The effect of finish rolling temperature on the microstructure and properties of non-quenched and tempered Bainite steel

Abstract

Abstract Non-quenched and tempered steel has excellent properties, and the production process of it is energy-saving, low-cost and eco-friendly. With these advantages, non-quenched and tempered steel attracts more attentions and has been widely used in recent years. Existing non-quenched and tempered micro alloyed steel is roughly divided into three categories: ferrite-pearl steel, bainite steel and martensitic steel. The strength and toughness of bainite steel is better than ferrite-pearlite steel, but less research of bainite steel has been done, especially its rolling process. In this paper, the effect of finish rolling temperature on the microstructure and properties of bainite non-quenched and tempered steel is studied. The results show that with different finish rolling temperature, the mechanical property of bainite steel has distinctions, along with the phase composition, the morphology and distribution of lath bainite, the amount of retained austenite, the difference between grain boundary orientations, recovery and recrystallization phenomena, and phase relation of lath bainite and parent phase. When finish rolling temperature is high (870°C), the ductility and toughness of bainite steel sample is better than others while the strength is lower. The amount of lath bainite is about 85%, which is the highest among the samples, and the width of lath bainite is small. With finish rolling temperature decreases, the strength of bainite steel sample increases at the cost of toughness. The diffusion of carbon atoms is slowed down due to the lower temperature, and as a result, the stability of austenite decreases, and martensite phase transformation occurs in the following cooling process. The portion of high-angle grain boundaries decreases as the finish rolling temperature decreases, while the phase relation of lath bainite and parent phase remains the same.