Abstract
Acicular ferrite is a microstructure nucleating intergranularly on non-metallic inclusions and forming an arrangement of fine, interlocking grains. This structure is known to improve steel properties, especially steel toughness, essentially. The formation of acicular ferrite is mainly affected by steel composition, cooling rate, inclusion landscape and austenite grain size. In recent decades, extensive research has been conducted to investigate these factors. The present paper provides an overview of the impact of published results and the state of knowledge regarding acicular ferrite formation. Special attention is paid to the effect of carbon, manganese and titanium addition to steel, as well as the optimum size, number and composition of non-metallic inclusions. In addition, the reactions during the nucleation and growth of acicular ferrite needles are briefly addressed. Further, characteristics of acicular ferrite and bainite are summarized, which should help to distinguish these similar structures.
Highlights
At the beginning of the 1990s, Takamura and Mizoguchi (1990) introduced the concept of Oxides Metallurgy, which focuses on the use of oxidic inclusions to improve the quality of the final product and stabilize the production process
Most of these early studies focused on weld metals (Abson, Dolby, & Hart, 1978; Bonnet & Charpentier, 1983; Cochrane & Kirkwood, 1978; Devillers, Kaplan, Marandet, Ribes, & Riboud, 1983; Düren, 1983; Evans, 1980, 1982, 1983; Ricks, Barritte, & Howell, 1981; Ricks, Howell, & Barritte, 1982; Saggese, Bhatti, Hawkins, & Whiteman, 1983), so using acicular ferrite to improve the toughness of weld fusion zones is quite popular today
Acicular ferrite has been of increasing interest to steel producers; the excellent combination of toughness and strength makes acicular ferrite favorable for HSLA steels
Summary
At the beginning of the 1990s, Takamura and Mizoguchi (1990) introduced the concept of Oxides Metallurgy, which focuses on the use of oxidic inclusions to improve the quality of the final product and stabilize the production process. They described, inter alia, that by controlling the nature and distribution of the inclusions, the formation of intragranular ferrite, the so-called acicular ferrite, can be promoted, which significantly increases the steel’s fracture toughness. Comprehensive efforts have been devoted to an explanation of acicular ferrite formation. The present work provides an overview of the state of knowledge on acicular ferrite formation and its four main influencing factors
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