Conventionally, square cross section billets from a furnace are forged into much smaller cross section round workpieces by passage through a series of mill roll stands.
Typically the workpiece (the original square billet) emerges from the first reduction stand with a rectangular section and emerges from the second roll stand with a square section of reduced section area. Following this, sequential pairs of roll stands shape and reduce the workpiece in section by alternately producing oval and circular cross sections of reduced sectional area. A typical mill may utilize a number of roll stands to obtain the desired reduction in cross sectional area.
The change in cross sectional shape which must occur during a typical rolling process as described above, requires substantial energy to produce the change in shape because the transformation from oval (or elliptical) cross section to circular cross section and vice-versa requires that the hot steel undergoing transformation must "flow" substantial distances to produce the change in shape.
To understand some of the dynamics of the metal flow in the prior art method of reduction, the change in shape from an oval to circular cross section is produced by rollers having deep semi-circular or semi ellipsoidal profiles which forces the metal in the workpiece to undergo massive plastic flow during rolling to form the resulting cross section.
Because of the shape of the cavities formed between the mating reducing rolls required to produce sequential ellipsoidal and circular cross sections there is substantial difference in the linear velocity at the largest diameter of roller profile when compared to the velocity of the surface of the roller profile closest to the roller central axis. This means that when the workpiece passes between such rollers, there is a considerably differential in relative velocity between the metal undergoing plastic deformation and elongation and the contacting surface of the rollers producing the change in profile of the workpiece. The differential in the relative motion between the workpiece surface (which undergoing a constant change during passage) is greatest at or near the periphery of the reducing rolls.
It will be understood that there is substantial energy required to produce the plastic flow of the metal undergoing the profile change, and because of the particular shape of the profile of the rollers, there is opportunity for substantial surface erosion of the surface of the roller at places where the relative motion between the workpiece surface and the contacting surface of the roller is greatest, i.e. at or near the periphery of the roller. The "wear" produced in the roll tends to produce an "undercut" in the roller profile just below the exterior surface of the roll such that the width of the semi circular or semi ellipsoidal profile in the roller increases at a point on the roller just below the maximum diameter of the roll. This phenomena tends to produce a lip on the ridge on the outermost portion of the roller profile which in time may become sufficiently proud so as to damage the workpiece as it exits from the roller bight.
It is therefore seen that the traditional classical method of producing the desired reduction in cross sectional area of a billet requires excessive energy which may be subsequently reduced by the judicious selection of the various section shapes formed during a reduction process. Simultaneously, the shape of the profile in the reducing rollers may be changed to a profile which does not produce the drastic plastic flow which leads to prior art surface erosion of the rollers which provide the cross section reduction used in the classical reduction process.