Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon, Manganese Transformation-Induced
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I.
INTRODUCTION
TRANSFORMATION-INDUCED-PLASTICITY (TRIP) steel is a representative high-strength steel that utilizes phase transformation to control the mechanical properties.[1–5] Strain-induced martensitic transformation of metastable austenite plays a major role in improving the mechanical balance (tensile strength 9 elongation), allowing TRIP steel to be actively applied in the automotive industry. Currently, the tensile strength of commercially produced TRIP steel reaches approximately 1000 MPa. However, when the tensile strength exceeds 800 MPa, the elongation tends to decrease to less than 15 pct and the mechanical balance is significantly deteriorated.[6,7] Recently, Matlock et al. suggested a guideline for improving the mechanical balance of higher-strength multiphase steel, taking into account the characteristics of constituting phases.[8] They emphasized that a microstructural control ensuring higher stability as well as a sufficient fraction of austenite would be essential to obtaining a higher tensile strength with desirable elongation. Low-carbon, manganese TRIP steel (Mn TRIP steel) based on an alloy system of Fe-0.1C-5Mn was first DONG-WOO SUH, Associate Research Professor, is with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, 790-784 Korea. Contact e-mail: [email protected] SEONG-JUN PARK and TAE-HO LEE, Senior Researchers, and CHANG-SEOK OH and SUNG-JOON KIM, Principal Researchers, are with the Structural Materials Division, Korea Institute of Materials Science, Changwon, Kyungnam, 641-010 Korea. Manuscript submitted April 3, 2009. Article published online December 3, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
introduced by Miller.[9] A retained austenite fraction of 20 to ~40 pct with optimized stability made it possible to exhibit an excellent mechanical balance after intercritical annealing. However, a prolonged heat treatment using a batch-type annealing process was required to obtain the desired properties; thus, the more modern continuousannealing conditions were not tested.[9–12] In the present study, we investigated the influence of Al on the microstructural evolution and mechanical behavior of Mn TRIP steel using the continuous-annealing process in order to obtain an exceptional mechanical balance. The Mn TRIP steels with different Al contents are prepared and heat treated under continuous-annealing conditions. The recrystallization of the cold-rolled structure consisting of martensite or ferrite and the formation of austenite during intercritical annealing are investigated, and the mechanical behavior of annealed sheets is examined with respect to the fraction and stability of retained austenite.
II.
MICROSTRUCTURAL DESIGN USING AL ADDITION
The microstructure of conventional TRIP steel consists of polygonal ferrite, bainitic ferrite, and retained austenite. This microstructure is obtained by means of a two-step heat treatment that includes intercritical annealing and austempering. During the intercritical annea
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