Effect Of Microstructure On Mechanical Behavior Of Dual-phase Weathering Steel Cu-P-CR-NI-MO

Martensite volume fraction, morphology, distribution and refinement of the ferrite areimportant factors that affect mechanical behavior of dual-phase steel. Different martensitemorphology and volume fraction of dual-phase weathering steel Cu-P-Cr-Ni-Mo weredesigned and produced by hot rolling and heat treatment. The influence of martensitemorphology and volume fraction on the mechanical behavior of dual phase weatheringsteel were investigated by means of microstructural observations by optical microscopy,scanning electron microscopy and transmission electron microscopy, and mechanicalproperty tests, such as impact, tension and in-situ tensile test.The research results show that, when the martensite volume fraction of dual-phaseweathering steel Cu-P-Cr-Ni-Mo is about5%, with the martensite morphology changingfrom fibrous gradually to island, the impact energy gradually increases from126J to165J,but tensile strength decreases slightly from656MPa to596MPa, the yield strengthdecreases rapidly from580MPa to356MPa, yield ratio reduces from0.88to0.6.Whenthe morphology of martensite in the dual phase steel is island-shape, the stress straincurves appear continuous yielding phenomenon. When the islands of martensite aredistributed in ferrite, with the increase of martensite volume fraction, tensile strength alsoincreases from588MPa to627MPa, the yield strength decreases rapidly from490MPato403MPa, and the yield ratio decreases from0.88to0.64, the elongation changes a little.And when the volume fraction of martensite is about8%and15%, the stress strain curvesappear continuous yielding phenomenon.Analyzed the microstructure of dual phase steel changes before and after the tensiletest find that, in the fine particle microstructure of dual phase steel with lower volumefraction of martensite islands, the martensite islands deformed obviously, microcracknucleation were mainly formed by the fracture of martensite; when the dual phase steelconsist of fine banded martensite with a lower volume fraction, the band structure ofmartensite is easy to crack repeatedly; In the higher volume fraction of martensite islandand coarse ferrite of dual phase steel, microcracks were mainly formed by the disappearance of binding force at phase boundary and ferrite grains. Concurrently, themost of the fractures of phase boundary were existed at the sharp corner of martensite.During the process of tensile deformation, new microcracks initiate in the plastic zone ofcrack tips and wave-like slip bands generated in ferrite grains in the plastic zone. Themicrocrack propagates by connected with each other gradually. When the crack meetsmartensite, it may pass through the martensite island or change its propagation directionand propagate kinking-likely. In the tensile deformed microstructure of the dual-phasesteel, martensite islands deform as torsion, with a trend of separation, and there is a highdensity of dislocation in the matrix ferrite. The dislocation density in interface ofmartensite and ferrite is higher, in the corner of martensite is the highest.