TPT July 2007

108

uly

ugust

2007

From the point of view of the material deformation, the differences

between the 2-roll RMM and the 3-roll FQM™ are significant.

The smaller difference of the peripheral speeds between the

groove bottom and the flange of the rolls leads to a more uniform

deformation in the pass of an FQM. It is more uniform both in the

steady phase and in the transient phases (biting and release).

This benefit results in:

• Less redundant transversal material flow

• Lower specific average and, particularly, peak pressures on

the mandrel and on the rolls

• Higher degree of stability of the mandrel in the pass

The effect of this technological improvement results in a range of

benefits.

3.1 Higher product quality

• Pipes with thinner wall thickness (higher diameter-to-wall

thickness ratio)

• Closer wall thickness tolerances

• Better diameter tolerances

• Higher pipe surface smoothness

3.2 Larger product range

• Steel grades – with critical hot workability – can be rolled

• Increased flexibility

3.3 Lower Production cost

• Higher yield

• Reduced tooling wearing

• Lower tooling inventory cost

• Higher efficiency in production and in automatic

process control

To expand upon these advantages of FQM™, it is worth explaining

the major process features influencing each improvement.

Tubes with higher D/t (diameter to wall thickness) ratios can be

rolled. The substantially higher uniform deformation, with lower

average and peak pressures, allow the rolling of tubes with thinner

wall thicknesses in relationship to the diameter.

Wall thickness tolerances are also considerably improved. The

process tolerances are improved because the 3-roll arrangement

ensures a considerably more uniform deformation, due to the lower

differences in peripheral roll velocity and the closer roll pass design

that can be used. The 3-roll arrangement also provides better

stability and centring of the mandrel in the pass.

The 3-roll pass also possesses a more favourable geometry. This

leads to a substantial reduction of wall deviations that result from the

operation of closing and opening the roll pass on the same mandrel

to produce several wall thicknesses. Tube end wall thickness can

also be notably reduced by AGC in order to compensate for the

end thickening, which takes place when stretch is applied in the

subsequent stretch-reducing mill.

The technology also achieves better OD accuracy

of inline finished tube. Higher temperature

uniformity across the section and along the mother

tube permits closer tolerances on sizing of the tube

outside diameter.

Achieving a smoother tube surface is also possible,

as the 3-roll pass rolling reduces slippage on the

tube surface. This eliminates any trace of a mark

on the outside surface. The inner surface quality,

already excellent in the RMM process, is also

improved.

These benefits mean that more sophisticated steel

grades can be successfully rolled. This advantage

is possible because of the higher compressive

circumferential stress conditions of the 3-roll pass.

Process flexibility is increased, meaning that small

lots of tubes can be processed in a wide range of

›

Figure 3

The 3-roll FQM mill (above) provides a more uniform deformation in

the pass than was previously experienced with the 2-roll RMM process

Figure 4 (below)

FQM – hot rolling is performed on a retained mandrel through a sequence on rolls

driven stands

Figure 5 (bottom)

: The FQM is an adjustable 3-roll stand longitudinal rolling mill, which uses the

retained mandrel technology typical of the 2-roll RMM, in combination with 3-roll pass design. The

working cycle is similar to the RMM process cycle