INFLUENCE OF STEEL-MAKING VARIABLES ON NOTCH TOUGHNESS

The relative importance of the process variables seems to be dependent on the test criterion chosen: one variable, for example, would mainly improve fracture appearance, whereas another would mainly affect impact energy. This divergence of effect between process variables would explain the lack of correlation between different test-criteria in comparing plates that have been processed in different ways. For plates having intermediate quality with regard to notch toughness, this point is especially important, since in obtaining this quality usually only one or two of the known process factors are controlled in order to obtain the required properties. In other words, intermediate quality plates may be made in different ways; which way should be chosen by the steelmaker? The answer to this question depends to a large extent on the choice of test criterion. This choice, however, is still different for different users and classification societies, because apparently insufficient common knowledge exists about the correlation between test criteria and service behavior. Further investigation in this field would seem to be very necessary, especially since the best way to classify steels should be based, in the author's opinion, on requirements that can be measured on the finished product. Specification of manufacturing process by the user should be avoided because this may lead to uneconomic production and may not always be sufficiently adequate. Examples supporting this view are given. The following process variables are discussed:-chemical analysis (especially manganese and carbon content); plate thickness; finish-rolling temperature (controlled rolling practice); deoxidation (semi-killed versus AL killed; normalizing and cross rolling.)