Identifying, visualizing and quantifying process disturbances at SSAB Oxelösund using multivariate modelling

Abstract: Modern process lines give rise to huge amounts of data which are stored in databases. Multivariateanalysis comprises useful tools to grasp useful information from the datasets. Inthe present study principal component analysis (PCA), projection to latent structures(PLS) and hierarchical PCA has been used to create models of five process steps at a Swedishsteelworks. The focus has been to identify and explain relations to quality problems ineach step, both within the step itself, but also from upstream processes using hierarchicalPCA.

The five process steps that have been modelled are the blast furnace, desulphurization inthe torpedo car, basic oxygen steelmaking in the LD–LBE-converter, secondary steelmakingin ladle and ladle furnace and, finally, continuous casting of slabs. Among the resultsachieved it is found that:

• PLS prediction of crude iron analysis from blast furnace discharge has been madewith a fraction of explained variance for external validation (Q2PS) above 20% for P, Cr, Cu, Ti, CaO, SiO2, MgO and basicity. The data resolution was relatively low.


• Hierarchical modelling revealed correlations between the process steps, e.g. thatLD-converter treatments registered as severe slopping heats have a titanium contentin the incoming crude iron that is higher than average.


• Heats with too high phosphorous content after LD-treatment can be identified ashaving low silicon content in the crude iron, which makes it impossible to createthe necessary slag amount for desired phosphorous cleaning effect.


• High sulphur content in the torpedo car demands a long treatment time. If the siliconcontent is low in such a batch, there is an evident risk that it will not have highenough temperature in the secondary steelmaking.


• Capturing reasons for quality problems during casting is difficult due to the lowvariation in data. The main variations exist between the steel qualities. However,the importance of casting properties such as oscillations for visual quality of theslabs, and temperature and steel analysis for slab inner quality have been recognized.