MicroNiobium Alloy Approach in Medium and High Carbon Steel Bar, Plate and Sheet Products
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TION
THE evolution of the MicroNiobium Alloy Approach relates to stabilization of the abnormal austenite grain growth that occurs during inhomogeneous heating of the steel slabs, billets, or blooms during the industrial reheat furnace operation. The MicroNiobium Alloy Approach mechanism relates to the integration of the physical metallurgy of abnormal grain growth, the Nb carbon nitride-pinning effect, and the influence of the process metallurgy of reheat furnace temperature practices, variability, and thermal inhomogoneity. The connection between inefficient slab, billet, and/or bloom reheat performance and the consequential result of mechanical property variability in the final hot-rolled product due to variations in grain size is often not related to both the laboratory and especially, in industrial operations. Transfer of these reheat time and temperature data from the laboratory experiment to actual industrial furnace operation are quite difficult to incorporate into the mill model. Considerable amounts of time and metallurgical resources involved in the study of chemistry thermomechanical controlled processing (TMCP) rolling regimes and thermal practices are considered as the root cause of the problem in not achieving desirable mechanical properties. However, the root cause problem often relates to the inhomogeneity of the initial furnace reheat
STEVEN G. JANSTO, Technical and Market Development Manager, is with the CBMM-North America, Inc., 1000 Old Pond Road, Bridgeville, PA 15017. Contact e-mail: [email protected] Manuscript submitted January 22, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
operation and thermal condition of the slab before hot rolling. This lack of homogeneous heating results in variations in austenite grain size, which translates into increased cost of diverts and scrap during the hot-rolling operation as well as variability in the final ferrite grain size and resultant mechanical properties.[1] These additional costs of quality are enormous. The mechanism of abnormal grain growth is well established and documented.[2] However, the relationship between reheat furnace temperature, solubility of the alloying elements at different carbon contents in actual industrial operations is difficult to correlate with laboratory studies and generated data. The traditional approach aims at maximizing the solubility of niobium (Nb) in carbon steels to ensure the alloy’s effectiveness in grain refinement, recrystallization kinetics, and subsequent precipitation, especially in low-carbon steels. Investigations of the effect of Nb in steel at very low concentrations have been quite limited.[3] In addition, due to limited solubility of Nb in these higher-carbon steels (exceeding 0.20 pct C), the application of Nb was not considered as a cost-effective solution. However, recent research and industrial trials have disproved this universal conclusion.
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MICRONIOBIUM APPLICATIONS
Most recently, the MicroNiobium Alloy Approach at Nb concentrations from 0.005 to 0.020 pct Nb have been successfully applied to the foll
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