Abstract
780MPa class high strength steel has a problem concerning about work hardenability because of many dislocations in the martensite matrix due to the mechanism of transformation itself. In previous report1), it is pointed out that poor work hardenability may decrease ductile crack initiation property. For this problem, three metallurgical methods were investigated aiming to improve uniform elongation. ([1]increasing Fe3C, [2]increasing Ni, [3]refinement of γ grain). In this paper, two newly developed steels are manufactured in laboratory scale taking those methods into consideration. Property of ductile crack initiation of developed steels is evaluated in two ways. One is the method by small round bar tensile specimen. The other is the method by specimen supposed to be real penstock structure. Finally, according to these two tests, superior properties of work hardenability and ductile crack initiation are clearly observed in developed steels. Through this study, it is known that increasing intragranular pile-up site on the assumption of martensite structure can control property of ductile crack initiation.
