基体组织对800MPa级双相钢强塑性机制的影响

采用两种热处理工艺制度,得到不同基体组织的800 MPa级双相钢,并系统地研究了基体微观组织特征及其对强塑性机制的影响。结果表明,基体组织对800 MPa级双相钢的塑性变形机制有显著影响,从而导致性能产生差异。(F+M)双相钢由多边形铁素体和约28%的第二相马氏体组成,屈强比0.540,而伸长率达到23.3%;(BF+γ)双相钢由贝氏体铁素体基体组织和约24%的第二相残留奥氏体组成,其屈强比为0.702,同时扩孔率达到56%。(BF+γ)双相钢在塑性变形过程中,厚度约为60~150 nm的γ相可有效分解裂纹尖端的应力集中,消耗裂纹扩展能量,同时诱导残留γ发生马氏体相变引起的体积膨胀还可弥合微裂纹产生的缝隙,在α相BF和残留γ两相的协调变形机制作用下,有益于提高其强度、塑性和扩孔性能。此外,(BF+γ)双相钢大角度晶界所占比例增加至63.1%,同时基体中存在较高的位错密度,均可有效弱化微裂纹扩展的驱动能,增加其继续扩展所需能量,缓解其在变形或扩孔过程中产生的应力集中。

Effect of matrix microstructure on mechanism of strength and ductility for 800 MPa grade dual phase steel

Two heat treatment processes were adopted to obtain 800 MPa grade dual phase steels with different matrix microstructure, and the microstructure morphology characteristics of the matrix and its effect on the mechanism of strength and ductility were studied systematically. The results show that the matrix microstructure has a significant indigenous effect on the plastic deformation mechanism of 800 MPa grade dual phase steels, resulting in differences in the properties. The (F+M) dual phase steel is composed of polygonal ferrite and about 28% second-phase martensite, with yield ratio of 0.540 and elongation of 23.3%. The (BF+γ) dual phase steel is composed of bainite ferrite matrix structure and about 24% second-phase retained austenite, with yield ratio of 0.702 and hole expansion ratio of 56%. During the plastic deformation process of (BF+γ) dual phase steel, the stress concentration at the crack tip can be effectively decomposed by the γ phase with a thickness of about 60-150 nm, and the crack propagation energy is also consumed. At the same time, the cracks generated by microcracks can be bridged by the volume expansion caused by the induced martensite transformation of residual austenite. Under the coordinated deformation mechanism of α phase BF and residual austenite phase, it is beneficial to improve the strength, plasticity and stretch-flange property. In addition, the proportion of large angle grain boundaries in (BF+γ) dual phase steel increases to 63.1%, and higher dislocation density exists in the matrix, the driving energy required for micro-crack propagation is weakened effectively, and the energy required for further propagation is also increased, the stress concentration generated during deformation or hole expansion is alleviated.