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ultra fine grained steels


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ultra fine grained steels

  1. 1. ULTRA FINE GRAIN IN PLAIN C-MnSTEELS WITH 0.15-0.3% C<br />R. Song, D. Ponge, D. Raabe<br />
  2. 2. ?<br />Why do we study ultra fine grained steel?<br />
  3. 3. The reason is<br />The development of industry needs a steel with advanced mechanical properties<br />Grain refinement is the only method to improve both <br /> strength and toughness<br />That is because ......<br />
  4. 4. Hall-Petch relationship<br />Ferrite Grain Size, d (µm)<br /> 40 10 5 3 1<br />900<br />800<br />700<br />600<br />500<br />400<br />300<br />200<br /> 0<br />- 40<br />- 80<br />-120<br />-160<br />-200<br />-240<br />Ultra fine grain<br />Normalizing<br />TMCP<br />50% FATT, (0C)<br />Yield Strengh (MPa)<br />2 6 10 14 18 22 26 30 34 <br />d-1/2 (mm-1/2)<br />
  5. 5. How to get ultra fine grained steel?<br />
  6. 6. High demands on the novel UF routes<br />Niikura et al.<br />
  7. 7. Aims <br /><ul><li>To obtain UF grain in plain C-Mn steels
  8. 8. To determine the relationship between micro-structure and mechanical properties of UF grained steel
  9. 9. To consider the industrial applicability</li></li></ul><li>Considerations<br />Lower cost elements <br />Easy recycling<br />Plain C-Mn steels<br />Fine cementite dispersion in a ferritic matrix<br />Method of getting UF grain<br />Recrystallized ferrite microstructure<br />Industrially applicable process parameters <br />Applicability in industry<br />
  10. 10. The effect of microstructure on strength<br />Ferrite Grain Size, µm <br /> 20 10 5 2 1 0.5<br />800<br />700<br />600<br />500<br />400<br />300 <br />0.15C-0.3Si-1.5Mn Steel<br />ferrite+ cementite<br />ferrite + pearlite<br />Yeild Strengh, MPa <br />+>300MPa<br />Conventional Grain Size <br />Ultrafine Grain Size<br />104 106 108 1010<br />Number of Grains in 1 mm3<br />K. Nagai<br />
  11. 11. The PonyMILL processing route<br />Conventional Hot Mill Line<br />Coiler<br />Run out table<br />Coil Handling<br />Coil Transfer<br />PonyMILL<br />Single High Reduction Stand<br />Un-Coiler<br />Re-Coiler<br />
  12. 12. Contents<br /><ul><li>Experiment and materials
  13. 13. Results and discussion</li></ul>Optimum hot rolling conditions<br />The effect of heavy deformation / coiling temperature on microstructure<br />The effect of heavy deformation strain on microstructure <br />Micro-hardness measurement<br /><ul><li>Summary</li></li></ul><li>Materials<br /> <br />C<br />Si<br />Mn<br />P<br />S<br />Al<br />N<br />Tnr* ℃<br />Ae3 ℃<br />0.15C<br />.17<br />.22<br />0.76<br />.004<br />.004<br />.031<br />.001<br />899<br />834<br />0.2C<br />.22<br />.21<br />0.74<br />.004<br />.003<br />.029<br />.001<br />925<br />820<br />0.2CM<br />.23<br />.22<br />1.52<br />.004<br />.004<br />.030<br />.001<br />926<br />797<br />0.3C<br />.31<br />.22<br />0.76<br />.003<br />.003<br />.030<br />.001<br />963<br />798<br /> Chemical compositions (wt%),with calculated Tnrand Ae3<br />Tnr* : nonrecrystallization temperature. Mn has not been considered in the calculation<br /> Ae3 : calculated by Thermo-Calc<br />
  14. 14. Experiment machine<br />The Hot Working Simulator<br />(WarmUMformSImulator)<br />W<br />SI<br />UM<br />“WUMSI”<br />
  15. 15. Experiments from WUMSI<br />Microstructure Investigation<br />Cuboid Sample<br />
  16. 16. Experimental routes<br />hot deformation <br />(conventional hot strip mill) <br /> =0.3, =10s-1<br /> holding compression <br /> 2min =4×0.4, <br />=10s-1 air cooling <br /> simulated final coiling<br /> <br />A3<br />5~12℃/s<br />50℃/s<br />PF<br />BS<br />heavy warm deformation<br />(PonyMILL)<br />Pearlite route BainiterouteⅠ Bainiteroute Ⅱ<br />
  17. 17. Optimum austenite deformation temperature<br />Optimization of deformation temperature in austenite region (WUMSI)<br />Water quenched microstructure after deformation at 860℃ of 0.15%C steel<br />Tg=Ae3+100℃ for 3 min<br />air <br />Tde compression<br />=0.3, =10s-1<br />water<br />
  18. 18. Selection of cooling rate to get desired initial microstructure (F+P or B)<br />Experiment schedule <br /> (deformation dilatometry)<br /> Changes in microstructure and hardness of experimental steels with different cooling rates<br /> Tg =Ae3+100℃ for 3 min<br /> air compression<br />Ar3<br /> cooling<br /> 64...2℃/s<br />M+B+F<br />F+P +B +M<br />UTS,<br />F+P+B<br />F+P<br />
  19. 19. DCCT diagram of the steels<br />DCCT diagram (ferrite + pearlite region) of 0.15%C, 0.2%C and 0.3%Csteel<br />DCCT diagram of 2CMsteel <br />BR II<br />PR<br />BR I<br />
  20. 20. Starting temperature of heavy deformation<br />Effect of heavy deformation temperature on flow curves and temperature increase in 0.3%C steel<br />500℃de<br />600℃de<br />700℃de<br />730℃de<br />
  21. 21. The effect of heavy deformation temperature on microstructure<br />5000C-coiling 5500C-coiling 6000C-coiling 7000C-coiling<br />5500C 6000C 6400C 7000C<br />bainite route I<br />ND<br />bainite route II<br />
  22. 22. (a) grain size: 3.50µm<br /> (b) grain size: 1.25µm <br />The effect of heavy deformation temperature on the microstructure in 0.3%C steel<br />7000C<br />85-95% are high angle boundaries<br />5000C<br />
  23. 23. Typical microstructure<br />1m<br />0.3%C deformed at 6000C in BR II<br /> <br /> <br /><ul><li>small grains
  24. 24. equiaxed grains
  25. 25. homogeneous cementite distribution</li></li></ul><li>The effect of strain on the microstructure<br />C-C<br />2.494<br />C-C<br />2.415<br />Q-C<br />2.188<br />Q-C<br />1.946<br />Y<br />X<br />Z<br />Q-Q<br />C-Q<br />Q-C<br />C-C<br />aspect ratio<br />Centre-Centre (C-C)<br />Quarter-Centre (Q-C)<br />Quarter-Quarter (Q-Q)<br />Centre-Quarter (C-Q)<br />strain<br />Effect of different local strain on grain size and aspect ratio<br />strain<br /> PR-5000C BR I-5000C <br />
  26. 26. Microstructure evolution during compression in PR<br />short pearlitic fragments<br />pearlitic ferrite<br />compression<br />compression<br />pearlitic ferrite<br />pro-eutectoid ferrite with subgrains<br />pearlitic cementite lamella<br />pro-eutectoid ferrite<br />new ferrite grains<br />1m<br />1m<br />2m<br />
  27. 27. SEM micrographs of 0.3%C steel after bainite routeⅠ<br />Substructure in large grains<br />subgrains<br />large grain<br />Heavy deformation at 500℃ and subsequent simulated coiling at 700℃<br />
  28. 28. Low angle misorientation<br />
  29. 29.  <br /> * deformation temperature (PR and BR II) or simulated coiling temperature (BR I)<br />Micro-hardness for different routes<br />
  30. 30. Summary I<br /><ul><li>Optimum hot deformation temperatures have been determined to get fine and homogeneous austenite
  31. 31. Three new process routes for heavywarmdeformation have been designed and employed to obtain UFG steel
  32. 32. Lower heavy deformation/ coiling temperature: finer ferrite grains but higher aspect ratio</li></li></ul><li>Summary II<br /><ul><li>The alignment of cementite particles affects ferrite grain shape (more elongated)
  33. 33. Pearlitic / bainitic ferrite grains: smaller, relatively equiaxed</li></ul> Pro-eutectoid ferrite grains: larger, higher aspect ratio, composed of subgrains<br /><ul><li>UFG is effective to increase hardness</li></li></ul><li> references<br /><ul><li>R. Song, D. Ponge, R. Kaspar, D. Raabe: Z. Metallk. 95 (2004) 513517, Grain boundary characterization and grain size measurement in an ultrafine-grained steel
  34. 34. L. Storojeva, D. Ponge, D. Raabe, R. Kaspar: Z. Metallkunde 95 (2004) 1108-1114, On the influence of heavy warm reduction on the microstructure and mechanical properties of a medium carbon ferritic-pearlitic steel
  35. 35. R. Song, D. Ponge, D. Raabe, R. Kaspar: Acta Mater. 53 (2004) 845858, Microstructure and crystallographic texture of an ultrafine grained C-Mn steel and their evolution during warm deformation and annealing
  36. 36. R. Song, D. Ponge, D. Raabe: ScriptaMaterialia 52 (2005) 1075-1080, Improvement of the work hardening rate of ultrafine grained steels through second phase particles
  37. 37. R. Song, D. Ponge, D. Raabe: ISIJ International 45 (2005) 1721-1726, Influence of Mn Content on the Microstructure and Mechanical Properties of Ultrafine Grained C-Mn Steels
  38. 38. R. Song, D. Ponge, D. Raabe: Acta Mater. 53 (2005) 4881-4892, Mechanical properties of an ultrafine grained C­Mn steel processed by warm deformation and annealing
  39. 39. R. Song, D. Ponge, D. Raabe, J.G. Speer, D.K. Matlock: Mater. Sc. Engin. A 441 , 2006) 1–17, Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels</li>