Processo Q&P (Quenching and Partitioning) Estudo de caso completo

1. Processo Q&P (Quenching and Partitioning)Estudo de caso completo Fernando Rizzo

2. Projeto de Cooperação Internacional NSF-CNPq (CIAM) , NSF-EPSRC J.G. Speer, D.K. Matlock , A. Streicher – Colorado School of Mines, USA F. Rizzo, A. R. Aguiar – PUC, Rio de Janeiro, Brazil D.V. Edmonds, Kejian He – University of Leeds, UK

3. Quenching and Partitioning: - Background - Fundamental Issues - Recent Results • J.G. Speer, D.K. Matlock, B.C. De Cooman and J.G. Schroth, Acta Mater., 51 (2003) 2611-2622. • J.G. Speer, A.M. Streicher, D.K. Matlock, F.C. Rizzo and G. Krauss, Austenite Formation and Decomposition, ed. E.B. Damm and M. Merwin, TMS/ISS, Warrendale, PA, USA, 2003, pp. 505-522. • John G. Speer, David V. Edmonds, Fernando C. Rizzo, David K. Matlock, Current Opinion in Solid-State and Materials Science, 8 (2004) 219-237

4. C = C g i g g g Ac 3 Temperature g g B + “Conventional” Processing of Steels with g M S M F Time CC and Isothermal Transformations TRIP Steels

5. The “Q&P” Process Quenching and Partitioning Provisional US Patent Application: September, 2003

6. The “Q&P” Process Step 1. Austenitize or Intercritically Anneal g g a g g g • more austenite • lower Cg • - higher Ms • less austenite • higher Cg • - lower Ms

7. Austenitize + QuenchIntercritical Anneal + Quench g g a g g g Step 2. Cool (quench?) below Ms • Ms -TQ controls martensite formation • intercritical annealing has more stable austenite • and higher carbon martensite

8. g g g g g g Step 3. Diffuse Carbon from Supersaturated Martensite • Phase compositions change • Phase boundaries stationary

9. Q&P Process Schematic

10. New Processing Concept (Sheet, Bar,…etc) Use carbon partitioning intentionally… from partially transformed martensite to untransformed austenite. Usually precluded because carbide precipitation occurs during tempering of martensite. Result: carbon-enriched austenite

11. Thermodynamics of Carbon Partitioning

12. Important Questions How much can we enrich the austenite? That is…what are the “equilibrium” martensite and austenite compositions? Or…when does partitioning stop?

13. “True” Metastable Equilibrium g g + Fe3C g + a a Temperature a + Fe3C % Carbon

14. “True” Metastable Equilibrium

15. “True” Metastable Equilibrium CANNOT Apply!! g g + Fe3C g + a a Temperature a + Fe3C Xa Xalloy Xg % Carbon • The equilibrium phase fractions are fixed by the lever rule • The actual phase fractions were fixed by cooling below Ms!

16. A New Equilibrium Condition was Hypothesized • “Constrained Carbon Equilibrium” (CCE) • Iron atoms are completely immobile (the phase boundaries are stationary). • Carbon atoms are completely mobile. • Carbon diffuses until its chemical potential (activity) is equal in ferrite and austenite. • Assume…competing reactions are precluded by Si/Al

17. Properties of “Constrained Carbon Equilibrium” • Not a unique condition at any temperature! • Depends on initial phase fractions/compositions

18. Properties of “Constrained Carbon Equilibrium” T0 A3 - Austenite may be more enriched or less enriched than ortho- or para- equilibrium

19. Properties of “Constrained Carbon Equilibrium” Mass balance: Carbon Constrained Equilibrium:

20. Key Characteristics of CCE (Fe-0.5C) - Almost all of the carbon should partition to austenite - Enrichment levels are potentially very high

21. Example CCE Calculations - 1.0%C Initial Austenite

22. g a a We have all the pieces to predictmicrostructure… Example Steel Composition: C=0.15 Mn=1.0 Si=1.5 Ms(oC)=539-423(%C)-30.4(%Mn)-12.1(%Cr)-17.7(%Ni)-7.5(%Mo) Ms=445oC (Steel) Intercritical Annealing T=810oC fg~ 22% Cg~ 0.68 wt. % Ms~222oC fa~ 78% TIA=810oC

23. g a a g a a Quench T = 150 oC Fraction of Martensite (Koistinen and Marburger) Final Microstructure fg~ 10% fM~ 12% fa~ 78% Phase Compositions After 450oC Partitioning Cg~ 1.5% CM~ .0019% Tq=150oC Tp=450oC

24. Q&P Process Design Methodology • ASSUME: • - Complete partitioning of carbon to austenite • No competing reactions (carbide formation)

25. Calculations for Experimental Al-Steel (at QT) aIA=0.5

26. Martensite Formation During Final Quench

27. Calculated Final Austenite Fraction in High-Al Steel

28. Effect of Intercritical Annealing Step

29. Effect of Manganese Content

30. Effect of Carbon Content

31. Example of DICTRA Simulation Solid-Solid Phase Transformations in Inorganic Materials 2005 Edited by J. Howe TMS (The Minerals, Metals & Materials Society), 2005 CARBON ENRICHMENT OF AUSTENITE AND CARBIDE PRECIPITATION DURING THE QUENCHING AND PARTITIONING (Q&P) PROCESS F.C. Rizzo 1, D.V. Edmonds2, K. He 2, J.G. Speer3, D.K. Matlock3and A. Clarke 3 1Department of Materials Science and Metallurgy; Pontifícia Universidade Católica-Rio de Janeiro; RJ 22453-900, Brazil 2School of Process, Environmental and Materials Engineering; University of Leeds; Leeds LS2 9JT, United Kingdom 3Advanced Steel Processing and Products Research Center; Colorado School of Mines; Golden, CO 80401, USA

32. Simulation Conditions • Steel composition: • 0.19C-1.59Mn-1.63Si wt% • Heat treatment: • Fully austenitized at 900oC, quenched to 293oC to produce 68% martensite and partitioned at 400oC. The thickness of the ferrite and austenite plates used in the simulation were 0.30 and 0.14 microns, respectively (obtained by TEM).

33. Carbon Concentration Profiles for ferrite and austenite

34. Carbon concentration profiles in a and g during partitioning under CCE at 400C, for a 0.19C-1.59Mn-1.63Si steel

35. Average carbon concentration as a function of time for a (0.30m) and g (0.14m) plates during partitioning at 400oC

36. Variation of (a) carbon flux and (b) carbon activity at the interface during partitioning. Time plotted in a log scale

37. Carbon concentration (wt%) at a and g interfaces as a function of time during partitioning at 400 oC

38. Carbon flux and concentration in the center of (a) ferrite and (b) austenite plates as a function of time

39. CONCLUSIONS For the scale of microstructure investigated, carbon depletion from the ferrite during partitioning at 400C occurs quite rapidly, around 10-1 seconds, while the austenite takes much longer, around 10 seconds, to achieve a uniform concentration. Due to its rapid depletion, the carbon concentration in the center of the ferrite plate starts to decrease after 10-3 seconds. After this time the driving force for carbide precipitation is gradually reduced. Carbon enrichment of the austenite will promote, initially, a substantial increase in the carbon concentration at the interface and a progressive stabilization of the plate, advancing from the interface to the center. Full stabilization is achieved when the composition of the central region reaches a carbon concentration corresponding to room temperature Ms.

40. Some Q&P Experimental Results

41. TEM micrographs of the Q&P microstructure produced in a 0.19%C-1.59%Mn-1.63%Si TRIP steel composition (a) (b) Quenching to 260°C and partitioning at 400°C for 100 s: (a) bright-field image and (b) dark-field image using a (200) austenite reflection.

42. Total elongation vs. ultimate tensile strength for TRIP, Dual phase (DP), martensitic (M), and Q&P sheet steel products