Optical sensors & laser precision for continuous casting process control

Continuous casting is the machine process by which molten metal is converted into semi-finished cast billets, blooms, or slabs that are then rolled in finishing mills. The development of continuous casting is aimed at improving yield, quality, productivity and cost efficiency. It facilitates the economical production of metal sections of superior quality by taking advantage of continuous and standardized production, as well as automation, which provides improved steel process control. Primarily used for steel casting, continuous casting is also used for aluminum and copper casting.

Here is why ProSpeed® LSV laser sensors allowing a non-contact inline measurement of casting speed and steel strip length are the metrology of choice for steel process control, especially in challenging industrial environments like on red-hot steel and continuous casting machine processes – wether for slabs, strip, cast billets, blooms or at the torch cutter.

Relevance of optical sensors for the continous casting process

There is a direct correlation between length and speed over the time constant. So, measuring speed in a continuous process also results in gathering the length of the measured product. Typically, all inline measurement methods use this correlation.

But why and where in the continuous casting steel process is the knowledge about the exact length and speed of the product relevant?

  • Precise control of the cut length on continuous casters (no. 1)
  • Crop-optimization in hot strip mills (no. 2)
  • Reliable control of throughput times at reheating furnaces (no. 3)

Continuous casting emission calculation and savings potential

Taking a 5-strand billet caster with 220 square millimeter billets cut to 6-meter length each and considering the density of steel as 7,85 kg/dm³, the cast billets produced way 379,94 kg/m. A typical caster would operate at 900 mm/min cast speed at a 24h/day operation. Bringing together the numbers results in 1,080 cuts a day, considering all 5 strands. Using measurement wheels or similar technology in a rough environment will lead to an average accuracy of 1% whereas using an LSV realistically will provide an accuracy of 0,1%. So, the average improvement per cut is roughly 24mm. The resulting figures of this improvement are shown in table 1.

Cut-length in continuous casting

The task of cut-to-length is an absolute must in every continuous caster. Measuring the cast length and triggering the cutting process needs to be implemented in every casting mill. Generally, no matter the shape, subsequent processes or final end product, the strand needs to be cut after casting. Otherwise, the following handling of the product is impossible. As the general casting speed varies, measuring the length of the product to determine when to cut is inevitable.

The product length is important for following processes especially when a certain final length of the product is needed e.g., in rail production. Generally, the better the consistency of incoming length, the more accurate the following processes can be regulated and the higher the quality of the end product. Reliable precision in the length of a product are key especially for SPQ-steel (superior product quality) in product trading business.

Process control in continuous casting

For certain process steps during continuous casting knowing the speed of the product is an essential control parameter. One important part of the process is the cool-down section where the transition between liquid and solid phase of the material takes place. The speed determines major characteristics of metallurgy and material properties and therefore knowledge of the casting speed is very important. Another example is speed synchronization of a flying torch or saw in order to precisely claw the cutter to the material and perform a clean cut with minimal tolerances and minimum impact on the equipment.

Weight determination in steel casting

Another major usage of length information is to determine the weight of the product much earlier in the process than possible with a weighing system. In a clean casting process the outer shape of the product is determined by the coquille early in the process and does not change during the following process. The specific weight of the product is a result of its ingredients and determined even earlier in the mold. Knowing the length of the product even before the cut can lead to very low tolerances in final weight as process fluctuations can be addressed early in the process leading to optimal process consistency. If planned upfront the mill can also save the integrated scale and use a speed measurement method instead.

Relevance of casting and global role of steel production

According to the World Steel Association roughly 7 % to 9 % of the entire global CO2 emissions are directly related to the production of steel.1  This results in around one third of the total emissions generated worldwide, in all industries combined.2 The World Steel Association, which represents more than 80 % of global steel production including casting, rolling and more, agreed on a program called “Step Up” in order to address specific actions aiming in the direction of improving the steel industries sustainability impacts.3

Read the full article

World Steel Association 2021, Climate change and the production of iron and steel, accessed 1 February 2023, https://worldsteel.org/publications/policy-papers/climate-change-policy-paper/

2 Hannah Ritchie and Max Roser, n.d., Emissions by sector, accessed 1 February 2023, https://ourworldindata.org/emissions-by-sector

3 World Steel Association, n.d., Step up programme, accessed 1 February 2023, https://worldsteel.org/steel-topics/technology/step-up-programme/

Images courtesy: Images courtesy of Polytec GmbH unless otherwise specified. 

Optical length and speed measurement in steel, aluminum and metals production