EFFECTS OF WIDMANSTATTEN FERRITE ON THE MECHANICAL-PROPERTIES OF A 0.2-PCT-C 0.7-PCT-MN STEEL

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925-degrees-C or 1150-degrees-C to produce nominal austenite grain sizes of 60 and 200 mum, respectively. The plates were then cooled at rates in the range of about 2-degrees-C/min to 400-degrees-C/min to produce mixed polygonal ferrite/Widmanstatten ferrite/pearlite microstructures. The percentage of Widmanstatten structure (a Widmanstatten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and Widmanstatten structure. Both yield and tensile strength increase with an increase in the volume fraction of Widmanstatten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/Widmanstatten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.