Cast iron indefinite chill roll produced by the addition of niob

Details Cast iron indefinite chill roll produced by the addition of niob Cast iron indefinite chill roll produced by the addition of niob

1997-12-05

2000-01-11

Yee, Deborah

Metal treatment

Stock

Ferrous

148545, C22C 3708, C22C 3700, C21D 514

Patent

active

060131411

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a process for producing a chill roll having surface properties that are highly desirable for use in the hot rolling of steel. More particularly, the invention relates to the discovery that the introduction of niobium into a chilled-iron roll casting composition produces surface hardness values not previously attainable without interfering with the balance between carbide formation and free graphite dispersion that is necessary in such casting compositions.
2. Background of the Invention
In the continuous hot rolling of steel strip, a continuously moving steel workpiece (the strip) is passed through a rolling mill which commonly consists of several stands of rolls arranged in a straight line (in tandem). The strip cools as it passes through the rolling mill, such that each succeeding stand is at a lower temperature than its predecessor stand. Typically, when the strip reaches the rolls of the last few mill stands there is a tendency of the strip to weld or fuse to the rolls through which it passes because of the lower temperature of the roll. The results of such welding can be a catastrophic demolition of the rolling mill stands and surrounding structures, not to mention the grave threat to workers in the area.
It is evident, therefore, that the selection of the proper grade of roll to be used in the latter stands of tandem style rolling mills is important. The problem of roll selection is complicated by the fact that mill conditions vary widely, but in general the finishing rolls on a tandem hot mill should have an outer skin which is dense and hard, and yet provide sufficiently low friction in the areas that contact the workpiece.
Since the early days of steelmaking, rolling mill rolls have been cast in a manner to ensure that the liquid iron on the outer surface of the roll is cooled to produce the desired structure and properties. One technique for attaining this rapid cooling is to insert metal rings or segments, called "chills", in the mold, close to the surface to be contacted by the molten iron. The production of the chill roll shells typically involves a two step process, in which an outer shell in formed that possesses the aforementioned qualities necessary for use in a rolling mill followed by the formation of an inner core composed of a material that provides additional strength to the chill roll, such as cast iron. The outer shell is formed by either a static or spin pour, as is well known in the industry, an example of which is U.S. Pat. No. 5,355,932 issued to Nawata et al.
Most early chill rolls were cast using ordinary low silicon iron alloyed with nickel and chromium and chilled at a very high rate to suppress the formation of graphite, which was thought to be detrimental to the roll due to the softness imparted to the alloy by the graphite. The chilled outer surface is very hard and, when fractured, has a white fracture face for a distance beneath the surface (known as the chill zone), signifying that the formation of free graphite in that area had been suppressed by the rapid cooling. The white iron zone sometimes is referred to as "white cast iron", as contrasted with iron containing graphite that has a grey fracture face, known as "grey iron".
In the 1930s, it was discovered that the introduction of finely dispersed graphite into the white iron zone substantially reduced roll breakage despite providing for a softer outer shell. The region of the finely dispersed graphite in the alloy is termed "mottled." The presence of graphite in the outer shell greatly improves the ability of the roll to withstand the thermal shocks associated with hot rolling steel strip, reduces the friction between the roll and the strip thereby lowering the applied stress on the strip, and greatly reduces the potential for fusing of the strip to the roll. As a result, white cast iron chill rolls were largely superseded by a roll characterized by finely dispersed graphite near the outer surface of the roll and the lack of a definite c

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English language translation of Japanese reference No. 62-136556 A.
English language translation of Japanese reference No. 57-149452 A.

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