For on-line observation of the production of high chromium steels in the AOD converter, a model-based system was developed and installed at the AOD converters of ThyssenKrupp Nirosta in the Bochum and Krefeld steel plant. This observation system calculates dynamically the decarburisation rate based on measured values for exhaust gas flow rate and analysis. A balance calculation, which starts from the analysed start value, gives the current carbon content. The carbon removal within the AOD process is with about 1.5 % much higher than the final carbon content of about 0.02%. Due to errors of the measured input values for the carbon balance, the accuracy in determination of the final carbon content from measured exhaust gas values is not sufficient. The error standard deviation of the observed final carbon content is with 0.09% four times higher than the carbon content itself. With the help of a thermodynamic calculation of the carbon content in the final phase of decarburisation, the accuracy was improved by a factor of 7.

.

Structure of the process model with required input data  

Parallel to the decarburisation calculation, an oxygen balance determines the part of blown oxygen which is used for oxidation of metallic elements like Aluminium, Silicon and Chromium, and  the part which is solved in the steel melt. From this balance, Chromium oxidation rate and summed Chromium loss are calculated. Furthermore the Nitrogen content is calculated continuously, to determine the optimal time to switch the inert gas supply from cheaper nitrogen to more expensive Argon. Thus the low aim nitrogen contents can be reproducibly achieved under minimum inert gas costs.

As important influence parameter for the carbon and nitrogen content calculation, the current temperature of the melt is also calculated on-line. A dynamic energy balance considers the energy input by decarburisation and metal oxidation, as well as the melting and reaction energies of the material additions during AOD treatment.

Accuracy of the process model
(Standard dev. of the modelling error)

Carbon content	0,01 %
Nitrogen content	0,007 %
Temperature	16 K

                                                                                                  Operator display for on-line observation of AOD treatment
   

Furthermore a dynamic control for a well-aimed supply of oxygen to minimise the chromium loss was developed. The figure below shows the distribution of oxygen supply for decarburisation and metal loss for a dynamically controlled AOD heat. The dynamic control starts when the oxygen demand for decarburisation decreases while the chromium loss increases. The oxygen supply is then controlled depending on the decarburisation rate, observed from off-gas measurements or from a thermodynamic calculation. As additional criterium for oxygen control the achievement of a heat-individual aim temperature at the end of decarburisation has to be ensured. Thus a well-aimed chromium loss is allowed as long as the observed melt temperature is below its target value. In comparison to a step-wise control of the oxygen supply the chromium loss can be lowered, and thus the consumption of silicon and slag formers for chromium reduction can be decreased.

AOD example heat with dynamically controlled oxygen supply

On-line applications of the process model                                                                            

• ThyssenKrupp Nirosta, Bochum, Germany                                                                      
• ThyssenKrupp Nirosta, Krefeld, Germany