Strip casting

Metal founding – Process – Shaping liquid metal against a forming surface

Reexamination Certificate

Rate now

  [ 0.00 ] – not rated yet Voters 0   Comments 0

Details

C164S452000, C164S428000

Reexamination Certificate

active

06536506

ABSTRACT:

This application claims priority to and the benefit of Australian Provisional Application Number PQ8180, which was filed in Australia on Jun. 15, 2000.
TECHNICAL FIELD
This invention relates to the casting of metal strip and making of cast steel strip. It has particular application to the casting of metal strip by continuous casting in a twin roll caster.
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 “nip” 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 smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to form 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.
The setting up and adjustment of the casting rolls in a twin roll caster is a significant problem. The casting rolls must be accurately set to properly define an appropriate separation of the casting rolls at the nip, generally of the order of a few millimeters or less, There must also be some means for allowing at least one of the rolls to move outwardly against a biasing force to accommodate fluctuations in strip thickness particularly during start up.
Usually, one of the rolls is mounted in fixed journals, and the other roll in rotatably mounted on supports that can move against the action of biasing means to enable the roll to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing means may be in the form of helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units.
A strip caster with spring biasing of the laterally moveable roll is disclosed in Australian Patent Application 85185/98 and corresponding U.S. application Ser. No. 09/154213. In that apparatus, the biasing springs act between the roll supports and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the roll. The positions of the roll supports need to be set and subsequently adjusted after commencement of casting so that the gap between the rolls is constant across the width of the nip in order to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects.
Eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls or due to distortion when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or subsidiary fluctuations within the generally sinusoidal pattern.
With improvements in the design of the casting rolls for a twin roll caster, particularly by the provision of textured surfaces which enable control of the heat flux at the interface between the casting rolls and the casting pool, it has been possible to achieve dramatic increases in strip casting speeds. However, when casting thin strip at high casting speeds there is an increased tendency to produce both high and low frequency gauge variations.
DISCLOSURE OF THE INVENTION
We have found that the gauge variations in cast strip can be alleviated by reducing the casting roll separation force and that the defect can be practically eliminated if the roll separation force in minimized. In practice there is at least a certain force that is required to balance the hydrostatic pool pressure and to overcome the mechanical friction involved in moving the rolls. We have also found that the high frequency gauge variation can be overcome, and a unique cast steel strip can be produced, by reducing the strip stiffness in the region of the nip by allowing a quantity of mushy or molten metal to be passed through the nip between the two solidified shells of the strip, by maintaining a roll gap at the nip slightly greater than the gap determined by the fully solidified shell thickness. It is desirable for these purposes that the mechanical friction forces involved in movement of the casting rolls relative to each other is minimized. By achieving very low strip stiffness, the dynamic interaction of the rolls on the strip is uncoupled, and consequently periodic gauge variation regeneration can be substantially reduced if not eliminated.
In at least one aspect, the present invention combines the features of applying a constant casting roll separation force (which can be small) and establishing a constant roll gap that will enable molten metal to be passed through the nip to further reduce strip stiffness. In order to maintain the constant separation force together with a constant roll gap, the invention may also allow for roll eccentricity compensation.
According to the invention there is provided a method of casting metal strip including 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, confining the pool at the ends of the nip by pool confining closures and rotating the rolls such that shells of metal solidify from the casting pool onto the casting rolls and are brought close together at the nip to produce a solidified strip delivered downwardly from the nip The casting rolls are biased bodily toward each other, in at least some embodiments under a substantially constant biasing force, and are maintained with a substantially constant gap between them at the nip. This gap is such as to maintain separation between the solidified shells at the nip so that molten metal passes in the space between them through the nip and is, at least in part, subsequently solidified between the solidified shells within the strip below the nip.
The molten metal may be molten steel and the method may produce solidified steel strip at a casting speed of at least 30 meters/minute. The casting speed may be at least 60 meters/minute. The separation space between the solidified shells at the nip may be in the range 0 to 50 microns. This separation provides for maintaining a substantially constant gap with a small biasing force
Said biasing force may be substantially equal to or slightly more than the minimum force required to balance the hydrostatic pressure of the casting pool and to overcome the mechanical friction involved in moving the biased roll. For 500 mm rolls 1350 mm wide and 175 mm pool, putting aside mechanical friction that should be kept small, the hydrostatic force of the molten casting pool will be about 0.75 kN. The biasing force, therefore, may be in range 0.75 to 2 kN per chuck (i.e., per side), and the corresponding roll separation force in the range of substantially 0 to 1.25 kN. Roll separation force is the net force exerted on the strip.
The roll biasing force may be in the range of 0.75 to 1.2 kN and the corresponding roll separation force substantially 0 to less than 0.45 kN. For strip thicknesses above 1 mm the roll separation force may be less th

LandOfFree

Say what you really think

Search LandOfFree.com for the USA inventors and patents. Rate them and share your experience with other people.

Rating

Strip casting does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Strip casting, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Strip casting will most certainly appreciate the feedback.

Rate now

     

Profile ID: LFUS-PAI-O-3046640

  Search
All data on this website is collected from public sources. Our data reflects the most accurate information available at the time of publication.