The fracture toughness of an advanced ultrahigh-strength 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (in mass%) transformation-induced plasticity (TRIP)-aided steel with a bainitic ferrite and/or martensite structure matrix was investigated for applications in automobiles, construction machines, and pressure vessels. After the steel was austenitized and isothermally transformed via heat treatment at temperatures between 200°C and 350°C below the martensite-finish temperature, it exhibited a good combination of tensile strength (1.4 GPa) and total elongation (15%). In addition, the steel achieved a much higher plane-strain fracture toughness (KIC = 129-154 MPa m 1/2 ) than conventional structural steel such as SCM420 steel (KIC = 57-63 MPa m 1/2 ). Surprisingly, the fracture toughness was nearly the same as that of a maraging steel. Our results indicate that the high fracture toughness was associated with (1) a softened wide lath-martensite matrix with a low carbide content and carbon concentration and (2) effective plastic relaxation of localized stress concentration by the strain-induced transformation of fine metastable retained austenite in the narrow lath-martensite and retained austenite mixture, which suppresses void formation and cleavage crack initiation at the pre-crack tip.
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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.