Low-carbon cast microalloyed steel intercritically heat-treated at different temperatures: microstructure and mechanical properties

In this study, dual-phase (DP, ferrite + martensite) microstructures were obtained by performing intercritical heat treatments (IHT) at 750 and 800 degrees C followed by quenching. Decreasing the IHT temperature from 800 to 750 degrees C leads to: (i) a decrease in the volume fraction of austenite (martensite after quenching) from 0.68 to 0.36; (ii) similar to 100 degrees C decrease in martensite start temperature (Ms), mainly due to the higher carbon content of austenite and its smaller grains at 750 degrees C; (iii) a reduction in the block size of martensite from 1.9 to 1.2 mu m as measured by EBSD. Having a higher carbon content and a finer block size, the localized microhardness of martensite islands increases from 380 HV (800 degrees C) to 504 HV (750 degrees C). Moreover, despite the different volume fractions of martensite obtained in DP microstructures, the hardness of the steels remained unchanged by changing the IHT temperature (similar to 234 to 238 HV). Applying lower IHT temperature (lower fraction of martensite), the impact energy even decreased from 12 to 9 J due to the brittleness of the martensite phase. The results of the tensile tests indicate that by increasing the IHT temperature, the yield and ultimate tensile strengths of the DP steel increase from 493 to 770 MPa, and from 908 to 1080 MPa, respectively, while the total elongation decreases from 9.8 to 4.5%. In contrast to the normalized sample, formation of martensite in the DP steels could eliminate the yield point phenomenon in the tensile curves, as it generates free dislocations in adjacent ferrite.

Automated summary

By decreasing the IHT temperature, the volume fraction of martensite in DP steel decreases while the block size becomes smaller, leading to an increase in hardness but limited impact on material strength and hardness.