Multiphase Structures in Case Hardening Steels following Continuous Cooling

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• Multiphase Structures in Case Hardening Steels
  following Continuous Cooling
• H. Roelofs, S.Hasler, L. Chabbi, U. Urlau, Swiss
  Steel AG
• J. Chen, H.K.D.H. Bhadeshia, University of
  Cambridge

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Typical steel structure of a MnCr-case hardening
steel
How to master structure and hardness by rolling
conditions and steel composition?
ferrite
pearlite
bainite
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Development of new products

• measuring CCT diagrams
• hot rolling trials

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The model
Chemical composition (C, Mn, Si, Ni, Mo,
Cr, V) Cooling rate Austenitic grain
size (AGS)
Steel Structure
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Dilatometry experimental conditions
Steel composition (wt.-) Cooling rates
(K/s) 0.2, 0.3, 0.6, 0.8, 1.0, 1.6, 3.0,
6.0 Austenitic grain size (mm) 13, 23
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Comparison for AGS 13 mm
as calculated
as measured
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Comparison for AGS 23 mm
as calculated
as measured
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Industrial trials layout of hot rolling mill
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Challenges under real industrial conditions

• sampling for AGS determination is sometimes
  difficult
• AGS and phase contributions vary over the
  cross- section of the wire/bar
• the cooling curve is not linear

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Stelmor line Positions of
sampling 6.5 mm wire
sampling
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Input data for modeling
mean AGS as calculated by hot rolling simulation
model 33.8 mm
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Comparison calculation versus experiment 6.5
mm wire
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Garrett line Position of
sampling 23.5 mm wire
sampling
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Input data for modeling
large scattering AGS due to the hot rolling
deformation
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Comparison calculation versus experiment 23.5
mm wire
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Bar production line Position of
sampling 36 mm
sampling
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Comparison calculation versus experiment 36 mm
bar
excellent good modest
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Conclusions

• the predictions of the model agree qualitatively
  well with the measured data from industrial
  hot rolling distinguishing ferrite, pearlite and
  remaining phases
• there is a first indication that the model
  underestimates bainite and overestimates
  Widmannstätten ferrite (this is still under
  investigation)

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Outlook
The quantitative analysis of phase contributions
in modern high-strength bainitic-martensitic
steels is a very challenging task. Together
with advanced EBSD techniques this model will in
future be applied to bring more light into the
microstructure of such steels.
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High strength bainitic steel
Stelmor line, 13 mm wire rod
cooling rate 3.0 K/s
very fine steel structure
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High strength bainitic steel
Stelmor line, 13 mm wire rod
Estimated cooling rate 3.0 K/s
Rp0.2 1001 MPa Rm 1135 MPa A5 15 Z
68
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Thank you ! Muchas gracias !