Multiphase Structures in Case Hardening Steels following Continuous Cooling H. Roelofs, S.Hasler, L....
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Transcript of Multiphase Structures in Case Hardening Steels following Continuous Cooling H. Roelofs, S.Hasler, L....
![Page 1: 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.](https://reader030.fdocuments.in/reader030/viewer/2022032800/56649d4d5503460f94a2cb50/html5/thumbnails/1.jpg)
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
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pearlite
ferrite
bainite
Typical steel structure of a MnCr-case hardening steel How to master structure
and hardness by rolling conditions and steel composition?
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• 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
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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
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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 (m): 13, 23
C Si Mn Ni Cr Mo0.17 0.19 1.20 0.076 1.05 0.008
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as measured as calculated
Comparison for AGS = 13 m
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as measured as calculated
Comparison for AGS = 23 m
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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
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• 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
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Stelmor line Positions of sampling: 6.5 mm wire
sampling
phase transformation
Furnace
Precision Sizing Mill
Finishing Mill Stelmor
Water Boxes
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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
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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
Furnace
Precision Sizing Mill
Coilers Garrrett
Water Boxes
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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
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Comparison calculation versus experiment 23.5 mm wire
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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
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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
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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
C Si Mn Cr Mo 0.15 % 1.19 % 1.50 % 1.19 % 0.31 %
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0
50
100
150
200
23°C -20°C -40°C
Ch
arp
y im
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
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Thank you !
Muchas gracias !