Abstract
The final mechanical properties of hot-stamped steel are determined by the microstructures which are greatly influenced by the cooling process after hot stamping. This research studied the effect of the cooling path on the microstructures and hardness of 22MnB5 hot-stamped steel. The cooling path was divided into continuous and discontinuous (primary and secondary) processes. After cooling, the Vickers hardness along the thickness of the specimens was measured. The results indicate that, for a continuous cooling process, there was a critical cooling rate of 25 °C/s to obtain fully martensitic microstructure. For the discontinuous cooling process, the slower was the cooling rate, the higher was the degree of auto-tempering that occurred, and the greater was the amount of carbides that formed, regardless of the primary or secondary cooling rate. When the cooling rate was lower than the critical value, a higher primary cooling rate suppressed the auto-tempering of lath martensite and increased the quenched hardness. By contrast, the hardness was not sensitive to the cooling rate when it exceeded the critical value.
Highlights
Due to the increasing demand for vehicle weight reduction, safety improvement, and improved crashworthiness, ultra-high strength steels have become more and more popular to manufacture automobile structural components [1]
We studied the effect of cooling path on the microstructures and hardness of hot-stamped 22MnB5 steel
The ferrite disappeared when the sample cooled at a cooling rate of 8 ◦ C/s, and the microstructure mainly consisted of granular bainite and lath bainite, as shown in Figures 2d and 3d
Summary
Due to the increasing demand for vehicle weight reduction, safety improvement, and improved crashworthiness, ultra-high strength steels have become more and more popular to manufacture automobile structural components [1]. The steel sheet is press formed and simultaneously quenched hardened with the die to obtain an intended shape and high strength due to a fully martensitic microstructure. The ultra-high strength of hot-stamped steel is attributed to its fully martensitic microstructure. A slow cooling rate below martensite start (Ms ) temperature facilitates the formation of carbides, causing auto-tempering [12], reducing quenched hardness [13] and strength [14]. A lot of research has been conducted on the phase transformation of hot-stamped steels as well as the tempering of martensite under isothermal conditions. Few attempts have been made to explore the auto-tempering and phase transformation of hot-stamped steel in discontinuous cooling process. We studied the effect of cooling path on the microstructures and hardness of hot-stamped 22MnB5 steel
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