Effect of Surfactant Te on the Formation of MnS Inclusions in Steel
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MNS inclusions are not uncommon in commercial steels. Since the 1930s, effect of MnS inclusions on steel service properties has been widely investigated. MnS inclusions can facilitate the nucleation of intragranular acicular ferrite (IAF),[1] improve steel machinability,[2] and reduce crack formation by coating hard non-metallic inclusions.[3] However, MnS inclusions also adversely affect steel properties. They reduce steel toughness after hot rolling especially in the transverse direction due to their highly deformable nature.[4–6] Moreover, MnS inclusions, especially of large size, can trigger pitting corrosion of stainless steels.[7,8] These effects are closely related to the size, morphology, and spatial distribution of the inclusions. According to Sims and Dahl,[9] the morphology of MnS inclusions formed during solidification can be classified as randomly distributed globular form (Type I), rod-like or sheet-like forms (Type II), and angular
LICHUN ZHENG is with the Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, Box 2450, 3001, Leuven, Belgium and also with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan. Contact e-mail: [email protected] ANNELIES MALFLIET, PATRICK WOLLANTS, BART BLANPAIN, and MUXING GUO are with the Department of Materials Engineering, KU Leuven. Manuscript submitted September 6, 2016. Article published online August 14, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
form (Type III). Later, this classification was further refined.[10–13] Type I sulfides are frequently observed in rimming steels, where the O content is high.[14,15] Type II sulfides are located in the interdendritic regions, showing a chain-like pattern in two dimensions. They can also appear in bead-like form, depending on the contents of Mn and S.[12,16] Type II sulfides are dominant in amount.[10,13] Both their formation by a monotectic reaction[12] and by a eutectic reaction[9–11,13] have been proposed. The first mechanism has been ruled out because the melting point of MnS [1893 K (1620 C)][17] is much higher than that of iron [1808 K (1535 C)].[18] It is now generally accepted that Type II sulfides are formed via a eutectic reaction in the interdendritic regions where Mn and S contents are sufficiently high due to segregation. Type III sulfides are the result of a divorced eutectic reaction related to the presence of some alloying elements at high contents, such as C, Si, and Al.[10,11] Type II sulfides are the most susceptible to deform into stringers during hot rolling causing severe problems. Type I sulfides are the least deformable.[14,19,20] Therefore, significant effort has been invested to globularize MnS inclusions by deep desulfurization,[21] rapid solidification,[22] heat treatment,[23] and by adding surfactants, such as Se and Te.[24–27] In sulfur-containing free-machining steel, surfactant Te has been used as an additive to improve the machinability of the steel.[24–27] The most widely accepted mechanism behin
