Phase Field Modeling of Microstructure Banding in Steels
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INTRODUCTION
MICROSTRUCTURAL banding is a phenomenon that is of increasing concern in advanced high strength low-carbon steels because of their higher alloying content (e.g., for Mn) and the potentially adverse effect of banding on mechanical properties.[1] The phenomenon has been known for more than half a century.[2–5] The root cause of banding is microsegregation of substitutional alloying elements during dendritic solidification. The alloying elements with low partition ratios, i.e., less than unity (e.g., P, Nb, Si, Mn, and Cr) are rejected from the first formed d-ferrite dendrites during casting, which results in the formation of interdendritic regions of higher solute content. The segregation is aligned into longitudinal bands during hot rolling, and upon subsequent cooling of the steel from austenite (c), alternating bands of proeutectoid ferrite (a) and pearlite (or martensite) may form.[6] The quantity of the alloying content is an important factor for segregation. Therefore, Mn that is usually present in rather high concentrations (1 to 2 wt pct) plays an important role in segregation and the associated banding phenomenon despite its relatively high partition ratio (e.g., compared with P and Nb).[1] MEHRAN MAALEKIAN, R&D Manager, is with AIM Metals & Alloys LP, Montreal, H1E 2S4, Canada. HAMID AZIZI-ALIZAMINI, formerly Research Associate with The University of British Columbia, Vancouver, V6T 1Z4, is now Post Doctoral Fellow with McMaster University, Hamilton, Canada, and MATTHIAS MILITZER, Professor, is with The University of British Columbia. Contact e-mail: [email protected] Manuscript submitted November 17, 2014. Article published online November 16, 2015 608—VOLUME 47A, JANUARY 2016
Mn stabilizes austenite and lowers both equilibrium and continuous cooling transformation temperatures, Ae3 and Ar3, respectively. For example, an increase in Mn content from 0.7 to 1.0 wt pct lowers the Ae3 temperature by 13 K (13 C) in a 0.17 wt pct C steel. Thus, during cooling of a steel with bands of high and low Mn content, ferrite tends to form in low Mn bands. Carbon is rejected from ferrite as it grows, thereby leading to carbon enrichment of austenite in high Mn regions which then transform to pearlite and/or other transformation products (bainite, martensite). From a technological perspective, ferrite/pearlite banding may cause hydrogen-induced cracking and may lead to a reduction in impact toughness.[2] In addition to microsegregation level, other parameters such as austenite grain size and cooling rate also influence the development of microstructural banding. The first quantitative description of band formation was presented by Kirkaldy et al.,[7] who developed a semi-empirical expression for the critical cooling rate necessary for suppression of intense banding in terms of the equilibrium transformation temperature difference between layers (e.g., high and low Mn bands), the mean carbon diffusion coefficient, and the average band spacing. Three decades later, Grossterlinden et al.[2] developed a finite differ
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