Special Features of Beam Design
In design of rolling beam, the first consideration is to displace the metal from the web as rapidly as possible; of course enough flange metal is to be made available to permit proper proportioning of web and flange draught at later passes.
In early passes, designer should take advantage of the higher plasticity of metal, because of higher temperature of the stock. The metal should be displaced quickly from the center for the formation of the web and flanges. It is stated that the cutting-in action of the first shaping pass of a beam, the center is held back by less reduced outer pass and the outer parts are dragged along by center. An attempt to roll in a too wide groove will cause too great an elongation; with the result that height of the flange of groove will get reduced drastically. Another factor affecting the height is the clearance between the bloom and side of the pass, if the difference between the incoming stock and pass is too high, then the pressure of knifing action will force the metal to the side of the pass and metal which could be moved for upward movement for flange formation, has gone in for spread. The clearance between the pass and bloom is also equally important in developing flanges of equal height. First question arises in the design of the first shaping pass is “How deeply a groove can be cut in the roll or how much reduction can be given”? In actual practice, this reduction is usually limited by the ability of roll to bite.
Ragging is used to improve the ability of roll to bite. However, in some instances, particularly in conventional design, the strength of roll is also an important consideration. The strength of roll is most important, as flanges are cut so deeply into the roll, by that roll gets weakened. The fillet at the bottom of flange also plays an important role in control of the strength of roll. If fillet is small, then the stress concentration developed will make the roll weaker at such places The power of motor and strength of housing also plays a vital role in some mill.
hi beam rolling, reduction takes place on account of two actions, i.e., one is the slabbing action or by indirect draught in the live hole with the tongue of one roll and the collar of the mating roll. The other is the direct reduction i.e., in the horizontal plane of the web. As there will be no slabbing action in the dead hole, here steel will be subjected to such an action, which reduces the height of metal rather than its thickness. Thus, flange thickness get reduce in live hole by indirect draught, while height will get reduce in dead part of beam design by direct dr aught. The thickness of flange of one side is always thicker than the other side in beam rolling, except in the finishing pass. This thickness variation of flanges will get decrease, as the finishing pass is approaching.
At first thought, it might appear that, if beam pass has all flanges designed with equal reduction, will all flanges will be filled equally? This is not true, as this design will produce heavy overfills in live flange and short flange in dead holes. The live flange has slabbing action and therefore, it offers the least resistance to the flow of steel.
Important considerations for Roll Pass Design of Beam
Selection of Bloom Size and Numbers of Passes
The selection of Bloom size and number of passes depends on the many factors-at one hand the Mill layout dictates, whether an odd or even number of passes are required and what length of stock can be handled in such layout. On the other hand, the availability of the power screw down on the roughing stand determines whether more than one pass can be taken in the initial passing’s, hence it also decides the number of groove required. The size of roll diameter and barrel length affects roll strength and biting condition, which alongwith the power of the motor limits the maximum reduction, which can be under taken in a pass. Rolling temperature also plays a vital role in deciding numbers of passes. If flanges are thicker in early passes, then heat can be retained for a longer tune, in turn, which may allow to increase no of passings, hi view of above stated factors, it is always advised to use beam shaped bloom/billet for light and medium structural mill.
We may compute the size of rectangular bloom/billet from the following consideration:
Bloom height, A = 2 x height of flange + 20 mm
Bloom width, В = width of the Beam - Bn
Bloom width depends on the total amount of spread, i.e., В B allowed from first to last, will be deducted from the web width of the beam.
However, experience in rolling medium and heavy rolling shows that actual widening found out to be less than the computed from above mentioned formula.
Bloom Area = 1.47 x A * В