Microstructure and Mechanical Behaviors of 0.1C-13Mn Metastable Austenitic Steel
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Grain refinement from a reverse transformation of martensite to austenite is reported to be useful to improve the mechanical properties of metastable austenitic steels.[1–3] The enhanced work-hardening characteristics during deformation are ascribed to the strain-induced martensite transformation of metastable austenitic phase. However, a wide application has been limited due to a considerable amount of Ni and Cr, which deteriorated the cost competitiveness. In this study, the fabrication of metastable austenitic structure through reverse transformation is attempted in the alloy system without Ni and Cr. Based on the Fe-Mn-C system, the metastable austenitic structures and their mechanical behavior are examined. The investigated alloy is 0.1C-13Mn steel (C: 0.08, Mn: 12.8, in wt pct). As the Ms temperatures of metastable austenitic steels containing Ni or Cr were calculated to be 60 C to 100 C, the Mn content in the investigated steel was controlled to keep the calculated Ms temperature approximately 100 C from following formula:[4]
DONG-WOO SUH and SEONG-JUN PARK, Senior Researchers, and SUNG-JOON KIM, Principal Researcher, are with the Department of Advanced Metallic Materials, Korea Institute of Materials Science, Changwon, Kyungnam, 641-010, Korea. Contact e-mail: [email protected] CHANG-HOON LEE, Senior Researcher, is with the Technical Research Laboratories, POSCO, Pohang, Kyungbuk, 790-785, Korea. Manuscript submitted July 31, 2008. Article published online December 17, 2008 264—VOLUME 40A, FEBRUARY 2009
Ms ð CÞ ¼ 539 423½wt pct C 30:4½wt pct Mn 7:5½wt pct Si þ 30½wt pct Al
½1
After hot rolling of vacuum-melted ingot to 4.5 mm thickness, the sheets were cold-rolled to 1.5 mm in thickness followed by annealing for 10 minutes at 580 C, 600 C, 620 C, and 640 C, which are close to the Ae3 temperature (605 C) calculated with the CALPHAD method.[5] The annealing was performed using an infrared heating furnace with heating and cooling rates of 1 C/s. The microstructure of annealed sheet was observed with a scanning electron microscope equipped with an electron backscattered diffraction (EBSD). The phase fraction was determined by X-ray diffraction (XRD) using Cu Ka radiation. Integrated intensities of (110)a, (200)a, (200)c, (220)c, and (10.0)e, (10.1)e, (10.2)e reflections were used for the calculation.[6] The mechanical properties of the steel were examined using a universal tensile testing machine at a crosshead speed of 2 mm/min with standard test coupons.[7] Figure 1(a) shows the XRD profiles of hot-rolled and cold-rolled sheets. It reveals that austenite and e-martensite remaining in hot-rolled sheet transform to a¢-martensite during cold rolling owing to the straininduced transformation. The reappearance of austenite peaks in Figure 1(b) implies that a¢-martensite is converted to austenite during annealing. For annealing temperatures of 580 C and 600 C, which are lower than the Ae3 temperature, a¢-martensite still remains even though the austenite is primary constituting phase. Meanw
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