Effect of V Precipitation on Continuously Cooled Sulfur-Lean Vanadium-Alloyed Steels for Long Products Applications
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INTRODUCTION
BOTH strength and toughness are important properties for heavy steel products used for structural applications such as buildings, bridges, offshore platforms, etc. The only strengthening mechanism that adds both strength and toughness to the steel is grain refinement. Today it is well documented that a high level of grain refinement can be achieved in microalloyed steels by proper thermomechanical treatment.[1–5] However, ferrite grain refinement in heavy shapes cannot easily be achieved during rolling and, thus, toughness cannot be improved, and there are several reasons why thicker sections give rise to coarser ferrite grains. There is usually a smaller overall deformation in transforming slab/beam blank than thick plate/beam. This limits the degree of austenite grain refinement that can be achieved. This condition is becoming increasingly severe with the tendency to apply near-net-shape casting processes.[6] Recently, continuous casting of beam blanks was successfully applied industrially.[7] This evolution was motivated by quality, financial, and environmental needs. The process, first used for light sections, is progressively extended to higher thickness ranges. Moreover, the rolling process is completed usually at higher temperatures, thus developing a larger austenite grain size. A reduction of the rolling temperature brings an increase in rolling loads and many mills are not designed to handle the additional stresses. Another justification to avoid low-temperature-controlled rolling is that a holding time is usually incorporated in the rolling schedule, which increases the rolling time and reduces the productivity of the mill. C. CAPDEVILA, Research Scientist, C. GARCI´A-MATEO, Tenured Scientist, J. CORNIDE, Postdoctoral Student, J. CHAO, Researcher, and F.G. CABALLERO, Research Scientist, are with the Materalia Research Group, Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalu´rgicas (CENIM-CSIC), E-28040 Madrid, Spain. Contact e-mail: [email protected] Manuscript submitted November 12, 2010. Article published online May 17, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
On the other hand, the slower cooling rate in thicker sections will always tend to contribute to a coarser ferrite grain size for a given prior austenite grain size. The development of accelerated cooling lines allowed refining of the microstructure in heavy plates and long products.[8] To overcome these difficulties, a strong structural refinement, by introducing acicular ferrite microstructure, combined with an optimum cooling process, can be used to develop tough, high-strength thick products. This is the goal of this article. Acicular ferrite, far from being organized, is better described as chaotic. The plates of acicular ferrite nucleate heterogeneously intragranularly and radiate in many different directions.[9,10] It is believed that propagating cleavage cracks are frequently deflected as they cross an acicular ferrite microstructure with many different orientations. These give rise to superior mechanica
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