Effect of Mn and Si addition on the dynamic transformation of austenite during strip rolling

Transcription

1 Hatchett Seminar London, July 16, 2014 Effect of Mn and Si addition on the dynamic transformation of austenite during strip rolling John J. Jonas Birks Professor of Metallurgy Emeritus McGill University Acknowledgements: Chiradeep Ghosh, Vladimir V. Basabe & Clodualdo M. Aranas, Jr.

2 Questions What is Dynamic Transformation? What are the microstructural changes that take place during DT? What can thermodynamics and kinetics tell us about these changes? What is the effect of Mn and Si addition on these phenomena? What is the effect of Nb addition?

3 Research Methodology 3

4 Strip Rolling Simulations Strip rolling simulations carried out on a 0.06C-0.3Mn-0.01Si steel with a cooling rate of 7-8 C /s, interpass times of 1s, and pass strains of 0.4 applied at a strain rate of 1s -1. 4

5 Strip Rolling MFS Plots Change in flow curve shape Why does the MFS during simulated strip rolling not increase with decreasing temperature (1s interpass times)? 5

6 Effect of Interpass Time in Strip Rolling.06C-.3Mn-.01Si The shorter the interpass time, the greater the amount of softening due to ferrite formation. The longer the time, the more re-transformation to austenite. 6

7 Effect of Interpass Time on Re-Transformation The shorter the interpass time, the greater the amount of softening due to ferrite formation. The longer the time, the more re-transformation to austenite. 7

8 MFS vs. 1/T Diagram Courtesy of T. Schambron 8

9 MFS vs. 1/T Diagrams Courtesy: E. Poliak, Arcelor-Mittal, USA ArcelorMittal, United States 9

10 What is going on?

11 Formation of Widmanstätten (displacive) ferrite V.V. Basabe et al., ISIJ Int., C. Ghosh et al., ISIJ Int., 2013

12 Widmanstatten Microstructures (0.09%C Steel)

13 Widmanstatten Microstructures (0.06%C Steel) Note that the Widmanstätten plates are only 200 nm thick and therefore cannot be seen using optical microscopy. The plates also coalesce into polygonal grains during and after rolling.

14 Conversion to Polygonal Ferrite

15 Shear Stress in Torsion Geometry associated with formation of a pair of self-accommodating Widmanstätten ferrite plates and the corresponding shear stresses.

16 Time Elapsed During Rolling ε = 0.5 (35% reduction). ε = 100 s -1 Time in the deformation zone = 5 ms Incremental transformation time = 100μs (for 1% strain) 16

17 Dynamic Transformation Austenite Diffusional transformation Dynamic transformation Diffusionless transformation Allotriomorphic ferrite Widmanstätten ferrite > A e3 Widmanstätten ferrite < A e3 Pearlite Bainite Martensite 17

18 Diffusion distance, nm Carbon Diffusion Distance during Formation of Widmanstätten Ferrite 100 Temperature range: C intervals Diffusivity in ferrite Increasing temperature Plate thickness: 200nm Distance: 100nm Time: 80μs Diffusivity in austenite Increasing temperature Time, s 1. D. A. Porter and K. E. Easterling: Phase Transformations in Metals and Alloys, (1988), Published by Van Nostrand Reinhold (International) Co. Ltd, Molly Millars Lane, Workingham, Berkshire, England. 2. J. Kucera and K. Stransky, Czech. J. Phys. B, 30 (1980), J. K. Stanley, Trans. AIME, 185 (1949),

19 Mean Diffusion Distance, nm Mean Diffusion Distance, nm Mean diffusion distances of carbon and Mn in ferrite C 753 C 763 C 773 C 783 C 793 C 803 C 823 C C 753 C 763 C 773 C 783 C 793 C 803 C 823 C Time, s Carbon Time, s Mn 19

20 No substitutional diffusion during displacive transformation O. Thuillier, F. Danoix, M. Goune and D. Blavette; Scripta Materialia, 2006

21 Times Required for Three Types of Transformation Velocity of sound in steel = 6000 m s-1 Therefore time taken to travel 100 nm in steel ~ 16 pico sec Substitutional FCC BCC C diffusion diffusion Displacive ~16 pico sec Paraequilibrium ~80 micro sec Orthoequilibrium ~10 sec 21

22 Thermodynamic Considerations C. Ghosh et al., Acta Mater., 2013 J.J. Jonas & C. Ghosh, Acta Mater. 2013

23 The Free Energy Obstacles to Dynamic Transformation

24 Effect of Mn & Si Addition On the Obstacles to Transformation +Si + Mn +Si +Mn LL:.01Mn/.24Si HL: 1.4Mn/.24Si LH:.01Mn/.95Si HH:1.4Mn/.95Si Wray, P.J.: Metall. Trans. A 15A (1984), 2041

25 Effect of Mn & Si Addition On the Obstacles to Transformation Note maximum because of approach to delta ferrite phase field.

26 Softening as the Driving Force Strain rate: 2.3 x 10-2 s -1 Data of Peter Wray replotted: Metall. Trans. A: 15A, 1984, 2041.

27 Softening as the Driving Force

28 Effect of Mn & Si Addition on the Driving Force These quantities can only be determined experimentally

29 Comparison of the Driving Forces and Obstacles to the Transformation

30 Summary of Driving and Opposing Forces 30

31 Effect of Nb Addition on Dynamic Transformation There is no partitioning of Nb during DT. Thus the W. ferrite is supersaturated in Nb. There is insufficient time for precipitation of NbCN and therefore no particle hardening. As a result of these considerations, determining the driving force for DT (i.e. the net softening) is more difficult than in plain C steels. Conversely, the thermodynamic obstacles to the transformation can be readily evaluated.

32 Conclusions Strip Mills 1. MFS vs. 1/T plots exhibit low slopes due to DT. 2. Strip mill (1s) simulations indicate that DT is initiated at the beginning of each pass. 3. The amount of DT ferrite formed & retained increases with decreasing interpass time. 4. Because of the formation of DT ferrite, the volume flow rate increases during rolling. 5. There is C but not substitutional diffusion during the displacive formation of W. ferrite.

33 Conclusions Thermodynamics 1. Stressing and straining raises the effective Ae 3 and Ae 1 temperatures. 2. Widmanstätten ferrite forms when the γ/α flow stress difference is large enough to overcome the free energy obstacles opposing transformation. 3. There is insufficient time for substitutional diffusion, so only displacive and paraequilibrium mechanisms can operate during DT. 4. C diffusion and partitioning (into austenite) during rolling can lead to undesirable ductility issues. 5. Mn addition somewhat opposes and Si addition significantly promotes the formation of Widmanstätten ferrite.

34 Softening as the Driving Force

35 Further Work 1. Effect of stress during transformation on variant selection? 2. Dependence of ferrite volume fraction on experimental conditions. 3. Modeling DT and DRX in combination. 4. Application to control of microstructure? Runout table modeling? 5. Application to gauge control. Other?

36 Mean Flow Stress (MFS) MFS = f(c,t,ε,ε) where: C = composition T = temperature of deformation ε = strain ε = strain rate MFS can be calculated by: Plate Mill Simulation: Dependence of mean flow stress (MFS) on 1000/T for 0.10%C-0.04%Nb- 0.30%Mo microalloyed steel. Here, interpass intervals of 30s was employed Jonas J. J., The Hot Strip Mill as an Experimental Tool, ISIJ, 2000; 40(8):

37 Pussegoda et al., Metall. Trans. A (1990) Ti-V steel Interpass time: 0.5s. Cooling rate: 10 C/s. ε p = 0.1.

38 Effect of Interpass Time T i = 1000 C 7 deformations ε = 0.4 per pass ε = 1 s -1 Strip rolling simulations showing the effect of increasing the interpass time. The MFS displays detectable load drops at short interpass times. 38

39 MFS Plots Plate Rolling T i = 1000 C 7 deformations ε = 0.4 per pass ε = 1 s -1 Plate rolling simulations showing MFS curves at longer interpass times. The MFS curves converge to the expected MFS for the plate mills. 39

40 Experimental Quench Times & CCT Diagrams 0.79C Steel

41 Effect of Strain on the Phase Diagram Ae 3 Zr? Ae 1 T exp Ti? Ae 3 T Ae 1 C exp increasing strain X %C

42 Gleeble Tests Prof. P. Karjalainen

43 Conclusions Critical Strain 1. The double differentiation method can be used to determine the critical strains for DT (as well as for DRX). 2. The DT critical strains of are well below those associated with DRX. 3. Free energy considerations call for three domains of transformation behavior: i) purely displacive ferrite formation (without carbide formation) at C levels up to about 0.1%; ii) ferrite formation followed by carbide to %; iii) ferrite formation accompanied by levels above %. formation at C levels up carbide formation at C

44 Conclusions - Microstructure 1. The displacive formation of Widmanstätten ferrite takes place during DT. 2. Appreciable C diffusion can take place during the 100 μs available (incrementally) during steel rolling; thus carbide formation generally follows ferrite formation (but see below). 3. There is insufficient time for substitutional diffusion and so only displacive and paraequilibrium mechanisms can operate during DT.

45 Conclusions 1. The displacive formation of Widmanstätten ferrite takes place during DT. 2. Appreciable C diffusion can take place during the 100 μs available (incrementally) during steel rolling; thus carbide formation generally follows ferrite formation (but see below). 3. There is insufficient time for substitutional diffusion and so only displacive and paraequilibrium mechanisms can operate during DT. 4. Straining raises the effective Ae 3 and Ae 1 temperatures and changes the nature of the phases.

46 Conclusions, continued 5. In the presence of Mn, DT ferrite, DT carbides and austenite (all 3) can all be present simultaneously. 6. Free energy considerations call for three domains of transformation behavior: i) purely displacive ferrite formation (without carbide formation) at C levels up to about 0.1%; ii) ferrite formation followed by carbide formation at C levels up to %; iii) ferrite formation accompanied by carbide formation at C levels above %.

47 Temperature, C Fe-C Phase Diagram 1000 γ 900 A cm α γ + α Ae 1 Ae 3 γ + cementite 600 α + cementite Weight percent, carbon 47

48 Early Strip Mill Simulations Samuel et al., THERMEC C-1.27Mn-0.2Si-0.11Nb-0.08V 2s interpass; 0.5 strain Karjalainen et al., ISIJ Int C-1.46Mn-0.04Nb 1 & 3s interpass; 0.24 strain

49 2013: 0.09C-1.3Mn-0.02Si-0.036Nb 1s interpass; s.r. 2s -1 ; cooling rate 8 C/s

50 Strip Mill Simulations (0.79%C Steel) T start = 907 C T Finish = 803 C Interpass time 1s. Cooling rate 8 C/s. ε p = 0.25; ε = 4 s -1. T start = 800 C; T Finish = 735 C J.J. Jonas et al. BAC2013

51 MFS (MPa) Strip Mill Simulations (MFS) T ( C) / T (K -1 ) + Pussegoda et al., Metall. Trans. A (1990) * Karjalainen et al., ISIJ Int. (1995)

52 Research methodology Temperature Torsion test Torsion simulation 1150 C 20 min. ε = 2 / s ε = C ε = 4 / s 20 s ε = s Test temperature 1 min. 1 C / s Ae 3 Testing atmosphere: Ar + H 2 atmosphere Time 52

53 Eight Steels Investigated Steel Composition Ortho. Ae C-0.2 Si-1.5 Mn C-0.26 Si-1.1 Mn Nb C-1.56 Si-1.56 Mn Nb C-0.01 Si-0.30 Mn C-0.02 Si-1.30 Mn Nb C-0.24 Si-1.30 Mn C-0.24 Si-0.70 Mn C-0.24 Si-0.65 Mn 733

54 Stress-strain Curves J.J. Jonas et al. SRI,

55 MFS vs. 1/T Diagrams

56 What is going on? What is the contribution of DRX? 56

57 Critical Strain Determinations using Double Differentiation C. Ghosh et al., SRI, 2013 J.J. Jonas et al., ISIJ Int., 2013

58 Two Sets of Minima

59 Double Minima 59

60 Critical Strains for DT & DRX Compression Tests

61 Critical Strain Strip Mill Simulations (Critical Strains) DRX DT Temperature, C

62 Widmanstatten Microstructures (0.09%C Steel)

63 Conversion to Polygonal Ferrite

64 How do Non-Equilibrium Phases Form? Austenite Reconstructive transformation Dynamic transformation Displacive transformation Allotriomorphic ferrite Displacive ferrite Widmanstätten ferrite Pearlite Metastable carbides Bainite Martensite

65 Free Energy Changes During Forward and Backward Transformations G T > Ae 3 deformation transformation deformation re-transformation austenite ferrite Phases H. Mahjoubi, M.S Thesis, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, 2010.

66 Free Energy-Composition Diagram Gibbs energy-composition diagram for 0.79%C steel at 803 C (Ae C) illustrating the Gibbs energy changes taking place during dynamic transformation.

67 Dynamic Phase Diagram 0.79C Steel Quasi-binary paraequilibrium dynamic phase diagram compared with the conventional undeformed diagram. 67

68 Effect of Mn on the Phase Diagram 0.21C-1.3Mn 0.79C-0.65Mn Calculated quasi-binary paraequilibrium phase diagram for the 0.21%C steel compared with that of the 0.79%C steel.

69 Effect of Mn & Si Levels on the Phase Diagram Calculated quasi-binary paraequilibrium phase diagram for the 0.06C-0.30Mn-0.50Si steel compared with that of the 0.79%C steel.

70 Strip Mill Simulations (1995) Cooling rate: 6 C/s; ε p = 0.25; ε = 2 s -1 ; 0.04%Nb steels Karjalainen et al., ISIJ Int., Vol. 35 (1995), pp

71 Free Energy-Composition Diagram 0.09%C Steel

72 Free Energy-Composition Diagram 0.06%C Steel

73 Free Energy-Composition Diagram 0.79%C Steel

74 Free Energy-Composition Diagram 0.21%C Steel

75 Carbide Formation & the Role of C Diffusion

76 Transmission Electron Microscopy [001] Ferrite Ferrite grain [111] Ferrite 0.79C Steel T = 763 C (Ae C) [660] cementite Carbides within the ferrite grain Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma 76

77 Conversion to Polygonal Ferrite

78 ~143.3 nm Partitioning of Mn & C Concentration profiles across the boundary ~22.34 nm Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma 78

79 No substitutional diffusion during displacive transformation O. Thuillier, F. Danoix, M. Goune and D. Blavette; Scripta Materialia,

80 Effect of DT & DRX on Rolling Load 10% drops Strain rate = 0.4 s -1

81 Strip Mill Simulations (0.79%C Steel) T start = 1070 C T Finish = 814 C Interpass time 1s. Cooling rate 8 C/s. ε p = 0.25; ε = 4 s -1. T start = 1000 C T Finish = 807 C C. Ghosh

82 Effect of DT & DRX on Rolling Load 10% drops Strain rate = 4 s -1

83 Mixed Grain Sizes-Strip Simulation F C Cooling Rate 6 C/s Interpass Time 3s.

84 Critical Strain -( ) -( ) Effect of Polynomial Order n = 2 n = 3 n = 4 n = 5 n = n = 7 n = 8 n = 9 n = 10 n = 11 n = 12 n = 13 n = 14 n = Stress (MPa) Stress (MPa) DT DRX Order of the polynomial Order of the polynomial Effect of polynomial order on the second derivative/stress relationship according to the partial curve method

85 -( ) Critical Strain -( ) -( ) Effect of Polynomial Order (Entire Curve Method) n = 3 n = 4 n = n = 6 n = 7 n = 8 n = 9 n = 10 n = Stress (MPa) Stress (MPa) n = 12 n = 13 n = 14 n = Stress (MPa) DT DRX Order of the polynomial

86 Critical Strain Strip Mill Simulations (Critical Strain) Critical Strain DRX T start = 907 C T Finish = 803 C DT Temperature, C T start = 800 C T Finish = 735 C Temperature, C

87 Presence of three phases concurrently Ternary orthoequilibrium phase diagram at 700 C for 0.21C steel

88 Presence of three phases concurrently Ternary orthoequilibrium phase diagram at 725 C for 0.79C steel

89 Hypo-Eutectoid steel Equilibrium phase diagram Dynamic phase diagram T Strain T A DT Ae 3 Ferrite and + Ae 1 Carbides Ferrite and Pearlite %C Strain

90 Stress, MPa Stress, MPa Stress, MPa Compression Flow Curves Low Carbon Steel 80 ε ε =0.25 s -1 ε =0.1 s-1 60 ε =0.5 s C 950 C C 1050 C Strain ε =0.1 s -1 ε =0.25 s -1 ε =1 s -1 ε =0.50 s -1 ε =0.25 s -1 ε =0.1 s -1 ε =0.5 s -1 ε =0.25 s ε -1 =0.5 s Nb-modified Steel ε =0.25 s ε =0.5 s -1 ε =0.5 s -1 ε =0.25 s 60 ε -1 =0.25 s -1 ε =0.05 s ε =0.05 s C 1050 C C 1150 C Strain ε =0.5 s -1 ε =0.25 s -1 ε =0.1 s -1 ε =0.05 s -1 ε =0.5 s Nb-modified TRIP Steel ε =0.5 s -1 ε =0.25 s -1 ε =0.5 s -1 ε =0.05 s ε =0.5 s -1 ε =0.05 s ε =0.05 s -1 ε =0.5 s ε =0.25 s ε =0.05 s C ε =0.05 s C 1050 C C 1150 C Strain * Torsion

91 Torsion Flow Curves

92 Gibbs free energy Effect of strain on Gibbs free energy of austenite T > Ae 3 ΔG = ΔG chem +ΔG Deformation α γ X X X X X %Carbon 92

93

94 Double Differentiation Poliak Method (1996) Ni (also 304 SS) ( / σ)( ( θ/ σ)) = 0 Note: Absence of phase change in Ni & SS

95 -( ) -( ) -( ) -( ) Double Minima Steel 1 (Compression) Strain rate = 0.1 s C 950 C 1000 C 1050 C Strain rate = 0.25 s C 950 C 1000 C 1050 C Strain rate = 0.5 s -1 Stress (MPa) 900 C 950 C 1000 C 1050 C Strain rate = 1.0 s -1 Stress (MPa) 900 C 950 C 1000 C 1050 C Stress (MPa) Stress (MPa)

96 -( ) -( ) -( ) -( ) Double Minima Steel 2 (Compression) Strain rate = 0.1 s C 1000 C 1050 C 1075 C 1100 C 1150 C Strain rate = 0.05 s C 1000 C 1050 C 1075 C 1100 C Strain rate = 0.25 s -1 Stress (MPa) 950 C 1000 C 1050 C 1075 C 1100 C 1150 C Strain rate = 0.5 s -1 Stress (MPa) 950 C 1050 C 1075 C 1100 C 1150 C Stress (MPa) Stress (MPa)

97 -( ) -( ) -( ) -( ) Double Minima Steel 3 (Compression) Strain rate = 0.1 s C 950 C 1000 C 1050 C 1100 C 1150 C Strain rate = 0.25 s -1 Stress (MPa) 950 C 1000 C 1050 C 1100 C 1150 C Strain rate = 0.05 s Strain rate = 0.5 s -1 Stress (MPa) 950 C 1000 C 1050 C 1100 C 1150 C 950 C 1000 C 1050 C 1100 C 1150 C Stress (MPa) Stress (MPa)

98 ~140 nm ~37.3 nm Atom Probe Tomography Carbon segregation to shear bands and sub-boundaries ~22 nm ~7.83 nm 98 Courtesy of Dr. Ilana Timokhina and Prof. Elena Pereloma

99 DT & DRX Critical Strains (Torsion) ΔT (Experimental Temperature Ae 3 ), C ΔT (Experimental Temperature Ae 3 ), C ΔT (Experimental Temperature Ae 3 ), C ΔT (Experimental Temperature Ae 3 ), C

100 Double Minima (Torsion)

101 Conversion to Polygonal Ferrite

102

103 Critical Strain Critical Strain Strip Mill Simulations (Critical Strains) T Start = 1070 C T Finish = 814 C DRX Critical Strain DT Critical Strain DRX T Start = 1000 C T Finish = 807 C DT DRX Critical Strain DT Critical Strain Temperature, C Temperature, C