Crystallography, Morphology, and Martensite Transformation of Prior Austenite in Intercritically Annealed High-Aluminum
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RODUCTION
LOW-ALLOY dual-phase (hereafter referred to as DP) steels are characterized by a microstructure consisting of fine recrystallized ferrite with evenly dispersed islands of martensite. This structure is typically developed by the annealing of a cold-rolled ferrite–pearlite microstructure at a temperature between Ac1 and Ac3 , followed by quenching to room temperature in a continuous annealing line. The phase fractions, morphology, and the crystallographic texture of the final DP product are inherited from the cold-rolled structure through ferrite recrystallization, austenite nucleation and growth, and finally martensitic transformation. The focus of this paper is on the nucleation and growth of austenite during intercritical annealing, with an emphasis on its morphology and crystallographic properties. Dilatometry heat treatments were carried out for two high-aluminum steels, followed by electron backscattered diffraction (hereafter referred to as EBSD) analysis. The contributions in this paper are as follows. It is shown that Markov clustering[1] combined with the iterative determination of the austenite–martensite
T. NYYSSO¨NEN, P. PEURA, and V.-T. KUOKKALA are with the Department of Materials Science, Tampere University of Technology, P.O. Box 589, 33101 Tampere, Finland. Contact e-mail: tuomo.nyyssonen@tut.fi Manuscript submitted February 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
orientation relationship (hereafter referred to as OR)[2] can be used to reconstruct the EBSD orientation map of austenite formed during intercritical annealing. The algorithm used for this purpose is described and made freely available. The accuracy of the OR determined from martensitic lath boundaries with the iterative method is discussed and compared with the OR observed directly between martensite and reconstructed austenite. Based on the reconstructed image maps and optical microscopy, the growth mechanisms prevalent in two intercritically annealed high-aluminum steels are identified, as well as the significant aspects affecting the martensite start temperatures determined through dilatometry. It is shown how the various ORs determined in this study deviate from the Kurdjumov–Sachs[3] orientation relationship.
II. INTERCRITICAL AUSTENITE MORPHOLOGY AND CRYSTALLOGRAPHY It has previously been reported by Garcia and DeArdo[4] that in a cold-rolled, 1.5 wt pct Mn steel, austenite preferentially nucleates at cementite particles on ferrite–ferrite grain boundaries. In various studies, the austenite grains have often been observed to bear a Kurdjumov–Sachs type orientation relationship with a neighboring ferrite grain.[5–7] Shtansky et al.[5] reported that the growth direction of a nucleated austenite grain is then toward an adjacent neighbor with an incoherent phase boundary, which has greater mobility compared to an ordered, semicoherent interface. Austenite growth
is initially rapid,[4,5] controlled primarily by the diffusion of carbon, but at later stages slows down as interstitial alloying elements start to partition
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