Effect of pre-strain on microstructure and bake hardening properties of Q&P steel
The bake hardening behavior of Q&P steel under different pre-strains(0％,3％,6％ and 9％)was analyzed.Microstructure evolution of the experimental steel during pre-strain and baking was characterized by means of scanning electron microscopy,X-ray diffraction(XRD)and electron backscatter diffraction(EBSD)technique.The results show that with the increase of pre-strain,the bake hardening(BH)value of the experimental Q&P steel first increases and then decreases.When the pre-strain is 6％,the BH value is the highest,which is 144 MPa.In addition,when the pre-strain is small,grain size is the main factor affecting the stability of retained austenite,and martensitic transformation preferentially occurs in larger retained austenite.With the increase of pre-strain,large-sized retained austenite is consumed completely.In the case where the size of remaining retained austenite is not significantly different,its location and surrounding microstructure distribution determine the stability of retained austenite.Finally,during the baking,there will be a slight decrease in the amount of retained austenite,which will also affect the occurrence of phase transformation-induced plasticity effects in subsequent deformation processes,which is not conducive to the overall plasticity of the material.
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