Effect of Cr on the phase transformation and interphase nanoprecipitation behaviours of high-strength microalloyed steels

In this work, the microstructures and mechanical properties of interphase precipitation strengthening micro-alloyed steels were investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy, atom probe tomography and tensile tests. Microstructural analysis shows that the studied steels coiled at 620 degrees C possess a microstructure consisting of a single-phase polygonal ferrite matrix and disc-liked interphase-precipitated carbides. Tensile test results show that with the addition of Cr, the yield strength increased from 644 MPa to 728 MPa and 748 MPa for 0Cr, 0.1Cr and 0.2Cr steels, respectively, without a substantial decrease of ductility. Thermodynamics analysis indicates that the addition of Cr increases the driving force of gamma -> alpha transformation, which was responsible for grain refinement and decrease of sheet spacing of nanoprecipitates. The strengthening contributions from solid solution, dislocations, grain boundaries and interphase precipitation were calculated, and the results indicate that the decrease of interphase-precipitated carbides sheet spacing and the refinement of grain size were responsible for the strength enhancement of Cr microalloyed steel.