EFFECTS OF SMALL AMOUNTS OF B, NB AND TI ADDITIONS ON NUCLEATION AND GROWTH-PROCESSES OF INTERMEDIATE TRANSFORMATION PRODUCTS IN LOW-CARBON3-PERCENT MN STEELS

The formation mechanism of intermediate transformation products and th e effects of B as well as carbide forming elements such as Cr, Nb and Ti on the isothermal transformation characteristics of low carbon 3 wt % Mn steels have been investigated and the following results have been obtained. (1) Widmanstatten ferrite and bainite exibit both the diffu sional and the displacive aspects in their formation processes. (2) Th e addition of the above described elements markedly retards the format ion of primary ferrite, Widmanstatten ferrite and bainite in the speci mens directly quenched to isothermal transformation temperatures from 1 200 degrees C, while the precipitation of coarse carbide particles i n austenite accelerates the following transformations. (3) Ferrite nuc leation on coarse carbide particles can be explained in terms of the d ecrease in the activation energy for nucleation by the lattice corresp ondence at the interface. (4) The coarse carbide particles precipitate d at austenite grain boundaries are related to the ferrite enclosing t hese particles with Kurdjumov-Sachs relationship.