Celox® for on-line process control in modern steelmaking Ing. R. Maes Product Application Manager Heraeus Electro-Nite International N.V. The masters when it comes to control techniques EN_brochure.indd 1 10/09/12 13:26 CELOX® FOR ON-LINE PROCESS CONTROL IN MODERN STEELMAKING EN_brochure.indd 2 10/09/12 13:26 CELOX® FOR ON-Line PROceSS CONTROL IN MODern STeeLMAKinG 1. ABOUT CELOX® 4 1.1 Measuring principle 4 1.2 Celox® and measuring system description 5 1.2.1 Celox® sensor 5 1.2.2 Celox® lance and accessories 7 1.2.3 Instrumentation 9 1.3 Formulae 9 1.4 Standard operating procedure 13 1.5 Sensor precision 14 1.6 Sample comparison 17 2. FAST MEASUREMENTS OF OXYGEN, CARBON AND ALUMINUM VIA THE CELOX® ROUTE 19 2.1 Oxygen determination 19 2.2 Carbon determination 20 2.3 Aluminium determination 21 3. CELOX® FOR PROCESS CONTROL IN MODERN STEELMAKING 23 3.1 Decarburization control in converters and EAF 23 3.1.1 Quick carbon determination (Quick-Tap practice) 23 3.1.2 Calculation of pre-deoxidation additions 24 3.1.3 Control of bottom stir efficiency 25 3.2 Decarburization control at degassing stations 25 3.3 Deoxidation control at the ladle treatment station 26 3.3.1 LCAK- steel grades (slab casters) 26 3.3.2 Mn-Si – killed steel grades (billet casters) 30 3.3.3 Successful continuous billet casting through oxygen control in mixed operations 34 3.4 Desulphurisation control 39 3 EN_brochure.indd 3 10/09/12 13:26 CELOX® FOR ON-LINE PROCESS CONTROL IN MODERN STEELMAKING 01 ABOUT CELOX® 1.1 MeaSurinG principLE Celox ® Principle Scheme + EMF - Thermocouple + R.T pO EMF = ln 2 steel n.F pO2 ref pO2 ref T pO2 steel Contact electrode Mo Solid reference Cr/Cr2O3 Solid electrolyte ZrO 2 (MgO) Bath contact electrode Fe FIG.1: Celox® MEASURING PRINCIPLE The determination of the oxygen activity in molten metal is based on an electro- chemical measurement. Magnesia stabilised zirconia is used as electrolyte. At high temperatures this solid electrolyte allows the transfer of oxygen ions between steel and the reference material in the electro-chemical cell. Schematically the cell can be represented as follows: Mo/Cr+Cr2O3//ZrO2.MgO//(O)Fe/Fe electrode/ pO2ref. //electrolyte //pO2steel/electrode From the generated EMF, the measured temperature and the known oxygen partial pressure of Cr/Cr2O3, the oxygen partial pressure of the steel can be calculated using Nernst’s law. Note that the electrodes (Mo – Fe thermocouple) generate a 24mV EMF within a temperature range of 1500 to 1600 °C. Hence, a correction of 24mV is required to determine the thermo- dynamical value. In absence of electronic conduction Nernst’s law is represented by the following formula: E= RT ln (pO2steel) 4F (pO2ref.) E= Electro-motive force (mV) R= Gas constant T= Measured temperature (K) F= Constant of Faraday pO2ref.= Oxygen partial pressure of the reference material (Cr/Cr2O3) pO2 steel= Oxygen partial pressure in the steel 4 EN_brochure.indd 4 10/09/12 13:26 1.2 CELOX® AND meaSurinG SYSTem DEScripTION FIG. 2: Celox® MEASURING SYSTEM Figure 2 represents the required hardware for an oxygen measurement in liquid steel: - Celox® sensor - Immersion lance with inner compensating cable. - Extension cable connecting the lance with the Multi-Lab® instrument. - Multi-Lab Celox® instrument. 1.2.1 Celox® Sensor The Celox® sensor includes an electrochemical cell for EMF measurement and a Pt/PtRh thermocouple for temperature measurement. Compensation wires connect oxygen cell and thermocouple to a connector making contact with the contact block of the immersion lance. (see fig.1). Both measuring units are assembled within a conical sand body, and covered by a steel and a cardboard slag cap in order to protect the sensor during immersion through the slag into the steel. For ensuring good contact with the molten metal, a steel ring is used. Doing this a Fe-Mo thermocouple is introduced in the measuring circuit influencing the EMF- output (24 mV between 1500-1650°C) of the sensor. The measuring head is pressed into a cardboard tube protecting the immersion lance. 5 EN_brochure.indd 5 10/09/12 13:26 CELOX® FOR ON-LINE PROCESS CONTROL IN MODERN STEELMAKING Sensor options Four types of sensors are available: Celox® standard (CE**01****) Celox® probe metal cap sand housing thermocouple EMF-conductor bath contact cardboard tube oxygen cell compensation wire cardboard cap FIG. 3: Celox® standard PROBE Used for measurements from 50 ppm to more than 1000 ppm in steel baths without aluminum. The Celox® standard can be used in EAF and BOF for the determination of the carbon content, but NOT for aluminum determination in the ladle. Celox® Al (CE**91****) Celox® Al probe metal cap Coated sand housing Thermocouple coated EMF-conductor bath contact cardboard tube oxygen cell compensation wire cardboard cap FIG. 4: Celox® AL PROBE 6 EN_brochure.indd 6 10/09/12 13:26 The Celox® Al sensor can be used for oxygen measurements from 1ppm to more than 1000 ppm. All quartz and sand components of the Celox® Al are coated with a special refractory material. This prevents the reduction of these components by the aluminum present in the bath, which could introduce errors in the measurement. Celox® Al can also be used to measure soluble Al in Al-killed steel grades and the carbon content in EAF and BOF. Celox® Ti (CE**92****) ImpORTANT In steel grades containing titanium (>0.02) a modified sensor named Celox®-Ti has to be used in case of soluble aluminum determination. Celox® tundish(CE**96****) Celox® Al probe metal cap coated sand housing thermocouple coated EMF-conductor bath contact cardboard tube oxygen cell compensation wire cardboard cap FIG. 5: Celox® TUNDISH PROBE For response time reduction at low temperatures (tundish) this Celox®-type does not contain a thermal shock shield on top of the oxygen cell. 1.2.2 Celox lance and accessories The construction of the lance is extremely important. Care must be taken to guarantee an insulation of min. 2 Megaohm between: The positive and negative legs of both, temperature and oxygen channels The positive legs of temperature and oxygen All legs and earth 7 EN_brochure.indd 7 10/09/12 13:26 CELOX® FOR ON-LINE PROCESS CONTROL IN MODERN STEELMAKING Polarity and colour coding of the contact block, mineral insulated inner cable, lance plug and socket, and outer compensating cable must be correctly maintained. The Heraeus Electro-Nite Celox® lance has been designed so that it may be used for temperature measurements only (Positherm®) and for temperature and oxygen (Celox®). For optimal protection of the internal compensation cable, the use of a telescopic cardboard tube is recommended for applications in converters and furnaces. The telescopic tube can be fixed firmly onto the lance by means of a conical coupling device (order number LC33016077) Automatic lances Automatic lances operate in a harsh environment with a lot of radiation heath, dust, fumes, moisture etc. In order to ensure reliable measurements, lance maintenance, repair, exchange, and lance checks are required on a regular basis. Heraeus Electro-Nite developed the Quick Connecting Lance system in order to enable a smooth and quick lance exchange without causing a major inconvenience at the treatment station. A composition drawing with reference numbers is given below. CONVENTIONAL LANCE QCL Connector 1 SALESREF NO. : LC24016006 PART NO. : 118.003.6- +5 330 0 1 2 3 2 3 QCL detachable part/S 4 7 SALESREF NO. : LC24016001 PART NO. : 108.057.6- QCL contactblock SALESREF NO. : LC24016005 PART NO. : 118.002.6- 5 4 5 6 6 QCL fixed part/S G 3/4” 8 SALESREF NO. : LC24016000 ~381 min.16 PART NO. : 108.056.6- Wire protection 7 9 QCL/S 8 max.30,5 FIG.6: COMPOSITION DRAWING QUICK CONNECTING LANCE SYSTEM 8 EN_brochure.indd 8 10/09/12 13:26 1.2.3 Instrumentation As instrument we recommend the Multi-Lab® Celox. This is an industrial, programmable micro-computer based instrument with an outstanding measuring performance. For more detailed information, please consult our Multi-Lab® Celox brochure. 1.3 FORMULae Oxygen activity in liquid steel grades The relationship between temperature (°C), EMF(mV) and oxygen activity, for Celox®, is given by the following formula: log a(O)= 1.36+0.0059*(E+0.54*(T-1550)+0.0002*E*(T-1550)) with E in mV, and T in °C and a(O) in ppm. Celox® Relation T - EMF- a(O) 1600 1400 1550 1600 1200 1650 1700 1000 1750 800 a(O) (ppm) 600 400 200 0 -50 0 50 100 150 200 250 EMF (mV) FIG. 7: Relationship BETWEEN EMF, a(O) AND Temperature In the Multi-Lab® an additional a(O)-formula can be used and modified to suit specific customer requirements: a(O)=e^(f1+f2/T)*(((e^(f3+f4/T))^1/4+(e^(f5+f6/T))^1/4)*e^(f7*E/T+f8/T+f9)– (e^(f3+f4/T))^1/4)^2+f10 with E=EMF(mV) and T= temp.(°C)+273 9 EN_brochure.indd 9 10/09/12 13:26 CELOX® FOR ON-LINE PROCESS CONTROL IN MODERN STEELMAKING 1: f1=8.27402 2: f2=16486.3 3: f3=56.2291 4: f4=-171243 5: f5=18.6325 6: f6=-86370 7: f7=11.6114 8: f8=290.28 9: f9=0.0000 10:f10=0.0000 Oxygen activity in Ni melts with a Fe-content <=30% In this case the relationship between a(O), EMF and temperature is given by the following formula: a(O)=10^(f1+(f2+f3*(E+f4+f5*T))/(T+273)) with E=EMF (mV) and T=temp.