The development of electric arc furnace (EAF) short-process smelting technology for interstitial-free (IF) steel has become a focal point of industry attention. IF steel generally requires the carbon (C) and nitrogen (N) contents to be controlled at extremely low levels—less than 30 ppm. However, the N content in molten steel produced by EAF short-process smelting is elevated (60–70 ppm), significantly exceeding the N requirements for IF steel. During continuous casting, the precipitation of nitrides, carbonitrides, and proeutectoid ferrite along grain boundaries is generally considered the primary factor that deteriorates the hot ductility of steel and causes surface cracks in slabs. However, the specific impact mechanism of elevated N content on the hot ductility of IF steel has not been conclusively established. This study employed a Gleeble-3800 thermal-mechanical simulator to investigate the hot ductility of IF steel with N content ranging from 13 to 68 ppm, within the temperature range of 700–1050 °C. Through experimental characterization and thermodynamic calculations, the fracture morphology of tensile specimens, the transformation of matrix phase, and the precipitation behavior of secondary phases were investigated. The results demonstrated that when the N content was below 56 ppm, the IF steel exhibited generally favorable hot ductility. However, when the N content increased to 68 ppm, significant widening of plastic troughs and deterioration in hot ductility were observed across the experimental temperature range, with the temperature range of the third brittle zone is approximately 822–930 °C. Due to the ultra-low C characteristics of IF steel, the growth process of ferrite precipitated along austenite grain boundaries toward the center of the austenite matrix proceeded rapidly, which was not identified as the primary factor contributing to the deterioration of hot ductility in IF steel. Within the temperature range of the third brittle zone (822–930 °C), the substantial precipitation of angular, micron-sized TiN, and Ti(C,N) particles along grain boundaries was determined to be the fundamental cause underlying both the formation of the third brittle zone and the impaired hot ductility in IF steel with elevated N content during solidification. To enhance the hot ductility of EAF short-process smelted IF steel, regulating the cooling regime in the continuous casting process to control the precipitation behavior of TiN and Ti(C,N) constitutes a key technical approach.
