Model Fe-Al Steel with Exceptional Resistance to High Temperature Coarsening. Part I: Coarsening Mechanism and Particle
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conventional approach for producing steel plate involves thick slab casting, reheating, and hot rolling. A recent alternative is the process of Thin Slab Casting Direct Rolling (TSCDR). The advent of TSCDR offers several advantages including reduced capital, energy, labor, and inventory costs.[1,2] However, the elimination of slab reheating and roughing deformation leaves fewer opportunities for grain refinement due to the small number of deformation passes available in the TSCDR process (5 to 7 passes). Studies of TSCDR show that thermomechanical processing can reduce the average grain size, but cannot eliminate the large grains present at the center of the slab, especially for heavy gage products.[3,4] This makes it extremely important to control the as-cast microstructure and the grain size evolution at TIHE ZHOU, Post-Doctoral Fellow, HATEM S. ZUROB, Associate Professor, and KASHIF REHMAN, PhD Student, are with the Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7 Canada. Contact e-mail: [email protected] RONALD J. O’MALLEY, Professor, is with the Department of Metallurgical Engineering, Missouri S&T, 284 McNutt Hall 1400 N. Bishop Ave., Rolla, MO 65409-0340. Manuscript submitted May 10, 2014. Article published online October 17, 2014 178—VOLUME 46A, JANUARY 2015
high temperatures in order to achieve acceptable final mechanical properties. Second phase particles have been used extensively to control the kinetics of grain growth at high temperature during TSCDR. In microalloyed steels, for example, dispersions of nano-scale Ti, V, and/or Nb carbonitrides are employed to limit grain growth.[5–7] When the steel is held at high temperature for an extended period of time, however, these precipitates become ineffective at pinning grain growth for two reasons: First, these nano-scale particles coarsen rapidly because of their small size and as a consequence the pinning force drops very rapidly. Secondly, most of these fine particles will dissolve at very high temperatures. In some cases, it is possible to use more stable precipitates, such as TiN. The problem then is that these particles form in the liquid and coarsen quickly to a large size which limits their effectiveness at pinning grain growth. Some authors proposed the use of oxide dispersions to pin grain growth at high temperatures.[8,9] While promising results have been obtained using this approach, the main challenges are how to produce fine dispersions of these particles in the caster. In the absence of particle pinning, austenite grains in excess of 2 mm may form prior to the onset of thermomechanical processing.[10,11] This work proposes a novel alternative, in which, a two-phase mixture of austenite and delta-ferrite is utilized to control the grain size evolution at high METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature. The basic concept is to design steels in which austenite and delta-ferrite co-exist over a wide temperature range. The particles of the minor phase would then pin grains of
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