Ultra-High Strength 50SiMnCr Spring Flat Steel and its Continuous Cooling Transformation Behavior

The development and application of ultra-high strength spring flat steel is of great significance in achieving automotive lightweight. Understanding the continuous cooling transformation behavior of this type of steel aids in developing its post-hot rolling cooling and quenching-tempering processes. On the basis of mechanical property testing， this study focuses on a newly designed ultra-high strength flat spring steel， 50SiMnCr， to investigate the effect of cooling rate on the microstructure and hardness of experimental steel by a combination of metallographic-hardness and thermal expansion method， thereby constructing continuous cooling transformation （CCT） curves. The results show that the strength and ductility of 50SiMnCr steel meet the requirements of the 2000 MPa level， with phase transition point Ac1=721 ℃， Ac3=758 ℃， Ms=220 ℃ and Mf=103 ℃. Ferrite and pearlite transformation occurs when the cooling rate is 0.05 ℃/s-0.5 ℃/s. Bainite transformation takes place when the cooling rate is increased to 1 ℃/s. Martensite transformation happens when the cooling rate is above 2 ℃/s. The hardness of the this steel increases with increasing the cooling rate.