Microstructure Evolution, Mechanical Properties and Strain Hardening Instability of Low and Medium Carbon Quenching & Partitioning Steels

The effect of quenching after martensitic finish (QAM(f)) or quenching & partitioning (Q&P) on microstructure evolution, mechanical properties, and strain hardening instability of low and medium carbon hot rolled steels were investigated. Two heats of low and medium carbon steels were cast in an induction open furnace. The chemical composition of low carbon steel is 0.16C-0.27Si-1.47Mn-0.02Al while medium carbon steel is 0.49C-0.30Si-0.91Mn-0.03Al. They were hot-rolled at 1200 degrees C for 30 min followed by air cooling. The microstructure after hot-rolled gives bands of ferrite and pearlite for 0.16 wt% low carbon steel. On the other hand, 0.49 wt% medium carbon steel produces coarse pearlite islands surrounded by ferrite phase. To enhance mechanical properties, it was necessary to modify the microstructure of low and medium carbon steels using QAM(f) or Q&P processes. The resultant matrix of microstructure after QAM(f) and Q&P processes contained ferrite, bainite, lath martensite, and retained austenite for 0.16 wt% low carbon steel, and polygonal ferrite, lath martensite, and retained austenite for 0.49 wt% medium carbon steel, respectively. In low carbon steel, QAM(f) process increased uniform elongation from 6.6 to 13.5% (105% increase) while ultimate tensile strength (UTS) improved slightly from 645 to 692 MPa (7% increase). However, in medium carbon steel, Q&P reduced uniform elongation from 12.4 to 4.8% (61% decrease) while increased UTS from 769 to 1242 MPa (61.5% increase). It is worthy to mention that QAM(f) process exhibited strain hardening instability zone (7.8% strain before necking) compared to hot-rolled process (0% strain before necking). On the other hand, Q&P process highly decreased strain hardening instability zone (0.77% strain before necking) compared to hot-rolled process (3.4% strain before necking).