Making of hot-rolled steel plates - 2

The reheating furnace is an important part of a steel plate mill. So-called reheating because slabs of higher thickness are procured in and then reheated from room temperature to temperature above 1200 degrees C. Using the more technical term, probably this is the austenitic region of the steel, whereby because of the microstructures of the steel in its Austenite form when heated to this high temperature, the thick slabs of 48mm, 58 mm, 68mm, 160mm, 200mm, 220 mm etc are "soft" and could be readily rolled into various smaller thicknesses such as 6mm, 8mm, 10mm, 12mm etc. to the limits of 45mm, 48mm, 50mm, 55mm etc. depending on the equipment used at the steel mill. (Mill stand and hot leveler)



The furnace is a well insulated enclosure where the internal temperature is raised through the burning of fuel. There are different types of fuel such as natural gas, diesel etc. Why is it that at our home, the burning temperature could only be raised to a certain extent/degree and no more? Is this something interesting for us to note? Theoretically if we burn natural gas with air, for complete combustion and without any loss of heat, the adiabatic frame temperature achievable is 1950 degree C. Due to losses through the surrounding atmosphere and other conductive, radiative, absorption losses etc, the furnance temperature could reach probably in the region of 1200-1350 degree C without much of a problem.



The construction and design of the furnace therefore is meant to minimize heat losses, make use of recycled energy etc so as to provide an efficient heating at the most economical cost. The walls are tiled with refractory bricks, insulation material etc to make it to be really good in heat insulation. In addition to that, rail structure is built inside to support and transport the slabs, and these rails are water-cooled so that under the heavy weight of such slabs, they are not deformed or melted by the heat. Because of this cooling requirement, the cooling pipes under the rails are sealed with refractory cements to protect them from severe heating, and therefore a cooling water recycling system has to be in place to cool the rails (sometimes they call it the skid).

The running of a reheat furnace is expensive and needs to be continuous as it takes up a lot of energy and time before it could reach that high temperature. Therefore a furnace need to run, if necessary non-stop though for practical reason and also for scheduled maintenance work to be carried out periodically, that this is not possible. When a furnace is shut down and it takes 5 days to cool down to room temperature and a lot of the insulating cement and refractory tiles will crack because of the contraction caused by this big drop in temperature range. Therefore, repair of such insulating material and the lining of the furnance is necessary to be carried out for every long shut down of the furnace.

The temperature control of the furnance is a function of gas flow rate, pressure, air suppy etc. If the temperature is getting too high, the slab surface is oxidized and produces scales which are not desirable for good quality steel sheets. And if the temperature is too low, the descaling process using high pressure water in the region of 100-200 bars will cool down the slab too much, and that becomes too hard for the rolling process, it might cause warpage or damage to the work rolls.

The reheat furnace and its temperature control is crucial to the making of quality steel sheets.