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Minas Gerais, Brazil

The production of defect free continuously cast microalloy Nb-bearing steel slabs, blooms and billets have increased in importance for both the producer and the end user. The minimization of surface and internal defects has a substantial impact on steel producing operating costs, internal and external cost of quality and delivery performance. A keen and thorough understanding of the hot ductility behavior of various steel grades is essential to successfully melt and cast high quality microalloy steels. This paper describes current steelmaking and casting considerations and recommendations to successfully cast high quality niobium-bearing slabs, billets and blooms. This study also integrates the process metallurgy and the physical metallurgy to better understand the hot ductility mechanisms and process metallurgy control practices in use today for the high quality production of Nb-bearing steels. © 2011 Allerton Press, Inc.

Misra R.D.K.,University of Louisiana at Lafayette | Jia Z.,University of Louisiana at Lafayette | O'Malley R.,LLC Sheet Mill | Jansto S.J.,CBMM Co.
Materials Science and Engineering A | Year: 2011

We describe here the precipitation behavior of copper and fine-scale carbides during thermo-mechanical processing and isothermal aging of copper-bearing niobium-microalloyed high strength steels. During thermo-mechanical processing, precipitation of e{open}-copper occurs in polygonal ferrite and at the austenite-ferrite interface. In contrast, during isothermal aging, nucleation of e{open}-copper precipitation occurs at dislocations. In the three different chemistries investigated, the increase in strength associated with copper during aging results only in a small decrease in impact toughness, implying that copper precipitates do not seriously impair toughness, and can be considered as a viable strengthening element in microalloyed steels. Precipitation of fine-scale niobium carbides occurs extensively at dislocations and within ferrite matrix together with vanadium carbides. In the presence of titanium, titanium carbides act as a nucleus for niobium carbide formation. Irrespective of the nature of carbides, copper precipitates and carbides are mutually exclusive. © 2011 Elsevier B.V.

Jia Z.,University of Louisiana at Lafayette | Misra R.D.K.,University of Louisiana at Lafayette | O'Malley R.,Nucor Steel Decatur LLC | Jansto S.J.,CBMM Co.
Materials Science and Engineering A | Year: 2011

We describe here the precipitation behavior and mechanical properties of 560MPa Ti-Nb and 770MPa Ti-Nb-Mo-V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo2C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4-20nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [001]NbC//[001]α-Fe, implying that the precipitation of NbC occurred during austenite-ferrite transformation. © 2011 Elsevier B.V.

Jansto S.G.,CBMM Co.
Materials Science and Technology Conference and Exhibition 2010, MS and T'10 | Year: 2010

Value-added applications of niobium (Nb) microalloyed steels continue to be developed meeting the increasing material demands within the automotive, pipeline and structural carbon steel segments. High quality production of these value-added Nb-bearing steels is realized through the synergistic balance of the process and physical metallurgy. Process metallurgy practices presented include development of the steel chemistries, proper melting and alloy additions, secondary refining, slab and billet reheat furnace operation, and alternative hot rolling considerations to successfully obtain a fine grain, homogeneous microstructure within production environments. Case examples of the necessary process metallurgy practices to meet the physical metallurgy objectives are discussed incorporating current niobium-bearing steel product applications. A sustainability structural steel study recently completed presents the positive environmental impact of Nb-microalloyed steel applications as it relates to more effective product design, reduced steelmaking emissions and reduced energy consumption. Copyright © 2010 MS&T'10®.

Jansto S.G.,CBMM Co.
AIST Steel Properties and Applications Conference Proceedings - Combined with MS and T'11, Materials Science and Technology 2011 | Year: 2011

The application of niobium (Nb) in high carbon steels enhances both the metallurgical properties and processability products such as steel bar, sheet and plate. Such process and product metallurgical improvements relate to the Nb-pinning effect of the austenite grain boundaries in microalloyed 0.25 to 0.95%C steels during the reheat furnace process prior to rolling. Consequently, Nb microalloyed high carbon automotive and long product steel applications have been developed. The Micro-Niobium Alloy Approach© is described and correlated to a variety of high carbon steel grades and applications. Metallurgical Operational Implementation (MOI©)) links the product requirements to the mill capability and resultant process metallurgical implementation. This integrative approach connects the Nb process and physical metallurgy necessary to achieve the desired ultra-fine grain, homogeneous high carbon steel microstructures that exhibit superior toughness, strength, fatigue performance and weldability.

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