Multiphase Structures in Case Hardening Steels following Continuous Cooling H. Roelofs, S.Hasler, L....

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

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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 !