Understanding Transformations in Copper Bearing Low Carbon Steels

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

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

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

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

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

Rolling Conditions Reheating 1573 K 1 hr Average cooling rate 0. 4 K s-1 (1073 - 773 K)

Chemical Compositions Chemical compositions were varied to determine the optimum value. C Si Mn Cu Ni 0. 02 -0. 06 0. 10 -0. 15 1. 00 -1. 50 0. 1 -1. 2 Mo 0. 0 -0. 3 Cr 0. 0 -0. 3 V B 0. 00 -0. 08 0 -11 ppm wt%

Experimental Results

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

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

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

Interphase Precipitation Ledge Mechanism Quasi-Ledge Mechanism

Copper Precipitation Temperature Cooling curve + + Cu -Cu w + Cu Fe wt% Cu Fe-Cu Alloy Cu Time

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.

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