ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Fracture Toughness of an Advanced Ultrahigh-strength TRIP-aided Steel
Junya KobayashiDaiki InaAsahiko FutamuraKoh-ichi Sugimoto
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2014 Volume 54 Issue 4 Pages 955-962

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Abstract

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 m1/2) than conventional structural steel such as SCM420 steel (KIC = 57–63 MPa m1/2). 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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© 2014 by The Iron and Steel Institute of Japan
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