Study On Mechanical Behavior Of High-strength Low-alloy Multi-phase Steel

Oil and gas pipeline passing through tundra, seabed, seismic area, land subsidence and slide areas are most likely to experience high degree of plastic deformation, which can cause failure accidents through buckling, distortion and fracture. High deformability pipeline steel which can sustain large deformation without failure has become an important development direction of high performance pipeline steel. The deformation capability of high deformability pipeline steel is closely related to strain hardening and failure behavior of multi-phase microstructure. Rational combination of soft and hard phase in multi-phase steel is an important technical route to achieve high plasticity of high deformability pipeline steel. The effect of microstructure and volume fraction of multi-phase steel on strain hardening capability/behavior, stress ratio and plastic damage behavior had been revealed by experimental investigation and numerical simulation analysis in this paper, which can provide experimental and theoretical basis for engineering application of high deformability pipeline steel.It was found that microstructure characteristic and constitutive relationship have significant influence on strain hardening capability of ferrite/bainite (F/B) multi-phase steel. The microstructures with various strength and strain hardening capability can be obtained by controlling composition and phase transformation. Yield ratio will be decreased with increasing strength difference between microstructure of F/B multi-phase steel. The strain hardening behavior of F/B multi-phase is closely related to the volume fraction of bainite. The effect of volume fraction of bainite on stress ratio, yield ratio, strain hardening exponent and uniform elongation in multi-phase steel had been reasonably explained by division of strain hardening process and characterization strain hardening capacity of each stage by modified C-J analysis. Relevant conclusions have been verified by the industrial and experimental datas, which processes high reliability.Volume fraction and morphology (appearance and distribution of microstructure) of microstructure in F/B multi-phase steel have obvious effect on strength and plastic damage. Since the effects are hard to measure by experiments, the effect of volume fraction and morphology of microstructure on the strength and plastic damage had been calculated and analyzed by micromechanical simulation method in this paper. The results show that volume fraction of microstructure has obvious effect on the strength and plastic damage of the multi-phase steel, which is also closely correlated to deformation mechanism and also presents volume fraction-related feature. However, morphology of microstructure has little influence on strength, but has great effect on the plastic damage. Finally, micro-strain distribution of a kind of F/B multi-phase steel during tensile test had been analyzed and predicted by numerical simulation in this paper. Moreover, the deformed microstructure and damage in the steel had been observed by experimental observation, the results agreed well with experiments.