Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-free bainite-martensite (CFB/M) ultra-high strength steel was established by applying controlled cooling of small samples. In addition, the influence of thermomechanical treatment temperature on the structure and properties was discussed. The experimental results showed that when deformed at 860 °C and below, ferrite transformation occurred due to strain. With the decrease of ausforming temperature, the quantity of ferrite increased and strength and toughness were deteriorated. Therefore, certain information was provided for optimizing technical parameter of ausforming process; firstly, the thermomechanical treatment temperature should not be lower than 860 °C in order to avoid ferrite formation induced by deformation; secondly, rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.