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L 09 L 09 Document Transcript

  • L 09 Rolling Process and production of various steel sections such as billet, bar, rod, channel, & Roll load calculation,12/17/2011 1
  • Fabrication of metals and alloys• Four basic manufacturing processes:• Casting–to give a shape by pouring in liquid metal into a mold that holds the required shape, and letting harden the metal without external pressure.• Forming–to give shape in solid state by applying pressure.• Machining–in which material is removed in order to give it the required shape.• Joining –where different parts are joined by various means.• Other important technique is powder metallurgy.10/27/2011 2
  •  Four important casting techniques are: •Sand casting •Die casting •Investment casting •Continuous casting  Four important forming techniques are: •Forging •Rolling •Extrusion •Drawing10/27/2011 3
  • Four Forming techniques are10/27/2011 4
  • Ingot is convertedinto sheet thru therolling process10/27/2011 5
  • 12/17/2011 6
  • 10/28/2011 7
  • 11/8/2011 8
  • 14.2 ROLLING ►Wrought steel is converted to useful products in two steps: 1. The molded ingot or the continuous strand is rolled into intermediate shapes- blooms, billets, and slabs. 2. These intermediate slabs are rolled further into plates, sheets, bar stock, structural shapes, strips, expended metal grates, & the like. ►The ingot remains in the mold until the solidification is about complete and the mold is stripped from the ingot. While still hot, the ingots are placed in gas fired furnaces called soaking pits, where they remain until they have attained a uniform working temp of about 1200 °C throughout. The ingots are moved to the rolling mill where, because of the variety of finished shapes top be made eventually they are rolled first into intermediate shapes such as blooms, billets, or slabs.11/7/2011 9
  • Rolling • Remember, however, that continuous casting eliminates some of the requirements for the ingot process. The continuous cast strand also follows The same rolling processes. The two, ingot casting& strand casting, have similar rolling operations to the final end product. • A bloom has a square cross section with a min size of 6 by 6 in. A billet is smaller than a bloom & may have any sq-section from 1½ in. Up to the size of a bloom. Slabs may be rolled from either an ingot or a bloom. They have a rectangular x-section area with a min width of 10 in. & a min thickness of 1½ in. The width is always three or more times the thickness, which may be as much as 15 in. Plates, skelp, and thin strips are rolled from slabs.11/7/2011 10
  • Rolling One effect of a hot working rolling operation is the grain refinement brought about by recrystallization, which is shown in figure 14.1. Coarse grain structure is broken up & elongated by the rolling action. Because of the high temp, recrystallization starts immediately and small grains begin to form. These grains grows rapidly until recrystallization is complete. Growth continues at high temp, if further work is not carried on, until the low temp of the recrystalline range is reached.11/7/2011 11
  • Rolling • In fig 14.1. AB and A B are the contact arcs on the rolls. The wedging action on the work is overcome by the frictional forces that act on these arcs & draw the metal thru the rolls. In the process of rolling, stock enters the rolls with a speed less than the peripheral roll speed. The metal emerges from the rolls travelling at a higher speed than it enters. At a point midway between A & B, metal speed is the same as the roll peripheral speed. Most deformation takes place in thickness, although there is some increase in width. Temp uniformity is important in all rolling operations since it controls metal flow &plasticity. • In rolling, the quantity of metal going into a roll and out of it is the same, but the area & velocity are changed. Thus,11/8/2011 12
  • Rolling Q1 + Q2 = A1V1= A2V2 (14.1) Where Q1 = Quantity of metal going into roll Q2 = Quantity of metal leaving the roll A1 = Area, ft2 , of an element in front of roll A2 = Area, ft2 , of an element after roll V1 = Velocity, ft/s, in element before the roll V2 = Velocity, ft/s, in element after the roll A1/ A2 = V2/ V1 (14.2) In the process of becoming thinner, the rolled steel becomes longer & may become wider, but it is constrained by vertical rolls set to restrict this sideways growth as the X-sectional area is decreased, the velocity increases as does the length of the material.12/17/2011 14
  • Rolling e.g. , a heated slab 7 in thick weighing more than 12 tons is reduced to a coil of thin sheet in a matter of minutes. The delivery velocity of the hot rolled product may be more than 3500 ft /min at the conclusion of rolling. • Most primary rolling is done in either a two high reversing mill of a three high continuous rolling mill. In the two high reversing mill, Fig14.2 A The piece passes through the rolls, which are then stopped & reversed in direction, & the operation is repeated. At frequent intervals, the metal is turned 90 deg on its side to keep the section uniform & to refine the metal throughout.12/17/2011 15
  • Rolling • About 30 passes are required to reduce a large ingot into a bloom. Grooves are provided on both the upper & lower rolls to accommodate the various reductions in X-sectional area. The two high rolling mill is versatile, since it has a range of adjustment as two size of pieces & rate of reduction. It is limited by the length that can be rolled & by the inertia forces that must be overcome each time a reversal is made. These are eliminated in the three high mill, Fig 14.2C, but an elevating mechanism is required. The three high mill is less expensive to manufacture & has a higher output than the reversing mill.11/8/2011 16
  • 11/8/2011 17
  • Rolling • Other arrangements of rolls used in rolling mills are shown in fig 14.2. Those that have four or more rolls use the extra ones for backing up the two that are doing the rolling. In addition, many special rolling mills take previously rolled products & fabricate them into such finished articles as rails, structural shapes, plates, and bars. such mills usually bear the name of the product being rolled and. In appearance, are similar to mills used for rolling blooms & billets. A mill specializing in rolling rails is able to roll a rail continuously about ¼ mile long.11/8/2011 19
  • Rolling Billets could be rolled to size in a large mill for blooms, but usually this is not done for economic reason. Frequently, they are rolled from blooms in a continuous billet mill consisting of six or more rolling stands in a straight line. The steel makes but one pass through the mill & emerges with a final billet size, approximately 2x2 in which is the raw material for many final shapes such as bars, tubes, and forgings. Fig 14.3 illustrates the no of passes and the sequence in reducing the X-section of a 4x4 in billet to round bar stock.11/8/2011 20
  • Fig 14.3 Diagram illustrates number of passes & sequences in reducing the cross section of a 4 by 4 in billet to round bar stock11/8/2011 21