Continuous casting of aluminum

Details Continuous casting of aluminum Continuous casting of aluminum

2002-02-19

2004-01-06

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S459000, C164S428000

Reexamination Certificate

active

06672368

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to continuous casting of aluminum alloys, more particularly, to continuous casting aluminum alloys between two cooled rolls at speeds of over 25 feet per minute
BACKGROUND OF THE INVENTION
Continuous casting of metals such as aluminum alloys is performed in twin roll casters, block casters and belt casters. Twin roll casting of aluminum alloys has enjoyed good success and commercial application despite the relatively low production rates achievable to date. The present invention is directed to a method of continuous casting aluminum which surpasses the productivity of twin roll casting and reaches a level comparable to or better than the productivity of belt casting.
Twin roll casting traditionally is a combined solidification and deformation technique involving feeding molten metal into the bite between a pair of counter-rotating cooled rolls wherein solidification is initiated when the molten metal contacts the rolls. Solidified metal forms as a “freeze front” of the molten metal within the roll bite and solid metal advances towards the nip, the point of minimum clearance between the rolls. The solid metal passes through the nip as a solid sheet. The solid sheet is deformed by the rolls (hot rolled) and exits the rolls.
Aluminum alloys have successfully been roll cast into ¼ inch thick sheet at about 4-6 feet per minute or about 50-70 pounds per hour per inch of cast width (lbs/hr/in). Attempts to increase the speed of roll casting typically fail due to centerline segregation. Although it is generally accepted that reduced gauge sheet (e.g. less than about ¼ inch thick) potentially could be produced more quickly than higher gauge sheet in a roll caster, the ability to roll cast aluminum at rates significantly above about 70 lbs/hr/in has been elusive.
Typical operation of a twin roll caster at thin gauges is described in U.S. Pat. No. 5,518,064 (incorporated herein by reference) and depicted in
FIGS. 1 and 2
. A molten metal holding chamber H is connected to a feed tip T which distributes molten metal M between water-cooled twin rolls R
1
and R
2
rotating in the direction of the arrows A
1
and A
2
, respectively. The rolls R
1
and R
2
have respective smooth surfaces U
1
and U
2
; any roughness thereon is an artifact of the roll grinding technique employed during their manufacture. The centerlines of the rolls R
1
and R
2
are in a vertical or generally vertical plane L (e.g. up to about 15° from vertical) such that the cast strip S forms in a generally horizontal path. Other versions of this method produce strip in a vertically upward direction. The width of the cast strip S is determined by the width of the tip T. The plane L passes through a region of minimum clearance between the rolls R
1
and R
2
referred to as the roll nip N. A solidification region exists between the solid cast strip S and the molten metal M and includes a mixed liquid-solid phase region X. A freeze front F is defined between the region X and the cast strip S as a line of complete solidification.
In conventional roll casting, the heat of the molten metal M is transferred to the rolls R
1
and R
2
such that the location of the freeze front F is maintained upstream of the nip N. In this manner, the molten metal M solidifies at a thickness greater than the dimension of the nip N. The solid cast strip S is deformed by the rolls R
1
and R
2
to achieve the final strip thickness. Hot rolling of the solidified strip between the rolls R
1
and R
2
according to conventional roll casting produces unique properties in the strip characteristic of roll cast aluminum alloy strip. In particular, a central zone through the thickness of the strip becomes enriched in eutectic forming elements (eutectic formers) in the alloy such as Fe, Si, Ni, Zn and the like and depleted in peritectic forming elements (Ti, Cr, V and Zr). This enrichment of eutectic formers (i.e. alloying elements other than Ti, Cr, V and Zr) in the central zone occurs because that portion of the strip S corresponds to a region of the freeze front F where solidification occurs last and is known as “centerline segregation”. Extensive centerline segregation in the as-cast strip is a factor that restricts the speed of conventional roll casters. The as-cast strip also shows signs of working by the rolls. Grains which form during solidification of the metal upstream of the nip become flattened by the rolls. Therefore, roll cast aluminum includes grains with multiaxial (non-equiaxed) structure.
The roll gap at the nip N may be reduced in order to produce thinner gauge strip S. However, as the roll gap is reduced, the roll separating force generated by the solid metal between the rolls R
1
and R
2
increases. The amount of roll separating force is affected by the location of the freeze front F in relation to the roll nip N. As the roll gap is reduced, the percentage reduction of the metal sheet is increased, and the roll separating force increases. At some point, the relative positions of the rolls R
1
and R
2
to achieve the desired roll gap cannot overcome the roll separating force, and the minimum gauge thickness has been reached for that position of the freeze front F.
The roll separating force may be reduced by increasing the speed of the rolls in order to move the freeze front F downstream towards the nip N. When the freeze front is moved downstream (towards the nip N), the roll gap may be reduced. This movement of the freeze front F decreases the ratio between the thickness of the strip at the initial point of solidification and the roll gap at the nip N, thus decreasing the roll separating force as proportionally less solidified metal is being compressed and hot rolled. In this manner, as the position of the freeze front F moves towards the nip N, a proportionally greater amount of metal is solidified and then hot rolled at thinner gauges. According to conventional practice, roll casting of thin gauge strip is accomplished by first roll casting a relatively high gauge strip, decreasing the gauge until a maximum roll separating force is reached, advancing the freeze front to lower the roll separating force (by increasing the roll speed) and further decreasing the gauge until the maximum roll separating force is again reached, and repeating the process of advancing the freeze front and decreasing the gauge in an iterative manner until the desired thin gauge is achieved. For example, a 10 millimeter strip S may be rolled and the thickness may be reduced until the roll separating force becomes excessive (e.g. at 6 millimeters) necessitating a roll speed increase.
This process of increasing the roll speed can only be practiced until the freeze front F reaches a predetermined downstream position. Conventional practice dictates that the freeze front F not progress forward into the roll nip N to ensure that solid strip is rolled at the nip N. It has been generally accepted that rolling of a solid strip at the nip N is needed to prevent failure of the cast metal strip S being hot rolled and to provide sufficient tensile strength in the exiting strip S to withstand the pulling force of a downstream winder, pinch rolls or the like. Consequently, the roll separating force of a conventionally operated twin roll caster in which a solid strip of aluminum alloy is hot rolled at the nip N is on the order of several tons per inch of width. Although some reduction in gauge is possible, operation at such high roll separating forces to ensure deformation of the strip at the nip N makes further reduction of the strip gauge very difficult. The speed of a roll caster is restricted by the need to maintain the freeze front F upstream of the nip N and prevent centerline segregation. Hence, the roll casting speed for aluminum alloys has been relatively low.
Some reduction in roll separating force to obtain acceptable microstructure in alloys having high alloying element content is described in U.S. Pat. No. 6,193,818. Alloys having 0.5 to 13 wt. % Si are roll cast into strip about 0.05 to 0.2 inch

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