Effect of temperature for the start of finish rolling on coiled steel X70 microstructure and cold resistance

Microstructural parameters, impact strength, and DWTT properties of coil steel X70 are governed to a considerable degree by a sufficiently high temperature for the start of rolling in 2000 mill finish stands (T _sfr). Results are provided for a study of steel microstructure in relation to cold brittleness threshold during DWTT (T ₉₀) and proportion of bainitic ferrite on T _sfr. Rolling with T _sfr close to a temperature for retarding recrystallization (T ₅ or T _NR) causes a difference in steel matrix grain size and an increase in volume fraction of bainitic transformation products with lath morphology (bainitic ferrite) due to partial austenite recrystallization. This structure leads to the worsening of steel impact strength and DWTT properties. The properties of coil steel may be improved by increasing steel niobium content and rolled product interim cooling.