On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades
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INTRODUCTION
The term ‘steel cleanness’ describes the number, size, chemical composition, and morphology of nonmetallic inclusions distributed in the steel matrix. Over many decades, intensive research on the improvement of steel cleanness, focusing on the reductions of inclusion size and frequency, has been performed. However, despite these efforts, the presence of nonmetallic inclusions in the steel matrix cannot be avoided completely. In general, nonmetallic inclusions are seen to negatively affect steel properties, such as ductility, fatigue strength and corrosion resistance.[1,2] Murakami,[3] for example, performed exhaustive work to describe the effect of nonmetallic inclusions on fatigue strength evaluating different influencing factors like inclusion shape and size, adhesion of inclusions to the matrix, and elastic constants of inclusion and matrix. However, specifically tailored inclusions can also be used for the creation of a favorable microstructure with enhanced properties. At the beginning of the 1990s, Takamura and Mizoguchi[4]
DENISE LODER, SUSANNE KATHARINA MICHELIC, ALEXANDER MAYERHOFER, and CHRISTIAN BERNHARD are with the Chair of Ferrous Metallurgy, Montanuniversitaet Leoben, 8700 Leoben, Austria. Contact e-mail: susanne.michelic@unileoben. ac.at Manuscript submitted December 21, 2016. Article published online May 4, 2017. 1992—VOLUME 48B, AUGUST 2017
introduced the concept of Oxides Metallurgy, which focuses on the use of inclusions to improve the final product quality—predominantly steel toughness—by controlling the inclusions’ nature and distribution and thereby influencing the microstructure. Nonmetallic inclusions are described as the major cause for the nucleation of the high-toughness phase acicular ferrite.[5,6] Acicular ferrite nucleates intragranularly on nonmetallic inclusions. The formed plates radiate in various directions and form a fine-grained, interlocking microstructure.[7] Due to the chaotic arrangement of acicular ferrite grains, mechanical properties, especially fracture toughness, are significantly increased. This effect was first observed for weld metals.[8] Comprehensive reviews on the formation of acicular ferrite have been done by Bhadeshia[9] as well as Sarma et al.[10] Up to now, C-Mn-(Ni) weld metals[11–15] were the most investigated material related to the formation of acicular ferrite. The nucleation of acicular ferrite is mainly influenced by the steel composition, cooling rate, prior austenite grain size, and inclusion landscape. In the recent decades, considerable research has been conducted to evaluate different inclusion types regarding their capability for acicular ferrite: Ti-containing inclusions such as TiN and TiC,[16] titanium oxides[17–20] partly in combination with rare earth metals,[17] or Manganese[19] have been intensively studied. The effects of many inclusion types have already been well described in the
METALLURGICAL AND MATERIALS TRANSACTIONS B
literature, with some popular types, however, being left out, with conflicting results. Apart from weld metals, mo
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