Development of Ferritic Rolling Process For The Production of Interstitial Free Automotive Steel

Abstract

Interstitial free (IF) grade steels have high transformation temperatures and their production through austenitic rolling, often results in non-uniform rolling, shape defects and lower yields. As a solution, soft hot strip and hard hot strip produced through ferritic rolling is projected as a direct replacement to austenitic cold rolled sheets. However, it is not industrially adopted due to hot rolling thickness limitations and formability variations. In the present work a new intermediate approach, consisting of ferritic hot rolling and subsequent cold rolling and annealing in the existing mills is proposed. However, in this approach, the combination of hot rolling temperature, cold reduction and annealing parameters governs the final microstructure and, texture and hence parameters must be optimized. In this work, the new route was first simulated and optimized for a Ti-Nb interstitial free steel (IF steel) using a thermo-mechanical simulator and hot strip mill model (HSMM) at different operating regimes. The best two ferritic rolling regimes were validated in the industrial hot rolling, cold rolling and annealing mill. Metallurgical and mechanical properties were compared between the ferritic and austenitic regime rolled sheets at each stage for structure-property comparison. Key observation was the gradual transition of recrystallization mechanism from oriented nucleation to oriented growth with the decrease in temperature of deformation in the ferritic region which changed the formability behaviour of the rolled sheets. As formability is an important requirement in auto steels, application-specific formability characteristics such as fracture criterion, stretch-flangeability, deep drawability and stretch formability was studied through the formability limit diagram, hole expansion ratio, earing and Erichsen cupping tests, respectively. High temperature ferritic rolled sheets show improved formability in all tests with higher formability limits in uniaxial tension of FLD, due to better ṙ, higher n-value, low Δr and stronger gamma fibre maxima at 111<121>. Low temperature ferritic rolled sheets show the lowest Δr and reduced ṙ but improved n-value and higher limits of biaxial tension in the FLD curve due to higher alpha fibre texture. Study 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. This intermediate route produced sheets have uniform and improved properties for all formability applications and lower cost due to reduced energy consumption. This new processing will help in the wider adoption of the ferritic rolling process on an industrial scale for developing high formability sheets in cold-rolled and annealed conditions.