Correlation of Microstructure and Fatigue Properties of Hot-Rolled Steel Strips for Automobile Structural Application

In this work, five different hot-rolled steel strips with different microstructures were produced through different microalloying design and adjusting final coiling temperature. The relationship between microstructure and mechanical properties, especially fatigue limit, was analyzed in detail. The results showed that the steels with mainly quasi-polygonal ferrite and pearlite have a similar yield strength as the steels with acicular ferrite and martensite, but they are higher than the DP (ferrite + martensite) steels that are strengthened by the second martensite phase. This is probably because the yield strengths of fine ferritic steels are further enhanced by the precipitation strengthening mechanism of nano-sized particles. Under the present experimental conditions, the fatigue limit is proportional to yield strength rather than tensile strength. It is also found that the precipitation strengthening mechanism is more beneficial to improving the fatigue stress than the strengthening mechanism by introducing martensite, although all these steels have the same fatigue crack initiation mode.