1. Understanding Transformations in Copper Bearing Low Carbon Steels Teruhisa Okumura Department of Materials Science and Metallurgy University of Cambridge 23 June 2003

2. Introduction ● Copper-bearing low-carbon steel ● Aim and method ● Transformations in steel ● Experimental procedure ● Results and discussion ● Conclusion ● Future work

3. Copper-Bearing Low-Carbon Steel Low Carbon- High Toughness - Good Weldability Copper - High Strength (Precipitation Hardening) Without heat treatments!

4. Aim and Method Understand transformations and precipitation Predict mechanical properties Reduce development cost and term

5. Transformations in Steel Fe    +   +  wt% C Fe-C Alloy Fe 3 C Temperature  +  Time  P ww Displacive  ' ' Bs Ws Ms bb Reconstructive Cooling curve CCT diagram Ae 3 Ae 1

6. Rolling Conditions Average cooling rate 0.4 K s -1 (1073 - 773 K) Reheating 1573 K 1 hr

7. Chemical Compositions Chemical compositions were varied to determine the optimum value. wt% C Si MnCu Ni 0.02-0.06 0.10-0.15 1.00-1.50 0.1-1.2 0.1-1.2 Mo Cr V B 0.0-0.3 0.0-0.3 0.00-0.08 0-11 ppm

8. Experimental Results

9. Lab. Experiment Micrography UTS 730 MPa vEo 9 J 670 MPa 10 J 671 MPa 108 J 509 MPa 271 J 40  m Toughness  w fraction UTS

10. Mill Experiment Micrography Chemical segregation of substitutional alloying elements (Mn) Solute enriched region Widmanstätten ferrite Solute depleted region allotriomorphic ferrite UTS 604 MPa vEo 256 J

11. Cu precipitates Solute enriched region (BF) Solute depleted region (BF) Solute enriched region (DF)

12. Interphase Precipitation Quasi-Ledge MechanismLedge Mechanism    

13. Copper Precipitation Fe    + Cu  +  wt% Cu Fe-Cu Alloy Cu Temperature  + Cu Time   -Cu  w w Cooling curve

14. Conclusion In Copper-bearing low-carbon steels, ● Correlation between UTS and toughness. ● Chemical segregation in the real products which may increase  fraction. ● Interphase Cu-precipitates which may reduce growth rates.

15. Future Work Established transformation model Extended transformation model Copper precipitation Microstructure Solute enriched region Solute depleted region Mechanical properties Physical and neural network modelling