The fracture morphologies of several advanced high-strength steels(DP590,DP780,DP980,Ml 180,and M1300) formed in uniaxial tension and piercing were observed by scanning electron microscope,and then quantitatively analyzed by image processing technique.The tension-induced fractographs are dominated by obvious uniform or bimodal size dimples,while shearing-induced fractographs have smooth surfaces and few dimples.The fracture zone of higher grade DP steels is smoother.As for Ml 180 and Ml300,the fracture zones consist of very small dimples and smooth brittle surfaces.The dimple size of M1300(~ 1.2 urn) is smaller than that of Ml 180(~ 1.6 μm).Moreover,in the tensile fracture,the quantitative correlation between average dimple diameter(d) and tensile strength(σ) can be represented by d = 10,502.32σ^(-1 21).However,the relation between dimple density and tensile strength is not monotonic due to the appearance of bimodal size dimples with increase of tensile strength.For shearing-induced fracture during piercing,the fitted empirical model between the percentage of burnish zone(f) and tensile strength can be described asf= 239.9σ^(-036).
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