The Behavior of Precipitates during Hot-Deformation of Low-Manganese, Titanium-Added Pipeline Steels
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IT is well documented that the formation and segregation of a variety of precipitates during continuous casting have significant effects on microstructural development and, consequently, on the mechanical properties of the steel product during and after hot rolling.[1,2] Manganese, in particular, is of great significance because MnS precipitates deform plastically along the rolling direction, resulting in poor toughness in the material.[3,4] During solidification, manganese tends to segregate toward the centerline, and large manganese sulfides form in this region. Therefore, if the segregation of manganese can be reduced, then it follows that the formation of MnS precipitates in the centerline regions will also be reduced. To achieve such a goal, it is vitally important to improve the understanding the mechanism and rate of MnS precipitation during solidification, however, importantly, the behavior of these precipitates during hot rolling. Several attempts, including improved casting conditions as well as appropriate adjustments to the chemical composition of steel, for instance the addition of microalloying elements such as titanium, niobium, and chromium,[5–7] have been made to reduce centerline segregation of precipitates in continuously cast slabs. Many attempts have been made in the past to encourage the formation of small, preferably spherically shaped and evenly distributed precipitates across the slab in a homogeneous manner.
ALI DEHGHAN-MANSHADI, Research Fellow, and RIAN J. DIPPENAAR, Professor, are with the Faculty of Engineering, University of Wollongong, Wollongong 2500, NSW, Australia. Contact e-mail: [email protected] Manuscript submitted July 7, 2010. Article published online October 28, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
Despite progress toward encouraging more uniform distribution of precipitates and to reduce the number of large sulfide precipitates forming at the centerline, industrially produced steel slabs still contain these large precipitates in segregated areas.[8] The most common and obvious strategy to reduce the precipitation of sulfide inclusions at the centerline of steel slabs is to reduce the sulfur content of the steel. However, desulfurization is costly and time consuming, adding significantly to production cost. An alternative approach to the reduction of manganese sulfide precipitation at the centerline is to decrease the manganese content of the steel, which is usually in the range of 1.0 to 1.5 in the high-strength pipeline steels[9,10] to about 0.3 pct. The important contribution of manganese to solid solution strengthening can be then compensated for by the addition of small amounts of alloying elements, such as Ti, Cr, and Nb. Such elements can also combine with manganese and sulfur to form sulfides that are harder than MnS. Therefore, those precipitates will deform less plastically during deformation of steel. The new precipitates can still prevent the hot shortness (i.e., formation of FeS). The sulfides of these alloying elements are usually smaller than MnS pre
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