The Steel slabs are cast at approx. 1450°C in the steel plant. These slabs are 0.225 m thick, 6-12 m long & 0.9-2.1 m wide. Each slab weighs around 20 – 30 tons. The slabs are cut from a continuous cast strand of steel with the desired cross section by gas cutting. At the time of cutting the slabs can be considered (fairly) homogeneous in temperature (800°C). They are then transported by train to partially roofed warehouses, which is approx. 2 km away, where they are stored in stacks (fig 1) and then transferred to the Hot Strip Mill (HSM), where they are rolled and coiled in the desired sequence. Around 800-1200 slabs/day are processed in HSM.
In the warehouses, the slabs are stacked on top of each other at room temperature. The ones on top cool down more than those at the bottom of the stack. They are also moved around several times, due to the sorting operation. The slabs are picked for processing in HSM, based on the material composition and size, which is decided by the customer requirements.
The slabs are fed with the determined production speed into 4 parallel furnaces, where they are reheated in 2-3 hours to 1200°C, which is required for the hot rolling operation. Depending on the slab width there are 20-30 slabs in each furnace. The furnaces are controlled in 3 heating zones having no individual adjustment. The challenge faced is that the heat content of the slab just prior to reheating is not known accurately hence, the reheating parameters are determined by the coldest slab. This limits the pace at which the slabs are fed into the furnace, in turn limiting the productivity of the plant.
To mitigate this problem, we have installed a surface temperature measurement (pyrometer) system, which scans the top surface of one slab at a time along its centreline, as the slab passes underneath just prior to entering the reheating furnaces, see fig. 2. Even with this technique, the temperature gradients in the slab are unknown, thus the average temperature (as a measure for the total heat content) is unknown. Simplified physical models indicate that difference between the surface temperature and the average temperature will be limited to approx. 40-60°C.
Another source of inaccuracy of the surface temperature measurement are oxides adhering to the slab surface. To counter this problem the phenomenon of thermal expansion (approx. 12 x 10-6 °C-1) was used to measure the change in length or width of a whole slab. This is currently done using a laser instrument with a resolution of approx 10-4 mm (0.2 mm on a 2 m range). With the total temperature drop of 500-700°C and a measurement before and after the drop, this would give an accuracy of ± 60°C. However, the irregularities in the shape of the slab, allows an accuracy of 2 mm on a 2 m range only. Thus, this method is also inadequate in solving the problem within the constraints of the industrial set up as a whole (interruption of the process flow, positioning, dirt etc).
We are, therefore, inviting solutions that will enable us to determine the heat content of the whole slab, before they enter the reheating furnace, with an accuracy of at least 4% at a temperature level of 100-400°C. Expressed more tangibly: the average temperature of the whole slab should be known with an accuracy of 5 °C (or less).
The solution should satisfy the following requirements: