What is Hot Rolling
Hot rolling is a hot working metal working process where large pieces of metal, such as slabs or billets, are heated above their recrystallization temperature and then deformed between rollers to form thinner cross sections.
The Hot Rolling Process :
The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. In continuous casting, rolling mill is mostly integrated to save energy. for larger work pieces gas-or oil-fired soaking pit for smaller work pieces induction heating. a finishing temperature is defined (50 to 100 degC above there crystallization temperature) If the temperature does drop below this temperature the material must be re-heated before more hot rolling.
Critical to the Hot Strip Mill is its walking-beam reheat furnace, state-of-the art equipment that replaced and now outperforms three older-(pusher-) style furnaces.
After exiting the reheat furnace, the slab passes through a descaling unit, an enclosure employing two pairs of spray headers that blast the intensely hot slab with 1,500 psi pressurized water to remove the 1/8-inch thick layer of oxidized iron that forms at the surface of the slab in the oxygen-rich atmosphere of the reheat furnace.
The roughing mill is made up of six independent rolling mill stands, the last four of which incorporate small vertical rolling mills called edgers. Slabs heated in the furnace until they glow bright orange-yellow are rolled through one stand at a time to produce so-called transfer bars suitable for finish rolling. Approximately 96% of steel is continuously cast (the remaining -ingots ).After hot rolling the cooling should be done uniformly else residual stress is generated due to shrinkage that affects the mechanical properties. Mill scale is the oxide layer formed on the surface due to reaction with atmosphere at high temperatures. Pickling is done with HCl to remove this scale.
At the very high temperatures at which the steel is rolled in the roughing mill, it is very plastic and ‘flows’ easily like cookie dough beneath a rolling pin. Consequently, as the slab is reduced from eight to nine inches thick to the final bar thickness of one to one and one-half inches, bars tend to spread width-wise by a few inches at their extremities, and by as much as an inch through the body.
It is Descaled by the low pressure headers. After descaling by the low-pressure headers, the bar is pinched by a pair of pneumatically-actuated rolls to mechanically loosen any remaining scale, which, as the processing temperatures cool off, becomes increasingly sticky even as it returns ever more slowly to the surfaces of the still red-hot steel.
The head-and tail-ends of nearly every transfer bar are cropped by a pair of large steel drums each with a shear blade extending along its length.
Prior to the finish rolling operation, the head-and tail-ends of the transfer bar will be sheared to square them up, helping to ensure proper threading and tail-out. A final two-stage descaling operation is performed to clean off the scale that has grown on the bar during roughing. Controlling Factors:-I. Temperature Control. Gauge Control. Flatness & Crown IV. Laminar Cooling V. Coiling VI. Coil Handling.
USAGES OF HR COIL
Hot rolled coils are primarily used for making pipes and have many direct industrial and manufacturing applications, including the construction of tanks, railway cars, bicycle frames, ships, engineering and military equipment and automobile and truck wheel caps, frames and body parts, pipes, LPG cylinders .