Korkealämpötilaprosessit

Transcription

1 Korkealämpötilaprosessit Pyrometallurgiset jalostusprosessit Lisäaineisto sulkeumien analysoinnista Inclusion analyses Many inclusions are not found until they cause problems in the final product - Reclamations Challenges in inclusion analyses - Huge amounts of produced steel vs. small samples - Representativity of samples? - Large (=harmful) inclusions are very rare - Huge amount of inclusions - Does average values tell anything? - Different methods measure/analyze different things - Size, composition, etc. - Complete size distribution not obtained using only one method - Some methods are time-consuming - Not fast enough for process control - Some methods do not give 3D-view on inclusions Online or offline correction 1

2 Sampling Samples from different process stages - Lollipop samples from molten steel (BOF, ladle, CC) - Metal cap protection (MCP) or Argon protection (AP) in order to get slag-free samples - Sample pieces from solid steel (slab, plate, sheet) Samples should be - homogeneous - representative Sample cooling rate has an effect on inclusions - Secondary inclusions = Inclusions formed after sampling - Fast cooling - small secondary inclusions - Slow cooling - heterogeneous nucleation and inclusion growth Sampling 2

3 Analysis methods On-line Off-line 2D OES-PDA (Optical emission spectrometry with pulse discrimination analysis) LOM (Light optical microscopy) SEM (Scanning electron microscopy) EPMA (Electron probe microanalysis) 3D LA-ICP-MS (Laser ablation, Inductively coupled plasma source - mass spectrometry) EE (Electrolytic extraction) EE+CCA (Coulter Counter analysis) EE+SPOS (Single Particle Optical Sensing) Ultrasonic scanning LSHR+US + Indirect inclusion analysis: Total oxygen content of steel Analysis methods 3

4 Analysis methods Analysis methods 4

5 Why 3D methods are needed if they are more expensive and time-consuming? Small inclusions are not detected from 2D samples due to interference of steel matrix Shape and real size of inclusions cannot be detected from 2D samples Total oxygen content (Indirect measurement) Amount of dissolved oxygen is very low in (Al-)killed steel (2...5 ppm) - Variations in total oxygen content are due to variations in amount of inclusions Measurement of total oxygen content is an indirect method to estimate the amount of inclusions in steel - Correlations between O tot and inclusion-related problems have been reported - Not accurate, but fast and cheap in comparison to actual inclusion analysis methods 5

6 Microscopy Magnification of polished sections of samples Different methods: - Light Optical Microscopy (LOM), Metallographic Microscope Observation (MMO) - Visible light, resolution approximately 200 nm - (Field Emission) Scanning Electron Microscope (FESEM) - Electrons, resolution approximately 1 nm - Back scattered electrons are used to analyze composition - Secondary elecgtrons are used to create an image - Limitations for samples - Electric conductivity - No volatiles - Must endure vacuum Image from back scattered electrons Image from secondary electrons Microscopy Chemical composition analysis with Electron Probe Micro Analyzer (EPMA) - Associated with SEM - Measurement on of either energy or wavelength - Energy-Dispersive X-Ray Spectroscopy (EDS) - Wavelength Dispersive X-Ray Spectroscopy (WDS) - Small volume is being analyzed (< m 3 ) - Depends on the accelerating voltage being used - Approximately same size as inclusions being analyzed - One analysis may contain elements from more than one phase (and from steel matrix) Automatic Image Analysis (IA) is used to study larger areas to improve representativity - Based on differences in lightness/darkness - Scratches etc. may be considered as inclusions 6

7 Sample Electrolytic extraction Electrolyte Principle of the method: - Iron is dissolved selectively into a solvent/electrolyte, whereas inclusions remain undissolved - Some solvents may dissolve some inclusions - Inclusions are filtered and may be studied as a whole The equipment - Steel sample as anode, Pt-ring as cathode - Ions may transport via salt bridge - Potentiostat is used to control the dissolution - Varying current is used to control the dissolution rate Inclusions Potentiostat Reference electrode After dissolution: - Inclusions are filtered from the solvent Salt bridge - Membrane filter (hole size e.g. 0.1 m), vacuum pump - Analysis with SEM (amount, size distribution, composition) - Amount and size distribution may be analyzed with: Electrolyte Sample - Single Particle Optical Sensing Method - Coulter Counter Analysis Pt-ring Electrolytic extraction TiN Ca-Al-Mg-oxide + CaS Restrictions and limitations: - Solvent/electrolyte must be chosen based on what kind of inclusions are studied - e.g. sulphides are dissolved into acids - Sample volume dissolved is very small - Size distribution lacks information about large inclusions - All the particles in the filter are not inclusions - iron precipitate, Pt-particles, KCl from salt bridge, etc. - Inaccuracy of EDS analysis - Inclusions may be smaller than the volume analyzed - Slow method CaS 7

8 OES-PDA Optical Emission Spectrometry (OES) - Atoms on the surface of steel sample are excited with plasma - When atom returns to the ground state, it emits radiation with a spectrum characteristic to each element - Intensity of different wavelengths is determined - Chemical composition of the sample may be determined - Average value from approximately sparks - Equipment must be calibrated for each sample type (e.g. each steel grade) Optical Emission Spectrometry with Pulse Discrimination Analysis (OES-PDA) - Analysis of inclusions (composition and size distribution) instead of average compositions of dissolved elements - Principle is similar to OES - Exception: Values of each spark is considered separately - Inclusions create high intensitey peaks that could be detected - Fast method (results ready 2-10 minutes from sampling) - No information about morphology - Suitable for small inclusions (< 12 m) only LA-ICP-MS Laser Ablation Inductively Coupled Plasma Mass Spectrometry ( Ablation = Evaporation material surface ) - Material surface is evaporated with a laser pulse - Particles detached from the sample are ionized with plasma (temperature approximately 8000 C) - Investigation of emission spectrums of detached elements Any solid material is suitable as a sample - No requirements on the sample size (few g is enough) - No preparation Possible to detect local variations in the composition - Resolution approximately 1 m - Possibility to detect inclusions from steel matrix 8

9 LA-ICP-MS More information about inclusion analyses - Karasev A: Proc. of the 9th International Conference on Molten Slags, Fluxes and Salts, Janis D, Karasev A & Jönsson P: 8th International Conference on Clean Steel, Karasev A, Inoue R & Suito H: ISIJ Int. 41(2001)7, Karasev A & Suito H: ISIJ Int. 44(2004)2, Karasev A, Suito H & Inoue R: ISIJ Int. 51(2011)12, Karasev A & Inoue R: Material transaction (JIM) 50(2009)2, Ericsson O: Doctoral Thesis, KTH, Stockholm, Zhang L & Thomas BG: ISIJ Int. 43(2003)3, Dekkers R: Doctoral thesis. Katholieke Universiteit Leuven,