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 investigate d with respect to structure coarseness. Large variation in impact resistanc e between heats is observed, which can not be explained in terms of an aver age grain size. This has been of major technical concern for the Swedish st eel industry. The EHS steels are purely ferritic. The high strength (HS) st eels have a complex microstructure consisting of primary ferrite and one or more other phases: pearlite, acicular ferrite, or ferrite with second phas e. Primary ferrite grain sizes as well as the cell size of pearlite, acicul ar ferrite and ferrite with second phase are measured to describe structure coarseness. Image analysis is successfully used to separate primary ferrit e and other microstructure constituents. Intercept length based size parame ters are utilized and size distributions are given. A generalized grain siz e is determined from the slope of the size distributions. This grain size i s used to interpret the toughness properties. The results show that differe nces in the size of the larger ferrite grains explain the variation in toug hness in both HS and EHS steels. (C) Elsevier Science Inc., 1999. All right s reserved.