Third Generation 0.3C-4.0Mn Advanced High Strength Steels Through a Dual Stabilization Heat Treatment: Austenite Stabili
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TRODUCTION
IT is generally agreed that for producing so-called third generation Advanced High Strength Steel (AHSS) with a better combination of strength and ductility than first generation steels, and which can be produced at a lower cost than second generation AHSS, it is necessary to stabilize a high volume fraction of retained austenite in a low-alloy steel, without the use of significant and expensive manganese and nickel alloying elements.[1,2] It is also clear that third generation AHSS should, where possible, utilize carbon as an economical austenite stabilizer,[3–7] recognizing that a high overall carbon content in the steel may be detrimental to weldability. Speer and coworkers[8–11] suggested a quench and partition (Q&P) process to address this problem by producing AHSS with a large quantity of carbon-enriched retained austenite in low-carbon alloys; it was suggested that this could be achieved through austenite stabilization by carbon uptake following carbon rejection from supersaturated martensite under paraequilibrium conditions.[9] This process requires that carbide formation and diffusional decomposition of austenite be suppressed by suitable alloying elements and heat treatment. The concept of paraequilibrium carbon partitioning during the Q&P process involves carbon diffusion from martensite to austenite following the martensitic transformation. The paraequilibrium concept[12] is now recognized as a constrained metastable equilibrium[13] that refers to conditions where the diffusion of substitutional HAO QU, formerly Graduate Student with Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106, is now Engineer with Momentive Performance Materials Inc., 22557 W Lunn Rd, Strongsville, OH 44149. GARY M. MICHAL, formerly Professor with Case Western Reserve University, is now deceased. ARTHUR H. HEUER, Professor, is with Case Western Reserve University. Contact e-mail: [email protected] Manuscript submitted September 29, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
elements is effectively frozen but the chemical potential of carbon in two or more phases can be equilibrated by relatively rapid diffusion of interstitial carbon. Although Q&P is one of the more promising methods for producing AHSS with improved strength and ductility, as shown in Figure 1,[14–21] the preferred heat treatment yielding the maximum amount of retained austenite is not clearly established for any particular steel. In addition, previous quench temperature selection methods based on the model of the kinetics of carbon partitioning[15,19,21,22] have poor predictability. Furthermore, the ideal constrained carbon-partitioning conditions have not been elucidated and the maximum extent of carbon enrichment in the austenite remains unknown. One recent study showed a good prediction of retained austenite content in Q&P steel through incorporating carbon partitioning from martensite to austenite across low-mobility martensite–austenite interfaces.[23–25] In the present work, simple modifications of the carbon-partitioning kinetic
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