Influence of Mixed Grain Size Distributions on the Toughness in High and Extra High Strength Steels

Continuously cast high and extra high strength (EHS) steels are investigated with respect to structure coarseness. Large variation in impact resistance between heats is observed, which can not be explained in terms of an average grain size. This has been of major technical concern for the Swedish steel industry. The EHS steels are purely ferritic. The high strength (HS) steels have a complex microstructure consisting of primary ferrite and one or more other phases: pearlite, acicular ferrite, or ferrite with second phase. Primary ferrite grain sizes as well as the cell size of pearlite, acicular ferrite and ferrite with second phase are measured to describe structure coarseness. Image analysis is successfully used to separate primary ferrite and other microstructure constituents. Intercept length based size parameters are utilized and size distributions are given. A generalized grain size is determined from the slope of the size distributions. This grain size is used to interpret the toughness properties. The results show that differences in the size of the larger ferrite grains explain the variation in toughness in both HS and EHS steels.