Effects of Prestraining on High-Cycle Fatigue Strength of High-Strength Low Alloy TRIP-Aided Steels

Abstract

The effects of prestraining on high-cycle fatigue strength of newly developed low alloy TRIP-aided steels with different matrix structures and different retained austenite characteristics were investigated for automotive applications. Prestraining to 10% in tension increased the fatigue limit of the TRIP-aided steels, especially in steel with a polygonal ferrite matrix. It was considered that the polygonal ferrite matrix brought on high a fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix resulting from a hard second phase on preslmining. On the other hand, the steel with bainitic ferrite lath matrix exhibited only a small increase inthe fatigue limit after prestraining. This was expected to be mainly due to "strain-induced martensite hardening" on prestraining, with small contributions of TRIP of 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 low alloy TRIP-aided steels.

Fatigue strength of newly developed high-strength low alloy TRIP-aided steels with good hardenability

“…According to Sugimoto et al, the TAM steel also exhibits high fatigue hardening [5] and high fatigue limit [4,6]. So, the steel is expected to be applied to automotive forging parts which need high notched fatigue strength.…”

Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

“…(iii) carbide-free and fine bainitic ferrite lath structure matrix without pro-eutectoid ferrite (iv) a long-range internal stress resulting from difference in flow stress between matrix structure and blocky second phase [5] It is expected that the (i), (iii) and (iv) mainly suppress a crack initiation and the (ii) mostly disturbs the crack propagation.…”

“…According to Sugimoto et al, [5] TBF steel is characterized by a continuous yielding and low yield stress. Also, the TBF develops a significant cyclic hardening due to mainly (i) an increase in the strain-induced transformation martensite fraction and (ii) a compressive long range internal stress in matrix structure.…”

“…So, commercial TBF steels with tensile strength of 980-1180 MPa are applied to some automotive members in Japan. The TBF steel also exhibits high fatigue limit [5] and low notch-sensitivity for fatigue, [6] as well as an excellent impact toughness [7] and high delayed fracture strength. [8] So, some applications to diesel engine common rail system which needs high inner pressure above 300 MPa can be expected, if the TBF steel has a high hardenability and high notch-fatigue limit.…”

Fracture strength and toughness of ultra high strength TRIP aided steels

“…Figure 16 [56] shows fatigue limit of three kinds of TRIP-aided steels with the same chemical composition of 0.17%C, 1.41%Si and 2.00%Mn before and after prestraining. These TRIP-aided steels represent higher fatigue limits than dual-phase (DP) steel.…”