Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. Graphic Abstract The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.