Faculty Publications

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Optimization of Annealing Parameters for Ferritic Hot-Rolled IF Grade Steel
(Springer, 2022) Kumar, D.S.; Sambandam, S.; Bhat, K.U.
Ferritic hot rolling of low carbon steel is now widely adopted by steelmakers for reducing energy costs and increasing the yield. These ferritic hot-rolled coils carry forward different grain morphology and texture which result in variation in properties after cold rolling and annealing compared to austenitic hot-rolled coils. These ferritic hot-rolled coils require different annealing treatments based on the hot rolling temperature for better results. In the present work, a Nb–Ti stabilized interstitial free (IF) grade steel was hot rolled at two different temperatures in the ferritic regime in an industrial hot strip mill and was subsequently cold rolled. These cold-rolled sheets were subjected to different continuous annealing cycles with soaking temperatures varying from 740 to 820 °C on a thermo-mechanical simulator for optimization of temperatures and study its effect on microstructure and properties. These coils were compared with simulated conventional austenitic hot-rolled coils. Ferritic rolled coils show better elongation and a higher percentage of equiaxed grains indicating better formability. The optimum continuous annealing temperature was found to be a function of hot rolling temperature in the ferritic regime. Elongation and grain size increased with an increase in temperature in all the samples, but the ferritic rolled coils show faster change due to higher stored energy. Comparison of elongation and microstructure indicates that temperatures above 780 °C should be sufficient for achieving complete recrystallization in ferritic rolled samples, compared to 810 °C required in the conventional austenitic rolled sheets which is an industrial advantage. Based on simulation studies, full-scale plant continuous annealing was carried out under the optimized temperature conditions where microstructure and properties matched closely with the simulation results and electron backscatter diffraction (EBSD) analysis confirmed improved texture. © 2022, ASM International.
Thermomechanical Simulation of Ferritic Rolling of Titanium-Niobium Interstitial-Free Steel
(ASTM International, 2021) Satish Kumar, D.; Sambandam, S.; Kuruveri, U.B.
Austenitic or two-phase rolling of ultra-low carbon steels face temperature control issues and generate shape defects. Ferritic rolling has been developed as a solution, and ferritic hot-rolled sheets are used as final products, replacing hot-rolled followed by cold-rolled sheets. However, it is not in regular industrial production because of mill limitations. Hence, ferritic hot rolling must be optimized for developing a ferritic cold-rolled and close-annealed sheet through subsequent processing. In this work, industrial ferritic rolling process was simulated for a titanium-niobium interstitial-free steel using a thermomechanical simulator. Multi-hit plane strain compression tests were carried out at three different regimes below the lower transformation temperature (Ar1). Steels were processed under high strain and strain rates as experienced during industrial hot rolling operation, and the results were compared with the conventional austenitic rolling. The flow stress of the material in the ferritic regime decreased with decreasing deformation temperatures but increased at temperatures below 700°C. Nonuniformity in grains and texture also increased with decreasing temperatures. High-temperature rolling in ferritic condition close to Ar1 temperature does not differ significantly from the austenitic condition, whereas the low-temperature ferritic rolled material had through-thickness microstructural nonuniformity and unwanted goss and brass fibers. The intensity of gamma-fiber {111}
Formability behaviour of ferritic and austenitic rolled Nb–Ti stabilized IF grade steel
(Springer, 2023) Satish Kumar, D.; Sambandam, S.; Udaya Bhat, K.
Recently, soft hot strip and hard hot strip produced through ferritic rolling are projected as a direct replacement to austenitic cold-rolled sheets for many forming applications. However, industrial hot-rolling mills, with final rolling thickness limitations cannot produce these thinner products and have to be subsequently cold-rolled to the desired application thickness and further annealed. Under ferritic rolling conditions, the hot-rolling temperature of these coils governs the final properties. The temperature difference in hot-rolled sheets generates the difference in the microstructure and texture of these coils after cold-rolling and annealing and variation in their formability behaviour. In the present work, an Nb–Ti stabilized IF grade steel was hot-rolled at two different temperatures in the ferritic regime and subsequently cold-rolled and annealed for structure-property comparison. As formability is an application-specific requirement, the annealed sheets were tested for different formability characteristics. Industrially rolled samples were tested for fracture criterion, stretch-flangeability, deep drawability and stretch formability through the formability limit diagram, hole expansion ratio, earing test and Erichsen cupping test respectively. These parameters were compared with those of the austenitic regime rolled sheets. High temperature ferritic rolled sheets show improved formability in all tests due to better r ˙ , higher n-value, low Δr and stronger gamma fibre maxima at 111<121>. Low temperature ferritic rolled sheets show the lowest Δr and improved n-value, but has reduced r ˙ and higher alpha fibre texture. High temperature ferritic rolled sheets show higher formability limits in uniaxial tension and low temperature ferritic rolled sheets in biaxial tension of the FLD curve. Various tests established that high temperature ferritic rolled sheets are best suited for deep drawing and stretching applications whereas low temperature ferritic rolled sheets should be preferred for stretch forming applications. © 2022, Indian Academy of Sciences.

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