SFB 761 Stahl ab initio

Transcription

1 SFB 761 Stahl ab initio MSE Nürnberg Uni. Prof. Dr.-Ing. D. Senk Dipl.-Ing. A. Lob Smelting and Soldification of Fe-Mn-C Steel

2 SFB 761 Stahl ab initio SubareaB1 Smelting and Soldification of Fe-Mn-C Steel Subtask: Hydrogen Problem

3 Abstract - Presentation of the System Fe-Mn-C - Hydrogen in Steel-making Prozess - Hydrogen in Austenitic Steel - Legality of Hydrogen Solubility - Hydrogen Solubility Examinations - Outlook for Further Examinations No. 1 Smelting and Soldification of Fe-Mn-C Steel Subtask: Hydrogen Problem

4 Steels of Interest TRIP-Steel (TRansformation Induced Plasticity): strain-induced ε hcp - and α bcc martensite formation in the γ fcc austenitic matrix under plastic deformation* Elements Content [wt%] Fe 75.0 Mn Chemical composition TRIP-Steel for SFB 761 C Al 0.1 S P Si 0.05 TWIP-Steel (Twinning Induced Plasticity): based on the formation of mechanical twins under plastic deformation Ni Cr steel system Fe-Mn-C: high strength combined with high ductility Elements Fe Mn C Al S P Si Ni Cr Content [wt%] Chemical composition TWIP-Steel for SFB 761 * for manganese contents > 15 wt% No. 2 System Fe-Mn-C

5 Austenitic and Martensitic Structure Weight% Temperature ( C) Fe-Mn-C system; chemical composition γ Fe, γ Mn Magnetic Transformation system Fe-Mn-C: 440 C fcc 1430 C RT ε-hcp 440 C Atome% Phase Diagram Fe-Mn Hansen, M.; Anderko, K.: Constitution of binary alloys, McGraw-Hill Book Company, 2.Ed, New York Toronto (1958), pp No. 3 Characteristics of the System Fe-Mn-C

6 Segregation-behaviour Segregation coefficient of manganese S Mn c = max = 1,16...1,35 c 30 0,9 min 28 0,8 depending on: - melting parameters - casting format - distance to the surface - C-content weight- % (M n,fe) ,7 0,6 0,5 0,4 0,3 weight- % (C ) Mn Fe C low Mn-distribution in interdendritic micro areas ,2 0, Distance (µm) Segregation profile of Fe-Mn-C system Fe-Mn-C: strong segregations Segregation leads to inhomogeneous solidification structure affecting also [H] concentration No. 4 Characteristics of the System Fe-Mn-C

7 One Task of Subarea B1 Determination of Hydrogen Chemically-induced hydrogen crack formation in the liquid System Fe-Mn-C Crack formation in steel No. 5 Hydrogen Examinations

8 Steelmaking-Process refractory in slags hydrated lime rusty scrap atmospheric humidity out break out (casting) crack formation residual humidity coolant decomposition CH 4 deoxidiser slags rusty scrap from this, high contents of hydrogen in steel induces problems for the steelmaking-process important steel treatment at this point No. 6 Sources for Hydrogen in Industrial Work Problems and Treatment

9 Continuous Casting Industrial hydrogen measurements in the tundish Continuous Casting, schematic breakout danger for hydrogen contents 10 ppm Continuous Casting No. 7 Critical Values for Hydrogen Contents

10 Solidification Temperature ( C) 1,6 1, Solubility switch of hydrogen induces the following hydrogen reactions 1 [ H ] { H 2} 2 [ H] s = [ H] s l hydrogen accumulation in the solid-liquid interface log [H] (ppm) 0,8 0,4 0 dissolved hydrogen will get gaseous and diffuses out of the solid steel H H H H H H H H H H H H H Solid-liquid interface, schematic H dissolved hydrogen accumulates at the solid-liquid interface and will be moved through the liquid phase -0, /T, T in K Solubility of hydrogen in iron hydrogen decreasing hydrogen increasing No. 8 Diffusion of Hydrogen

11 Dependence on Structure Hydrogen solubility in the fcc (Austenite) is higher than in the bcc (Ferrite) octahedron places in the fcc are greater than the tetrahedrons places Hydrogen solubility: fcc > bcc Solubility of hydrogen in pure iron Ratte, E.: Wasserstoffinduzierte verzögerte Rissbildung austenitischer Stähle auf CrNi(Mn)- und Mn-Basis. Berichte aus dem Institut für Eisenhüttenkunde, RWTH Aachen: Shaker Verlag, 2007 No. 9 Hydrogen Solubility

12 Dependence on Chemical Composition [H]max (ppm, wt) [Mn] [Al] for the system Fe-Mn-C the elements Mn, Al, C are of great interest [C] Alloying Elements (wt%) Hydrogen solubility in Fe-X-systems depending on chemical composition Weinstein, E.; Elliot, J. F.: Trans. Met. Soc., AIME 227 (1963), pp No. 10 Hydrogen Solubility

13 Hydrogen Trapping - H is free to move in the lattice, till H crosses a trap - strong interaction between Fe atom and hydrogen at trap - exothermal reaction bonds H at this position shrinkagehole cracks Multi phase steel Graphic representation of typical lattice defects Cavities in as-cast pores negative influence from hydrogen on solid steel depends on microstructure Energy of bonding for hydrogen, to leave the trap: kj molh kj molh plane traps ( G f T 30 )= active traps (-) mechanical qualities deep traps ( 55 G f T 61 )= innocent for mechanical qualities mechanical problems cracks blowholes No. 11 Hydrogen in Microstructure

14 Sieverts K = equilibrium constant a = activity f = activity coefficient (4) Gmelin, L.; Durrer, R.; Trenkler, H.; Krieger, W.: Gmelin-Durrer Metallurgie des Eisens. Weinheim: Verlag Chemie, (1978) (5a/b) to measure with Hydris or pin tube to calculate with measured temperature to be adjusted Hydrogen content depends on: - chemical composition - temperature - pressure to calculate with analysed chemical composition No. 12 Legality for Hydrogen Solubility

15 Sieverts; Parameters Temperature dependence K = equilibrium constant T = temperature (K) A = (K -1 ); constant B = 0,9201; constant Stone, R. P.; Plessers, J.; Turkdogan, E. T.: Die Genauigkeit der Bestimmung des Wasserstoffgehaltes in flüssigem Stahl mit dem Hydris-System. Stahl und Eisen 110 (1990), 11, pp Alloy element j Al C Cr Mn Ni P S Si Interaction parameter e j H Dependence on chemical composition Frohberg, M. G.: Thermodynamik für Werkstoffingenieure und Metallurgen. Leipzig: Deutscher Verlag für Grundstoffindustrie (1994) examinations of the dependence on chemical composition for hydrogen solubiltiy at first No. 13 Legality for Hydrogen Solubility

16 Hydrogen Solubility Trials for the System Fe-Mn-C exhaust duct Hydris -lance Hydris -probe refractory hood for inerting (protection against the atmospheric humidity) 50 kg medium frequency induction furnace at the bottom: porous plug Ar and Ar/H flooding No. 14 Hydris Measurements Functional principle of Hydris

17 Hydrogen Solubility Trials for the System Fe-Mn-C Steps for pin tube sample taking 1) sample extraction from the melt 2) quenching the sample into cold water rapid treatment of every step 4 cm Pin Tube 3) removing the fused quartz from the solidified sample Storage the sample in liquid nitrogen 4) leaving the solidified sample into liquid nitrogen Analysing No. 15 Pin Tube Measurements

18 Hydrogen Solubility Trials for the System Fe-Mn-C Hydrogen Content (ppm, wt) Pin Tubes Hydris Temperature Manganese Content (wt%) Comparison between Hydris - and Pin Tubes measurement for increasing manganese contents Both measurement techniques give conformable results Temperature ( C) critical hydrogen content for casting breakout at first the dependence of chemical composition will be checked No. 16 Own Results of RWTH Metallurgie

19 Further Hydrogen Solubility Trials Determination on: - temperature dependence - pressure dependence - examination of interaction parameter frequency: 3 khz maximum power input: 150 kw vacuum: mbar capacity: 100 kg special features: adjustable atmosphere programmable temp. charging during vacuum upcoming features: online and clean measurement = Hydris -System Vacuum-Induction-Furnace 4 at IEHK No. 17 Lookout

20 Summary - System Fe-Mn-C: - austenitic structure - segregation behaviour - Hydrogen = - poor mechanical qualities for steel - casting break out for high [H] contents - high solubility in austenitic structure - accumulation at sold-liquid interface - First experiments: - chemical composition dependence - Hydrogen solubility above critical values for casting break-out - Further experiments: - temperature dependence - pressure dependence No. 18 Project B1

21 Thank you for your attention No. 19