Precipitation Strengthening by Induction Treatment in High Strength Low Carbon Microalloyed Hot-Rolled Plates
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OVER the past several decades, the requirements for high strength plates have increased, especially in fields where weight reduction is always a priority, such as ship construction and car manufacturing. In response to those demands, new advanced steels and optimized thermomechanical processing schedules are being developed.[1] High strength low alloy steels (HSLA) and advanced thermomechanical controlled processes might be an option for improving the final levels of properties in hot-rolled plates. Using microalloyed steels and the appropriate thermomechanical treatment for processing plates can result in products with a good balance between strength and toughness due to the sum of the contributions of different strengthening mechanisms, such as grain size refinement, solid solution,
G. LARZABAL, N. ISASTI, J. M. RODRIGUEZ-IBABE, and P. URANGA are with the CEIT, Manuel Lardizabal 15, 20018 Donostia-San Sebastian, Basque Country, Spain and also with the Universidad de Navarra, Tecnun, Manuel Lardizabal 13, 20018 Donostia-San Sebastian, Basque Country, Spain. Contact e-mail: [email protected] Manuscript submitted September 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitation hardening, and dislocation strengthening. Three HSLA steels microalloyed with Nb, NbMo, and TiMo combinations were selected for this work. Each addition promotes different improvements in the steel. As it is well known, Nb is able to induce strain accumulation in the austenite prior to transformation,[2] thereby providing important microstructural refinement, and Mo addition enhances the formation of complex non-polygonal transformation products.[3] TiMo microalloyed steels have an interesting combination of high strength and good formability because of a wide dispersion of nanometer-sized precipitates in the ferritic matrix.[4] Recently, special efforts have been made to develop new thermomechanically controlled processes that promote an additional strengthening contribution making sure that the market requirements for high strength plates are met. The mechanism of precipitation strengthening has been widely employed in thin strip products through the optimization of the coiling strategy.[5,6] However, precipitation strengthening in thick plates is less frequently used owing to the limitation of precipitation during continuous cooling after hot rolling.[7] In the current context, the application of induction heat treatments instead of conventional heat treatments might be an option for enhancing the final mechanical properties in hot-rolled plates. Similar approaches have
already been applied to different products and steel grades.[8,9] A subsequent fast induction heat treatment could generate new nano-precipitates that would increase the yield strength of the hot-rolled plates without inducing any major change in the hot-rolled microstructure and without affecting productivity. This approach has previously been proposed for conventional heat treatment of thick plates in Nb microalloyed steels.[10,11] However, specific application
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