and the measured thickness values on the portion of the steel strip 3 vary periodically upon each rotation of the chilled rolls 1 , at least one of the chilled rolls 1 is resiliently deformed periodically to suppress increase/decrease of the nip G.

The relative relationship between rotational positions of the chilled rolls 1 and periodic thickness variations of the steel strip 3 obtained from the noncontact type sensor 6 may be obtained from the length of movement of the steel strip 3 from the roll nip up to the noncontact type sensor 6, a distance between the noncontact type sensor 6 and the mark sensing means 9 and a moving velocity of the strip 3.

Thus, the widthwise thickness distribution of the steel strip 3 delivered from the nip 6 can be equalised.

Figures 3 , 4 and 5 illustrate one particular design of components for a twin roll strip caster whereby bending forces can be applied to the casting rolls in performance of the method described above with reference to Figures 1 and 2. As shown in Figures 3 to 5, casting rolls 1 are formed as sleeves 21 mounted on stub shafts 22 which are rotatable in inboard and outboard journal bearings 23, 24 mounted within inboard and outboard casting roll chocks 25, 26. Roll chocks 25, 26 are carried by slides 27 moveable on guides 28. The slide 27 carrying the chocks 25, 26 and bearings 23, 24 for one of the casting rolls is moveable by hydraulic servo cylinders 29 and this roll can be set in a fixed position by those cylinders . The other casting roll is moveable laterally by movement of the respective slides 27 under the influence of the roll separation forces during casting and biasing springs 31 which apply biasing forces to the moving roll tending to

maintain a constant gap between the rolls at the nip.

Outer end chocks 32 are provided for roll positioning at one end of the stub shafts 22 and rotary couplings 33 are provided at the other ends of the stub shafts for flow of cooling water to and from the interior of the casting rolls .

Roll chocks 27 are fitted with brackets 34 which carry cylinder units 35 operable to produce roll bending. More specifically each inboard roll chock 27 is fitted with two of the hydraulic cylinder units 35 the thrust rods 36 of which bear against the respective inboard roll chock 26. Actuation of cylinder units 35 applies equal and opposite forces to the inboard and outboard roll 11 chocks so as to impart bending moments to each of the stub shafts 22 producing bending of the roll .

The action of the roll bending cylinder units 35 is to bend the rolls such that the outer ends of the stub shafts

22 tend to move toward one another producing the equivalent of a concave roll crown at the nip . The casting rolls may be ground before hand to have an initial convex crown so that the roll bending cylinder units 35 can be actuated to deform the rolls through a range of shapes varying from the initial convex crown to a concave crown .

Equal forces may be applied to the roll bending actuators 35 at each end of a casting roll to produce symmetrical bending of the roll to a gentle parabolic curve. This is appropriate for correction of thickness variations across the strip which are symmetrical about the centre of the strip. If those variations are not symmetrical and are greater to one side of the central part of the strip to the other, at least one of the rolls can be bent asymmetrically by applying greater forces through the

bending cylinder units 35 at one end of the roll than the forces applied by the actuators at the other end of the roll.

Figure 6 illustrates an alternative casting roll construction which enables the roll to be deformed for crown control . In this case the casting roll 1 comprises a copper sleeve 42 mounted on a central shaft 43 supported in journal bearings 44. Sleeve 42 has internal spirally formed water flow passages 45 to which cooling water is supplied through a rotary coupling 46 at one end of the roll shaft and water supply passages 47 and is returned by return passages 48 through the coupling 46.

The central part of the roll shaft 43 within the sleeve 42 is provided with a pair of tapered annular pistons 51 disposed within tapered annular piston cavities 52 formed within the two ends of the central part of the shaft. The tapered pistons 51 are actable by hydraulic fluid supplied through supply and return lines 53, 54 via the rotary coupling 46. The movement of the pistons in the tapered piston chambers produces distortion and bending of the central part of the roll so as to vary the crown. The supply of hydraulic fluid to the piston chambers 52 can be controlled by the profile control means 10 as previously described with reference to Figure 2.