HOT STEEL ROLLING
When rolling is carried out at elevated temperatures (hence "hot rolling"), the coarse-grained, brittle, and porous structure of the cast feed is broken down into a wrought structure, with finer grain size. This new structure of the rolled solid has enhanced ductility resulting from the breaking up of brittle grain boundaries and closing up internal defects, such as porosity. Temperature ranges for hot rolling are similar to those for other forming processes, for example for carbon steels temperatures generally lie between 800 oC to 1200 oC. Steel is heated in order to increase its plasticity and decrease its yield strength.
The microstructural transfromations that take place within the rolled solid present an additional and very significant benefit. Hot rolling process subjects the solid steel to a high degree of plastic deformation, thus improving its internal structure. The major microstructural changes occur immediatelly after the steel has left the rolls; phase transformations are mainly affected by the initial microstructure, chemical composition, the degree of strain and the cooling rate.
A further advantage, compared to forging or other processes based on plasticity, is its high efficiency of energy utilization (over 90 %).
Hot rolling of steel is relatively new technique, probably due to the fact that steel was not available in large quantities until the end of seventeenth century. By the end of seventeenth century, hot mills using grooveless (“flat”) rolls were being employed to reduce the thickness of cuboids into sheets. At the present time, hot rolling mills can produce up to one mm thickness, but the usual minimum thicknesses are limited at 1.5 mm especially when rolling velocity reaches 15 m/s and over. Steel wire can be hot rolled with diameters as small as 5 mm at velocities as high as 100 m/s. Perhaps the major drawback of hot rolling is the surface oxidation. Loss due to oxidation increases with the surface area and temperature of the rolled steel.
At the hearth of the rolling operations is the process during which the plastic steel passes between at least two rotating rolls (right circular cylinders). In a most accustomed configuration the axes of rolls are parallel and lie in the same plane, so called “exit plane”. Clockwise rotation of one roll and the simultaneous counter-clockwise rotation of the other roll are maintained by motor drive via set of spindles. Processed solid is drawn into the deformation zone by the friction forces developing along the contact interface between the rolled steel and the rotating rolls.
An idealised sketch of a rolling pass is depicted in Fig. 1.
Fig 1: Top and side views at the deformation zone
The entry solid (the “feed”) usually fits very close to the geometry of a cuboid (a curved “fillet” is introduced at junctions of the side surfaces which would otherwise intersect at an angle). A cuboid was anticipated to be the most rational output geometry for the previous manufacturing stage – casting –because this is the most practical form for storing, manipulating and transport.
More generally, the rolled solid has the geometry of a cylinder. In the case of a direct use of the cast feed along a continuous production line, when the steel temperatures fall down to the level of 1200 – 1300 oC, the conti-cast feed enters the first set of rolling mill rolls as shown in Fig 2. In such a case, the feed cross-section need not to have a rectangular shape.
Fig 2: On-line rolling
However, the most often encountered hot rolling practice involves a separate stage between the continous casting and rolling operations. During that stage, the cast feed cuboids are cooled down to the ambient temperatures and the whole heat remaning from the casting stage is lost. This allows for storing the feed and having separate time shedules for casting and rolling operations.
Before the rolling can commence, steel has to be re-heated in special furnaces to temperatures of 1200 – 1300 oC. Process that follows can be generally described as a sequence of rolling passes whereby the vertical cross-section area of the rolled cylinder is gradually decreased. To enable this series of passes, rolls are assembled within so called housing. Housing is a major frame within a more complex assembly called rolling mill stand. Rolling mill stand is equipped with a mechanism which allows for changing the height "h" (see Fig 1 above) between the passes.
There are many additional auxiliary components that together enable performing the process at an industrial scale within a rolling mill plant. Figure 3 shows an example of such a plant.
Fig 3: A layout (top view) of a rolling mill plant
In order to discuss the basic pronciples if rolling, we shall analyse relations during the single passing of a cuboid through a gap between the grooveless rolls. Single rolling pass will occur when the horisontal resultant of friction force overcomes the horisontal resultant of the steel deformation force. These components appear already at the instant when the front end of the cuboid touches upon the roll surface. Figures 4 and 5 show decomposition of relevant vectors and sliding velocities.
Fig 4: Force vectors at
the instant of bite
Fig 5: Velocity distribution in the deformation zone; the curved arrows indicate the direction of the resultant friction force that arises due to the difference between the surface (peripheral) velocity of the roll and the plastic flow velocity at the surface of the rolled steel
N = deformation force
T = friction force
The height reduction (draft) and the lateral spread are defined as follows (refer to Fig 1 above):
The maximum draft is delimited by the following factors:
- the ability of rolls to pull the rolled bar into the deformation zone (roll bite),
- the maximum force that can be resisted by rolls without the roll fracture, and
- the maximum reduction that can be sustained by rolled material without apperance of cracks in the rolled steel.
By introducing appropriate trigonometric substitutions and by taking in account numeric simplifications the bite conditions can be expressed as follows:
By reviewing Figures 1 and 4, it can be derived:
From trigonometry it holds
By substituting tan(a) = f and by combining the above equations it can be written
Roll radius R is much larger than the draft Δh. This allows us to take in account the numerical insignificance of the third member on the left side of the above equation, and write
In hot rolling, the value of the coeficient of friction f varies normally between 0.2 and 0.7. Higher values of f (within the above range) usually occur at the stages of production line where the larger roll diameters are employed, which allows for assuming the numerical insignificance of the fourth member on the left side of the above equation. Hence the following simplification can be used:
The above equation defines the maximum possible draft in terms of the coefficient of friction and roll radius. However the next question is whether the rolls can withstand the forces which will arise due to such reduction in the cuboid height.
In summary, the limitations on draft are
(a) the angle of bite,
(b) rolling loads (roll parting forces and torques) and
(c) metallurgical considerations.
Looking at each of these factors:
(a) Angle of bite is a function of roll diameter(s) at the point of first contact, the smaller the roll diameter the greater is the angle of bite; and the amount of draft taking place. When there is no assistance on entry other than the kinetic energy of the approach table, the maximum values of angle of bite can reach only about 22o. If there is some mechanical assistance, e.g. pinch rolls helping to drive the bar into the mill, then it is possible to have a bite angle which is greater, up to 25o. Angle of bite is also affected by the coefficient of friction of the roll material and surface condition. If the coefficient of friction is modified artificially by use of ragging or knurling on the working surface, then the angle of bite can be greater.
(b) Rolling loads depend on the deformation resistance of the rolled steel (which is strongly affected by the steel temperature), the strain and the strain rate. These loads are directly related to area of contact, and the greater the draft, the greater the area of contact, and consequently the greater the load. On flat rolls and wide slabs, small increases in draft (angle of bite) can give large increases in parting force. These forces must be kept within the mechanical limits of the mill rolls, housing, drives, etc.
(c) Metallurgical considerations are related to the plasticity of the rolled steel. Experience from many rolling mills suggests that the first pass on a bar, and especially so with continuous cast feed, should not exceed 10% reduction in height. In old mill the first pass had a partial function of descalling. An additional issue is a danger on this first pass of corner cracking because of high temperature and possible "hot-shortness" combined with localised spreading.
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