TRIP강의 기계적 특성에 미치는 잔류오스테나이트 안정도의 영향

Abstract

Dynamic mechanical properties of TRIP steels with the same volume fraction but different stabilities of retained austenite were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 10^(-2) to 6×10²s^(-1) and ultimate tensile strength and uniform elongation were evaluated. Two steels (0.10 and 0.15C) were processed to have TRIP microstructures containing different austenite carbon levels, and thus stabilities. Low stability retained austenite needs less mechanical driving force to transform to martensite, and thus the retained austenite in the low stability microstructures is found to transform earlier in the deformation process. Consequently, a higher initial strain hardening rate leading to higher strength but lower uniform elongations were found when compared to the conditions with higher stability retained austenite.

Strain control during high speed tensile testing was accomplished using a "stopper" attachment designed to limit strain within the gage section to an amount preset before testing. Strain was controlled successfully up to the highest strain rate examined, 200s^(-1). The methodology allowed, for the first time, the extent of austenite transformation to be monitored at incremental strains during a high-rate test. The extent of transformation at low strains appeared to increase slightly at increased strain rates, presumably due to the higher flow stresses (greater mechanical driving force) encountered.

At all strain rates, the extent of martensite transformation was considerable after only a few strain percent. The volume fractions of retained austenite in the deformed samples measured by neutron diffraction were a few percent higher than those measured by X-ray diffraction. The greater sensitivity of neutron diffraction may be useful in future studies to examine transformation of austenite late in the deformation process, when the remaining amount of austenite is low.