Improvement of microstructure and properties of air-cooled 46MnVS5 forging steel rod for fracture splitting connecting by thermomechanical control process (TMCP) and by niobium micro alloyingVerbesserung von Gefuge und Eigenschaften eines luftgekuhlten 46MnVS5 Schmiedestahlstabes fur Bruchtrennverbindungen durch ein thermomechanisch kontrolliertes Verfahren und durch Mikrolegieren mit Niob

Microstructure control and properties of air-cooled high-strength 46MnVS5 forging steel rod for fracture splitting connecting were systematically investigated by quantitative metallographic analysis, thermomechanical simulation and industrial trials etc. The results indicate that the critical cooling rate for bainite transformation in the steel is about 1.5 degrees C/s-2 degrees C/s during continuous cooling. Cooling parameters are optimized in two sections. That is, the specimen is firstly cooled to 750 degrees C at a rate of 4 degrees C/s, then continuously cooled to 570 degrees C at 1 degrees C/s, 1.5 degrees C/s. Addition of 0.024 wt.% niobium improves the fracture splitting performance by 10 %-20 % reduction of decarburization sensitivity. Finally, the optimized parameters for the niobium micro alloyed steel in the laboratory were successfully applied to produce the connecting rod with excellent fracture splitting performance on the traditional production line.