Effect of heat treatment and cleanness of ultra low carbon bainitic (ULCB) steel on its impact toughness.

The small variations in sulphur and carbon concentrations can have a major influence on the impact transition temperature (ITT) of ultra low carbon HSLA100 steel which has been quenched in water and tempered (WQ&T). Since the average carbon concentration is very low thus sensitivity of ITT to heat treatment parameters depends also on the yield strength increase due to precipitation effect of epsilon_Cu phase. The regression analysis has been used to establish equations taking into account those parameters. The properties of a mixed microstructure formed from partially austenitic regions have been also considered. The fine austenite grains transform into more desirable fine bainitic ferrite phases with lower hardness values and higher toughness. On the other hand, if cooling rate is sufficiently large, then the carbon enriched austenite transforms partially into hard martensite and some of the remaining untransformed austenite being retained to ambient temperature. Because hard martensite islands are located in much softer surroundings consisting of tempered ferrite, they do not cause a general reduction in impact toughness tests. Due to further grain refinement of microstructure the measured toughness on Charpy V specimens can be very high at low temperatures. The very detrimental effect of sulphur in ULCB steel has been confirmed by presented results.