Korkealämpötilaprosessit

1. Korkealämpötilaprosessit Pyrometallurgiset jalostusprosessit Lisäaineisto sulkeumien analysoinnista

2. 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

3. Inclusion analyses: 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

4. Inclusion analyses: Sampling

5. Inclusion analyses: Analysis methods + Indirect inclusion analysis: Total oxygen content of steel

6. Inclusion analyses: Analysis methods

7. Inclusion analyses: Analysis methods

8. Inclusion analyses: Analysis methods

9. Inclusion analyses: 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

10. Inclusion analyses: 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 Otot and inclusion-related problems have been reported • Not accurate, but fast and cheap in comparison to actual inclusion analysis methods

11. Inclusion analyses: 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

12. Inclusion analyses: 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 (< 10-30 m3) • 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

13. Inclusion analyses: Electrolytic extraction 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 After dissolution: • Inclusions are filtered from the solvent • 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: • Single Particle Optical Sensing Method • Coulter Counter Analysis Electrolyte Sample Potentiostat Inclusions Reference electrode Salt bridge Electrolyte Sample Pt-ring

14. Inclusion analyses: Electrolytic extraction 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 Ca-Al-Mg-oxide + CaS TiN CaS

15. Inclusion analyses: 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 3000-4000 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

16. Inclusion analyses: 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

17. Inclusion analyses: LA-ICP-MS

18. More information about inclusion analyses • Karasev A: Proc. of the 9th International Conference on Molten Slags, Fluxes and Salts, 2012. • Janis D, Karasev A & Jönsson P: 8th International Conference on Clean Steel, 2012. • Karasev A, Inoue R & Suito H: ISIJ Int. 41(2001)7,757. • Karasev A & Suito H: ISIJ Int. 44(2004)2,364. • Karasev A, Suito H & Inoue R: ISIJ Int. 51(2011)12,2046. • Karasev A & Inoue R: Material transaction (JIM) 50(2009)2,341. • Ericsson O: Doctoral Thesis, KTH, Stockholm, 2010. • Zhang L & Thomas BG: ISIJ Int. 43(2003)3,271. • Dekkers R: Doctoral thesis. Katholieke Universiteit Leuven, 2002.