A Quantitative Study of Interphase Precipitation in a Commercial Microalloyed Steel

The role of interphase precipitation as a strengthening mechanism in microalloyed steels is not very well understood. It is known to be important, but there is a lack of quantitative information due to the fact that interphase precipitation occurs in only part of the ferrite grains. This paper presents an investigation 'grain-by-grain' of a commercial hot strip steel, in an attempt to determine the number of ferrite grains strengthened by interphase precipitation, and thus to quantify the real effect of this precipitation mode on yield strength. Microhardness measurements and transmission electron microscopy observations were employed as the most important experimental techniques, both showing that interphase precipitation had occurred in about one half of the total number of ferrite grains. Well established theoretical models were used to determine the extent of precipitation and dislocation hardening. Somewhat similar contributions of between 50 to 100 MPa were found to come from interphase precipitation, precipitation in austenite and dislocation strengthening. Interphase precipitation and dislocation strengthening were strongly reduced by a subsequent normalizing heat treatment.