Abstract
Both ausforming and transformation temperature affect the successive bainitic transformation and microstructure. The individual influence of each case is clear, whereas the combined effects are still unknown. Thermomechanical simulation and metallography were used to investigate the combined effects of ausforming and transformation temperature on bainitic transformation and microstructure. The kinetics of isothermal bainitic transformation in non-deformed and deformed materials was analyzed. A lower transformation temperature can lead to more bainite formation without deformation. However, ausforming with small strains can partially compensate for the decrease of bainite amount caused by the decreased undercooling. The larger the applied strain is, the smaller the difference between the final amounts of bainite with different undercooling. Ausforming at a relatively higher temperature is more favorable to the acceleration of subsequent isothermal bainitic transformation. The results in the present work provide reference for optimizing the fabrication technology of medium-carbon nanobainite steels.
Highlights
For the past decade, advanced high strength bainitic steels have been widely investigated as a third generation automotive material
The results indicate that ausforming at a relative higher temperature is more favorable to the acceleration of isothermal bainitic transformation
The combined effects of ausforming and transformation temperature on bainitic transformation and microstructure investigated by thermomechanical simulation and metallography
Summary
For the past decade, advanced high strength bainitic steels have been widely investigated as a third generation automotive material. High carbon nanobainite steels with a microstructure of lath-like nanoscale bainite and film-like retained austenite (RA) exhibit a tensile strength of ~2.5 GPa and fracture toughness greater than 30 MP·m1/2 [1,2,3]. Aluminum and cobalt are added into high-carbon nanobainite steels to reduce the heat treatment time [4,5,6], and the time can be shortened from several days to ~10 h despite being impractical for industry. Low carbon bainitic steels can be designed to shorten the incubation period and improve weldability and impact toughness [7]. A medium-carbon Si-Mn-rich alloy could be a desired alternative to produce high strength nano-structure bainite steel. Many works have already been conducted on transformation, microstructure and property of medium-carbon bainite steels (MCBS) [8,9,10,11,12]
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