ultra fine grained steels
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ultra fine grained steels ultra fine grained steels Presentation Transcript

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