multiphase structures in case hardening steels following continuous cooling h. roelofs, s.hasler, l....
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Multiphase Structures in Case Hardening Steels following Continuous Cooling
H. Roelofs, S.Hasler, L. Chabbi, U. Urlau, Swiss Steel AGJ. Chen, H.K.D.H. Bhadeshia, University of Cambridge
pearlite
ferrite
bainite
Typical steel structure of a MnCr-case hardening steel How to master structure
and hardness by rolling conditions and steel composition?
• measuring CCT diagrams
• hot rolling trials
• simulating the steel‘s microstructure on a computer
„Complete Calculation of Steel Microstructure of Strong Alloys“
Excellent agreement for isothermal treatments
Development of new products
Chemical composition (C, Mn, Si, Ni, Mo, Cr, V)
Cooling rate
Austenitic grain size (AGS)
Steel Structure
Modified Avrami model including simultanuous nucleation and growth rates of
• allotriomorphic ferrite, • pearlite, • widmanstätten ferrite,• bainite and • martensite
The model
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 (m): 13, 23
C Si Mn Ni Cr Mo0.17 0.19 1.20 0.076 1.05 0.008
as measured as calculated
Comparison for AGS = 13 m
as measured as calculated
Comparison for AGS = 23 m
Furnace
Precision Sizing Mill
Cooling Bed (16 – 64 mm)
Descaler
Shears
Coilers Garrrett
(16 – 38 mm)
Finishing Mill Stelmor (5.5 – 16 mm)
Water Boxes
Industrial trials: layout of hot rolling mill
• 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
Challenges under real industrial conditions
Stelmor line Positions of sampling: 6.5 mm wire
sampling
phase transformation
Furnace
Precision Sizing Mill
Finishing Mill Stelmor
Water Boxes
Distance from surface Austenitic grain size (m)
X Min Max Stddev
surface: 0 20.3 15.2 26.3 2.8 mid-radius 1/2 25.4 22.7 29.4 2.7
centre: 1 36.2 26.3 55.7 8.0
Average over
radius 27.3 21.4 37.1
C Si Mn Ni Cr Mo 0.16 0.21 1.21 0.13 0.98 0.015
Steel composition (wt.-%):
Cooling rate (K/s): 0.34 K/s (800-500°C)
mean AGS as calculated by hot rolling simulation model: 33.8 m
Input data for modeling
Comparison calculation versus experiment 6.5 mm wire
Garrett line Position of sampling: 23.5 mm wire
sampling
Furnace
Precision Sizing Mill
Coilers Garrrett
Water Boxes
Distance from surface Austenitic grain size (m)
X Min Max Stddev
surface: 0 26.0 22.7 31.3 4.0 mid-radius 1/2 31.8 26.3 35.7 3.7
centre: 1 49.5 31.3 167.1 20.9
Average over
radius 35.8 26.8 78.0
C Si Mn Ni Cr Mo 0.20 0.19 1.23 0.11 1.00 0.017
Steel composition (wt.-%):
Cooling rate (K/s): 0.16 K/s (800-500°C)
Input data for modeling
large scattering AGS due to the hot rolling deformation
Comparison calculation versus experiment 23.5 mm wire
Bar production line Position of sampling: 36 mm
FurnacePrecision Sizing Mill Cooling Bed
Water Boxes
C Si Mn Ni Cr Mo 0.17 0.19 1.22 0.09 1.03 0.018
Steel composition (wt.-%):
Cooling rate (K/s): 0.80 K/s (800-500°C)
sampling
Distance from surface Austenitic grain size (m)
X Min Max Stddev
surface: 0 19.3 14.1 28.3 4.2 mid-radius 1/2 47.8 27.8 71.4 13.2
centre: 1 66.9 29.4 250.7 31.8
Average over
radius 44.7 23.8 116.8
excellent
good
modest
Comparison calculation versus experiment 36 mm bar
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)
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.
High strength bainitic steel Stelmor line, 13 mm wire rod
cooling rate ~ 3.0 K/s
very fine steel structure
C Si Mn Cr Mo 0.15 % 1.19 % 1.50 % 1.19 % 0.31 %
0
50
100
150
200
23°C -20°C -40°C
Ch
arp
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pac
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ISO
-V in
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Rp0.2 = 1001 MPa
Rm = 1135 MPa
A5 = 15 %
Z = 68 %
High strength bainitic steel Stelmor line, 13 mm wire rod
Estimated cooling rate ~ 3.0 K/s
Thank you !
Muchas gracias !