Investigation of hot ductility behavior of micro-alloyed steel and the effect of strain rate and dynamic phase transformation on the 2nd ductility minimum

Continuous casting of steel is widely used to manufacture semi-finished long or flat products. Various stresses are present during slab casting: stresses arise from friction between the mold wall and the solidified shell, thermal stresses on the strand surface, and stresses from bending and straightening operations. Steels present a minimum ductility point during continuous casting in the solid-state condition. This work aims to answer the metallurgical reasons for the occurrence of the ductility minimum in a micro-alloyed steel by investigating the microstructural evolution. The samples are in situ melted via induction heating in the BETA250-5^® thermomechanical simulator machine, followed by hot tensile tests conducted at different temperatures and strain rates. The ductility drop is analyzed in the range of 650 °C–1100 °C at different strain rates, 10⁻² s⁻¹ to 10⁻³ s⁻¹. Furthermore, the study investigated the development of the ferrite phase at the prior austenite grain boundaries, the thickness of ferrite, dynamic phase transformation, and the influence of the test conditions on these parameters. The fracture mechanism and ferrite phase thickness are determined from metallography investigations using light optical microscopy and scanning electron microscopy. Finally, the microstructural changes are correlated to the ductility minimum using the measured results.