Journal
ISIJ INTERNATIONAL
Volume 54, Issue 4, Pages 955-962Publisher
IRON STEEL INST JAPAN KEIDANREN KAIKAN
DOI: 10.2355/isijinternational.54.955
Keywords
fracture toughness; ultrahigh-strength steel; TRIP-aided steel; microstructure; retained austenite; martensite; bainitic ferrite
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Funding
- Adaptable and Seamless Technology Transfer Program through Target-riven R&D from the Japan Science and Technology Agency
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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 degrees C and 350 degrees 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 (K-IC = 129-154 MPa m(1/2)) than conventional structural steel such as SCM420 steel (K-IC = 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.
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