The Dependence of Fracture Resistance on the Size and Distribution of Blocky Retained Austenite-Martensite Constituents
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ADVANCED high-strength steels (AHSSs) have been investigated for decades, which are driven by the continuously increasing requirement for weight reduction and safety from automotive industry. AHSSs have been developed from the first-generation AHSSs (for example, dual-phase steel[1] and transformation-induced plasticity (TRIP) steel[2]), second-generation AHSSs (such as twinning-induced plasticity steel[3,4]) to third-generation AHSSs (for instance, carbide-free bainite (CFB) steel[5] and quenching and partitioning (Q & P) steel[6]). Researchers have studied the enhancement of tensile properties by adjustment of microstructures via the development of various thermo-mechanical processes.[7–10] Since the proposal of CFB steel by Bhadeshia et al.[10] and Q & P steel by Speer et al.,[9] they have been intensively studied. For Q & P steels, the DEZHEN YANG and ZHIPING XIONG are with the School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China and also with the National Key Laboratory of Science and Technology on Materials under Shock and Impact, Beijing 100081, P.R. China. Contact email: [email protected]; [email protected] Manuscript submitted August 31, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
effect of quenching temperature, partitioning temperature and time on microstructure (especially for retained austenite, RA), and tensile properties has been systematically investigated,[6,11] which has shown an ultimate tensile strength (UTS) of 1000 to 2000 MPa and a total elongation (TE) of 10 to 30 pct.[12] For CFB steels, the evolution of microstructure and tensile properties with chemical composition, isothermal holding temperature, and holding time has also been thoroughly explored, leading to an extreme high UTS of 1500 to 2500 MPa and a good toughness of 5 9 105 to 1.2 9 106 J/ m2.[5,13–15] However, the fracture resistance of AHSSs has not attracted enough investigations. Jacques et al.[16] demonstrated a higher transformation rate of RA-to-martensite during double edge-notched tension (DENT) testing than uniaxial tension testing. They concluded that a larger amount of higher carbon RA improved tensile properties but led to a lower fracture toughness due to a network distribution of brittle martensite and high-strength bainite in the low-alloyed multi-phase TRIP steels. In addition, their following paper demonstrated a monotonic increase of the fracture toughness at cracking initiation, quantified by J-integral and essential work of fracture (EWF), with decreasing volume fraction of RA; however, the fracture toughness was independent of RA stability because of
its full transformation in the fracture process zone.[17] de Diego-Calderon et al.[18] showed that the RA greatly improved crack growth resistance (J0.2 = 3.13 9 104 to 5.42 9 104 J/m2) in Q & P steels but its volume fraction (7 to 24 pct) did not significantly affect initial fracture toughness (Jc = 0.87 9 104 to 1.01 9 104 J/m2). Wu et al.[19] demonstrated that martensite transformation of less st
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