Effects of martensite morphology and volume fraction on quasi-static and dynamic deformation behavior of dual-phase stee
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I. INTRODUCTION
DUAL-PHASE steels composed of ferrite and martensite have a better deformability than other high-strength low alloy (HSLA) steels with similar strength.[1–5] They are also characterized as having a continuous yielding during plastic deformation, whose strength increases with increasing the martensite volume fraction in accordance with a rule of mixtures like in composite materials.[2] Several important points regarding the deformation mechanism in dual-phase steels have been documented. Fischmeister and Karlsson[6] reported that the load transfer of each phase is the primary factor determining strength. Gurland[7] calculated the load transfer stress in relation with microscopic parameters. Interpretation of the fracture behavior of dual-phase steels has been based on the observation of fracture surfaces. Stevenson[8] reported that cracks initiate first in martensites under low strains and then propagate into ferrites. On the other hand, according to Kim and Thomas,[9] cleavage fracture occurs at ferrites in a coarse martensitic structure, whereas voids initiate at ferrite/martensite interfaces in a fine martensitic structure. Suh et al.[10] observed from the in situ scanning electron microscope (SEM) fracture test that the deformation behavior varies with the martensite morphology. These studies on dual-phase steels have been primarily concerned with phenomena occurring only under either static or quasi-static loading, but very few studies have been done on the their dynamic deformation and fracture behavior. The present study is concerned with investigating the quasi-static and dynamic deformation mechanisms of dualphase steels fabricated by different heat-treatment conditions. Torsional tests were conducted on four dual-phase steel specimens having different martensite morphology and SANGHO KIM, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. Manuscript submitted June 22, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
volume fraction both under quasi-static and dynamic loading. Their deformed microstructures and fracture surfaces were observed to study various factors determining the deformation characteristics. II. EXPERIMENTAL An HSLA steel bar (diameter: 25 mm) with a tensile strength level of 500 MPa was used in this study, and its chemical composition is 0.21C-0.21Si-0.72Mn-0.01P0.01S-1.1Cr-Fe (wt pct). In order to obtain dual-phase steels with different morphologies, two kinds of heat treatments were used. For the intermediate quenching treatment (referred to as “IQ” hereafter), specimens were held at 1000 8C for 1 hour and subsequently quenched into water, while for the step quenching treatment (referred to as “SQ” hereafter), specimens are heat treated directly into the (a 1 g) region after the austenitization. By varying the annealing temperature in the (a 1 g) region, four dual-phase steel specimens of 770 IQ, 790 IQ, 740 SQ, and 760 SQ, which have different marte
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