Comparative Study of Blooming Mills

A comparative study of six different types of Blooming Mill, operated at different parts of world shown in Table 4.2, below:

Table 4.2

Contd...

Roll Pass Design for Blooming Mill

Pattern of Reduction

The ingot as cast is tapered from one end to other for ease of stripping from ingot mould. The cast structure may be very coarse with marked trails crystalline zone and it may be porous due to blow holes.

Fig. 4.12 Shows the placement of roller table vis- a-vis working roll of Blooming Mill.

The function of early passes is to eliminate the taper of ingot and help to break-down the cast structure into more workable one and densities the material by closing up of internal blow holes. In majority of cases, during early passings, when cross-section of work piece is more and temperature of metal is also high, a considerable high degree of deformation is recommended, which is only be limited by angle of bite and strength of rolls. But in case of high carbon or special steel, high deformation at early stage may lead to surface defects due to the presence of sub-cutaneous blow holes, near to the surface and will get exposed due to high reduction. Once it appears, these defects can only be removed by dressing the bloom by scarfing or chipping. Dressing of bloom will reduce the yield and increase the cost of production.

With initial draughting, cast structure become stronger, as it gets refined by working, though the resistance to deformation which may not necessarily increases at this stage. Higher draught is always advisable at intermediate passings; there also, it is limited by strength characteristics of roll and capacity of mam drive motor.

Iii final passings, the draught has to be reduced extensively, since resistance to deformation increase with the drops of metal temperature.

Arrangement of Grooves in Blooming Mills Rolls

Usually three to five grooves are made in blooming mills rolls depending upon the character of manufacture of finished products and local conditions of prevailing. Generally, following two types of arrangements are used.

5.2.1 Series or Russian System

The bullhead pass in the form of shallow box pass is located not at the center of rolls, but at the end of barrel as shown in Fig. 4.13 (a). Passes are arranged in the order of rolling. This design leads to reduction of rolling time and corresponding increase of the output of Blooming mill, as stock move along the barrel of rolls.

Other advantages are:

• • hi shallow bullhead pass, groove is cut in roll upto certain depth. Such depth of pass even in insignificant amount, makes the easier rolling for operator. It reduces the height as the top roll is required to be raised or lowered. This may help in increasing the output of blooming mill by at least by 5-10%.
• • The shallow pass in place of barrel pass prevents the requirement of stepped entry roller before Blooming mill.

Fig. 4.13 Pass Arrangements on Rolls of Blooming Mill.

5.2.2 Symmetrical or American System

The first bullhead pass, having maximum effective diameter is at the center of roll body and other passes are arranged on the both side of bullhead pass. This system provides symmetry in distribution of passes and rolling load around center of roll. Hence, this system is called as symmetrical or American system [Fig. 4.13(Z?)] above. The main characteristics of this system are as follows:

• • Bullhead pass is used for breaking down pass for the ingot or slabs in initial stage.
• • The roll load during rolling in bullhead pass has equally distributed on roll necks; this reduces the non-uniform wear of roll bearings.
• • Rolling in bullhead pass can be performed at higher draught.
• • Since, bullhead pass does not have any groove cut, it helps in avoiding weakening of roll, as stress concentration at center of roll is highest but any cut at center will make it further weak.
• • Scales, which are embedded in ingot, got shedded in initial passings being bullhead pass is at center. This design ensures less possibility of scales getting into the roll neck.
• • Absence of collars in bullhead pass, allows length of roll barrel to be used more effectively.

Major disadvantage of this system is that after earlier passing in bullhead pass, the stock has to be shifted from left side of bullhead pass to right of it. This increases idle time of rolling, in turn total rolling time gets expanded. In addition, absence of collars in bullhead pass affects on the stability of stock, especially after tilting.