Low Density High Strength and High Toughness Steel with Ferrite and Martensite Laminated Structure

The impact toughness of high strength steel decreases with increasing strength in general and thus leads to extensive research to improve the toughness of high strength steel. In this study a series high strength low carbon and low alloy steels alloyed with manganese and aluminum were designed, hot deformed in the ferrite and austenite dual phase region and air cooling down to room temperature. The microstructure observation indicates that the hot deformation introduces a fibered ferrite phase into the martensite matrix in the bar samples or laminated ferrite phase into the martensite matrix in the plate samples. Charpy V-notch toughness measurements at room temperature reveal that the designed steels are remarkably enhanced high up to 150-250Jin bar steel and 200-450J in plate steel with tensile strength varied from 1.0-1.5GPa, both of which are nearly 10 times higher than that of the conventional low alloy steels with similar strength level. This finding gives us another way to produce high strength steel with ultrahigh impact toughness by simple alloying design and hot deformation in the ferrite and austenite dual phase region.