Effects of Prestraining on High Cycle Fatigue Strength of High Strength Low Alloy TRIP Steels
Sung-Moo SONG, Koh-ichi SUGIMOTO, Shinsuke KANDAKA, Asahiko FUTAMURA, Mitsuyuki KOBAYASHI and Setsuya MASUDA
Abstract:The effects of prestraining on high cycle fatigue strength of newly developed low alloy TRIP steels with different matrix structure and different retained austenite characteristics were investigated for the automotive applications. The prestraining to 10% in tension increased fatigue limit of the TRIP steels, especially in steel with polygonal ferrite matrix. It was considered that the polygonal ferrite matrix in the steel brought high fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix, resulting from hard second phase on prestraining. On the other hand, the TRIP steel with bainitic ferrite lath matrix exhibited a small increment in fatigue limit after prestraining. This was supposed to be mainly owing to strain-induced martensite content increased by prestraining, with small effects of TRIP of retained austenite and compressive internal stress in the matrix. In addition, a very interesting finding was obtained that the internal stress is the most effective parameter among some parameters to increase the fatigue strength in the low alloy TRIP steels. Key Words:High cycle fatigue, Prestrain, Low alloy TRIP steel, High strength steel, Transformation induced plasticity, Retained austenite, Internal stress