Control of Microstructure during Solidification & Homogenization of Thin-Slab Cast Direct-Rolling (TSCDR) Microalloyed Steels

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Microalloyed Steels Tihe (Tom) Zhou Supervisors: Dr. Hatem.

1 Control of Microstructure during Solidification & Homogenization of Thin-Slab Cast Direct-Rolling (TSCDR) Microalloyed Steels Tihe (Tom) Zhou Supervisors: Dr. Hatem. S. Zurob, Dr. Nikolas. Provatas February 27, (Part 1) 1/37

2 Outline Introduction Objectives Literature Review Solidification δ To γ Phase Transformation Austenite γ Coarsening Process 4. Experiment Approach 5. Preliminary Results Heat Transfer Model Effect of Cooling Rate Effect of Dipping Time Verification of the simulation process 6. Summary and Future work 2/37

Introduction Thin Slab Casting Direct Rolling Process (TSCDR) Holding furnace Rolling Stands caster Layout of the most common SMS-Demag Compact Strip Production Mill Started In 1989 Nucor Third

3 Introduction Thin Slab Casting Direct Rolling Process (TSCDR) Holding furnace Rolling Stands caster Layout of the most common SMS-Demag Compact Strip Production Mill Started In 1989 Nucor Third Revolution of Production of Steel Slab thickness from 50mm to 70mm 50 Installations, 55 million tons, 14% of the world output C. Klinkenberg et al, Mater. Sci. Forum, 2005, , 235 J. Muller et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 240 3/37

4 Introduction Benefits of TSCDR Process: Low Capital Costs: Shot length, Less high and less space for casting and rolling equipment, eliminating rough rolling mills Energy Savings: No re-heating stage, short production time, economically variable at low capacities, more flexible than CCS, products can change easier and quicker Environmental Advantages: Electric arc furnace using scraps, reduce energy consumption D. Shi et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 59 G. Megahed et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 292 4/37

5 Introduction TSCDR Process for Microalloyed Steel Comply with American Petroleum Institute (API) standards Microalloy steel used for oil and gas pipelines must meet the requirements: High strength, High toughness, Low ductile-to-brittle transition temperature, Good weldability, Corrosion resistance. Examples of plants Developing Nb-(Ti)-API tube grades C. Klinkenberg et al, Mater. Sci. Forum, 2005, , 235 5/37

6 Introduction Challenges of TSCDR Microalloyed Steels - Non-uniform as Cast Microstructure/Grains Dendrite morphology and SDAS (Second Dendrite Arm Spacing ) of cast structure 400 um Wang et al, Mater. Sci. Forum, 2005, , 29 6/37

7 Introduction Challenges of TSCDR Microalloyed Steels - Non-uniform as Cast Austenite Grains Austenite grains in the as cast slab Wang et al, Mater. Sci. Forum, 2005, , 29 7/37

Introduction Challenges of TSCDR Microalloyed Steels - Rapid Coarsening of Austenite Grains Microstructure evolution during the solidification

8 Introduction Challenges of TSCDR Microalloyed Steels - Rapid Coarsening of Austenite Grains Microstructure evolution during the solidification and subsequent cooling γ 1520ºC 1500ºC 1480ºC γ γ 1460ºC γ 1430ºC N. S. Pottore et al, Metall. Trans A, 1991,vol. 22A, pp, γ γ 1380ºC 8/37

9 Introduction Challenges with TSCDR Microalloyed Steel - Persistence of Large Austenite Grains Average grain size reduced by thermo-mechanical processes not eliminate the non-uniformity large grains still exist refinement limited by number of passes P. Uranga et al., Mater. Sci, Forum, 2005, , /37

10 Research Objectives Obtain finer & more uniform microstructure Simulation of initial solidification process Study the mechanism of coarsening in the solid state - delta dendrite coarsening - delta grain growth - gamma phase transformation - austenite grain coarsening Find methods to refine microstructure and prevent grain coarsening 10/37

11 Literature Review Microstructure Evolution (1) Liquid/Solid, (2) δ-ferrite/γ-austenite, (3) γ-austenite/α-ferrite 1. Liquid 2. Delta dendrites 4. Austenite Grains 5. Alpha ferrite grains C% 3. Delta grains 11/37

Literature Review Solidification Theories and Models - Heat flow - Mass flow - Solute redistribution - Liquid-solid interface - Processing parameters and microstructure parameters M. C.

12 Literature Review Solidification Theories and Models - Heat flow - Mass flow - Solute redistribution - Liquid-solid interface - Processing parameters and microstructure parameters M. C. Flemings, Solidification Processing, McGraw-Hill Inc., 1974 W. Kurz and D. J. Fisher, Trans Tech Publication Ltd., Switzerland, 1998 Bruce Chalmers, John Wiley & Sons, Inc., 1964 W. C. Winegard, Institute of Metals, London, /37

13 Literature Review Solidification Heat Transfer Model k T = T t ρc = (α T ) - Merton C. Flemings Model - First finite difference Model - Incorporated two-phase changes - 3-D Mode: steady and unsteady state - Thin slab casting process Model α: thermal diffusivity cm²/s K: thermal conductivity cal/(cm-ºc-s) ρ: density g/cm³ c: specific heat, cal/(g-ºc) M. C. Flemings, Solidification Processing, McGraw-Hill Inc., B. G. Thomas, Metall. Mater. Trans B, 2002, Vol. 33B, No. 12, 795. M. Gonzalez et al, Metall. Mater. Trans B, 2003, Vol.34B, No. 8, 455 S. Louhenkilpi, Mater. Sci. Eng. A , 2005, 135 J.E. Camporredondo S et al Metall. Mater. Trans B, 2004, Vol.35B, No. 6, /37

14 Literature Review Solidification - Affects influence As-cast Microstructure From liquid to solid Control Processing Parameters - Increase cooling rate (thinner slab 20mm) 2. Addition of inoculation substances - Particles tend to accumulate at liquid steel 3. Electromagnetic stirring (EMS) fields - Need special set-up 4. In TSCDR process core reduction - Break the dendrite arms - Homogenize the as cast microstructure Microstructure evolution in the solid state: Coarsening, transformation 14/37

15 Literature Review Grain Growth Model 4γ ℜ = 2 dt 0 r (λ + α C) 0 ℜ :dimensionless grain size γ: surface energy r0 :initial grain size Grain size (μm) t C: solute concentration t: time λ and α: constant Time (S) Which stage dominate the coarsening process? Dendrite arm spacing, delta grains, delta to austenite phase transformation, austenite coarsening before and inside the soaking furnace H. S. Zurob et al, Acta Meter. 2002, 50, /37

16 Literature Review Delta δ to γ Austenite Phase Transformation - Nucleation of γ-grains At the Triples Points of δ GBs H. Yin et al Acta mater. 1999, Vol. 47, No. 5,1523 Along the δ GBs 16/37

17 Literature Review Austenite γ-grains refinement methods Thermomechanical treatment during δ to γ austenite phase transformation F. Zarandi and S. Yue, Mater. Sci. Forum, 2006, , μm 17/37

18 Literature Review Austenite γ-grains refinement method 2. Various oxide particles act as heterogeneous nucleation sites for austenite. H. Suito et al, ISIJ Int., 2006, Vol. 46, /37

19 Literature Review Austenite γ-grains refinement method 3. Precipitates pin grain boundaries C. J. Tweed et al, Acta Metall., /37

20 Literature Review Literature Summary From the literature review, the best chance to produce fine and uniform initial microstructure is in the solid state. Research approach: Step 1: Produce as cast microstructure which is resemble the Thin Slab Cast slab in the industry Step 2: Examine the coarsening process of delta grains, delta to austenite phase transformation and austenite grains. 20/37

21 Temperature (oc) Experimental approach Experiment Process References Method Liquid Current Method 15300C δ-ferrite 15000C 11500C γ-austenite α-ferrite Time (hr) Real time simulation of the TSCDR Process in the industries is impossible 21/37

22 Experimental approach Equipment Pressure Gauge Gas Inlet Furnace Chamber Gas Outlet Diffusion Pump Mechanical Pump ADL Model-MP Crystal Growing Furnace Courtesy Mr. John Thomson Sketch of ADL Model-MP Furnace Courtesy Dr. Dmitri Malakhov 22/37

23 Experimental approach Experiment Set-up Dipping bar Thermocouples Thermocouples Doll Pin Dipping Block Crucible Graphite Longitudinal Transverse 23/37

24 Preliminary Results - Heat Transfer Model h h h q q q Complicated solution - geometries - variation of the properties with temperature - unstable S-L interfaces K: conduction C: convection, R: radiation 24/37

25 Preliminary result Heat Transfer Model (cont.) Temperature Measurement Dipping Bar Thermocouple Temperature( C) top bottom 100 center Time(s) 25/37

26 Preliminary result Chill Heat Transfer Model (cont.) Solid Liquid k T = T = (α T ) t ρc T 2T = α t z2 Boundary conductions: z=0, T = T0 z=s, T=TM T T0 x = erf ( ) Ti T0 2 αt γ α st α: thermal diffusivity cm²/s S= 2 K: thermal conductivity cal/(cm-ºc-s) ρ: density g/cm³ c: specific heat, cal/(g-ºc) γ: depends on the properties of mold and solid 26/37

27 Preliminary result Etching Solution & Pre-heat Treatment ( Ø (Cu),t= 6s ) Pre-Heat Treatment: 970 C, 30 mins; 3%Picral, 1 min 27/37

28 Preliminary result Effect of Cooling Rate Processing and Microstructural Parameters λ 1 = A1G mv n G: Temperature gradient, V: Solidification rate λ 2 = B1 (G V ) n B1 and n are constants Using the heat transfer model: Control the processing parameters, the microstructural parameters can be predicted R. W. Cahn et al, Physical Metallurgy, 1983, /37

29 Preliminary result Effect of Cooling Rate Ø (Cu), 6s Ø0.5 3 (Fe-pipe), 6s Ø0.5 3 (Cu), 6s Ø0.5 3 (Cu), Al2O3 coating 6s 29/37

30 Preliminary result Effect of Dipping Time Ø (Cu), t= 4s, 7s, 10s, 15s 4S 7S 10S 15S 30/37

31 Preliminary result Preliminary Results Ø1 - Ø0.5 3 (Steel), t= 7s 31/37

Preliminary Verification result of the solidification simulation process Wang et al, Mat. Sci. Forum, 2005, 500-501, 29 1.

Primary arm spacing has the same scale as the references: 100~150um: In all the samples TSCDR: 120~180um: H. J. Diepers, Mater. Sci. Forum 2006, Vol.

32 Preliminary Verification result of the solidification simulation process Wang et al, Mat. Sci. Forum, 2005, , Microstructures are the same as the references 2. Primary arm spacing has the same scale as the references: 100~150um: In all the samples TSCDR: 120~180um: H. J. Diepers, Mater. Sci. Forum 2006, Vol. 508, Solidification rate is approximately 1 mm/s TSCDR is1mm/s. J. E. Camporredondo, Metall. and Mater. Trans B, 2004, vol.35b, 541. A. H. Castillejos et al, 33rd McMaster Symposium on Iron &Steelmaking 2005, P,47 This set-up successfully simulates TSCDR Process. 32/37

33 Summary 2. Unique etching technique was established to reveal the dendrite microstructure of the as-cast microalloyed steel 3. Secondary dendrites arms were generated during the simulation process 4. 1-D heat transfer model can predict the relationship between the processing and microstructural parameters 5. Experimental set-up successfully simulated the initial solidification stage in the thin slab casting process 33/37

34 Future Work 2. Build a set-up to study the kinetics of grain growth and its contribution to the final coarsening - δ-ferrite growth - δ-ferrite to γ-austenite phase transformation - Simulate the coarsening process of γ-austenite grains 2. Refine δ-ferrite and γ-austenite grains 34/37

35 Future work Experimental Set-up Dipping bar Thermocouples Thermocouples Heating elements Alumina crucible Solidified shell Graphite crucible 35/37

36 Acknowledgements Dr. Hatem. S. Zurob Dr. Nikolas. Provatas Ms. Connie Barry Mr. Martin Vanooste Dr. Sumanth. Shankar Dr. Mani. Subramanian Mr. Jim Garrett Mr. John Thomson Mr. Doug Culley Mr. John Roddaand Mr. Ed McCaffery Financial support Steel Research Center, McMaster University Courtesy John Thomson 36/37

37 Thank you Questions?? 37/37

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