Multiphase Structures in Case Hardening Steels following Continuous Cooling

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. Bhadeshia, University of Cambridge

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

3. 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 • measuring CCT diagrams • hot rolling trials

4. Modified Avrami model including simultanuous nucleation and growth rates of • allotriomorphic ferrite, • pearlite, • widmanstätten ferrite, • bainite and • martensite The model Chemical composition (C, Mn, Si, Ni, Mo, Cr, V) Cooling rate Austenitic grain size (AGS) Steel Structure

5. C Si Mn Ni Cr Mo 0.17 0.19 1.20 0.076 1.05 0.008 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

6. Comparison for AGS = 13 mm as calculated as measured

7. Comparison for AGS = 23 mm as calculated as measured

8. Finishing Mill Stelmor (5.5 – 16 mm) Cooling Bed (16 – 64 mm) Furnace Shears Descaler Coilers Garrrett (16 – 38 mm) Water Boxes Precision Sizing Mill Industrial trials: layout of hot rolling mill

9. 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

10. Finishing Mill Stelmor Precision Sizing Mill Furnace Water Boxes phase transformation Stelmor line Positions of sampling: 6.5 mm wire sampling

11. Steel composition (wt.-%): Cooling rate (K/s): 0.34 K/s (800-500°C) C Si Mn Ni Cr Mo 0.16 0.21 1.21 0.13 0.98 0.015 Input data for modeling mean AGS as calculated by hot rolling simulation model: 33.8 mm

12. Comparison calculation versus experiment 6.5 mm wire

13. Coilers Garrrett Precision Sizing Mill Water Boxes Furnace Garrett line Position of sampling: 23.5 mm wire sampling

14. Steel composition (wt.-%): Cooling rate (K/s): 0.16 K/s (800-500°C) C Si Mn Ni Cr Mo 0.20 0.19 1.23 0.11 1.00 0.017 Input data for modeling large scattering AGS due to the hot rolling deformation

15. Comparison calculation versus experiment 23.5 mm wire

16. Steel composition (wt.-%): Cooling rate (K/s): 0.80 K/s (800-500°C) C Si Mn Ni Cr Mo 0.17 0.19 1.22 0.09 1.03 0.018 Water Boxes Precision Sizing Mill Furnace Cooling Bed Bar production line Position of sampling: 36 mm sampling

17. Comparison calculation versus experiment 36 mm bar excellent good modest

18. 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)

19. 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.

20. High strength bainitic steel Stelmor line, 13 mm wire rod cooling rate ~ 3.0 K/s very fine steel structure

21. 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 %

22. Thank you ! Muchas gracias !