Taneo Hatomura

EFFECT OF ACCELERATED COOLING CONDITION ON FERRITIC AND SUBSEQUENT SECOND PHASE TRANSFORMATION FROM DEFORMED AUSTENITE

ABSTRACT This paper describes the result of a study concerning transformation behavior of an accelerated cooling after controlled rolling. In the conventional accelerated cooling, when a finish-cooling temperature is lowered below 450°C, the tensile strength (T.S.) is increased accompanying with decrease in the yield strength (Y.S.) due to the formation of martensite as a second phase. On the other hand, by applying the new cooling pattern, in which the cooling in ferritic transformation range is accelerated and subsequent cooling in bainitic transformation range is furthermore controlled, both T.S. and Y.S. are increased and the impact toughness is not deteriorated. The improved combination of T.S., Y.S., and toughness, which can not be realized by conventional accelerated cooling, is thus obtained. This behavior can be explained in terms of both the ferrite grain refinement and the formation of lower bainite-like transformed products, which are transformed from 500°C to 400°C as the second phase.