INFLUENCE OF MESOTEXTURE ON EFFECTIVE GRAIN SIZE AND MICROSTRUCTURE-TOUGHNESS RELATIONSHIP IN THERMOMECHANICALLY CONTROL-ROLLED (TMCR) MICROALLOYED STEELS

摘要

Prediction of impact toughness in ferrite-pearlite structural steels requires on input of overage ferrite grain size. This is due to the fact that impact energy values comprise of both crack initiation and propagation components and that propagation energy may be influenced by crack path deviations at groin boundaries. However, it has been shown that single cleavage facets in thermomechanically control rolled (TMCR) microalloyed steels can consist of multiple groins with low angle boundaries (up to 12 degrees) between them. Thus the average microstructural unit experienced by the crack front (i.e.. the cleavage facet) is significantly larger than the overage metallographic, 2-D grain size. It is shown in this paper that 2-D grain size is insufficient information in TMCR microalloyed steels for the prediction of toughness. In these steels it is necessary for adequate toughness predictions to use an "effective grain size" based on the metallographic grain size and mesotexture. This paper also shows that the existing models for fracture stress predict values close to the measured fracture stess when both crack initiation in carbide and the crack propagation through grain boundaries are considered. A normalized steel has also been studied in order to compare and contrast the predictability of Charpy toughness in different microstructures.