Hot Rolling Process:
The process of hot rolling consists of passing the hot metal (ingot) between two rolls revolving in opposite directions. Rolling reduces the thickness or changes the cross-section of a long metal workpiece by applying compressive forces through a set of rollers. Rolling may be (a) flat rolling and (b) shape rolling. In flat rolling, the end products of rolling have rectangular cross-sections such as slabs, plates, sheets, etc. In shape rolling, the end product may have various cross-sectional shapes such as I-beams, channels, angles, squares or hexagonal rods and tubes, etc. The initial breaking down of an ingot (or of a continuously cast slab) is done by hot rolling into the shape of a bloom. The cast ingots have typically dendritic structure with coarse and non-uniform grains and also have some porosity and are hard. Hot rolling being done above the recrystallization temperature, converts the cast structure to a wrought structure with fine grains and increased ductility.
Rolling is a mass production process. Rolling (including both hot and cold rolling) accounts for about 90% of all metals produced by metal working processes. The initial breaking down of an ingot or of a continuously cast slab is always done by hot rolling. The traditional method of casting ingots is being rapidly replaced now by continuous cast slabs or other castings in steel plants. The product of first hot rolling is called a bloom, or slab or billet.
Flat rolling a thick flat to thinner strip (i) and showing a corresponding increase in the width (spreading) of the flat rolled strip (ii).
Shape rolling: showing various stages in shape rolling an H-section in above Fig.
Bloom has a square cross-section (at least 150 mm side); a slab is usually rectangular in cross-section; and a billet is usually square with cross-sectional area smaller than bloom. Blooms are processed further by shape rolling into I-beams, rail roads, etc. Slabs are rolled into plates and sheets. Billets are rolled further into rods, bars, seamless pipes, wire rod, etc.
Some of the hot rolled products are shown in, It may be emphasized here that the hot material, which is in the plastic condition, resists the change of shape (during rolling or other hot-working operations) with considerable force, although quick application of deforming force heats up the metal adding to its plastic flow. Even then, it is necessary to go through a series of stages before the final shape can be produced. Each stage requires a different set of tools or equipment (i.e. set of rollers with different profiles). Reheating of the metal is usually required between stages. The sequence of reducing a billet to a round bar stock in different stages of rolling
Showing the schematic outline of various flat rolling and shape rolling processes. White hot steel ingots (continuous cast products) are passed through rolls which form the plastic steel into slab, billet and bloom for further processing.
During hot rolling of blooms, billets or slabs, the surface of the hot metal is first ‘conditioned’, i.e. prepared for subsequent operations by removing heavy scales with a gas torch (called scarfing); lighter scale is removed by grinding. Also prior to further processing by cold rolling, the scale developed in hot rolling is always removed by pickling with acid or blasting with water or by grinding.
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