Microstructure and Mechanical Properties of Medium-Carbon Si-Rich Steel Processed by Austempering after Intercritical Annealing

Abstract

In the present paper, the medium-C Si-rich steel with a quenched martensite microstructure was heated to intercritical annealing temperatures at 750 °C, 760 °C and 770 °C after warm rolling deformation to obtain ferrite with varying volume fractions. Subsequently, bainite/ferrite multiphase microstructures were attained via austempering near Ms temperature. The microstructures of the test steel after different heat treatments were characterized by scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction, and corresponding tensile and impact properties were tested. The results showed that, with the increase of intercritical annealing temperature, the austenite content increased, which limited the growth of ferrite grains, and the grain size decreased from ~1.6 μm to ~1.4 μm. In addition, the degree of ferrite recrystallization was almost complete. At the same intercritical annealing temperature, compared with austempering above Ms, prior athermal martensite (PAM) was obtained after austempering below Ms, which effectively refined the size of bainite ferrite lath. Moreover, with the increase of intercritical annealing temperature, the bainite content of the test steel increased after austempering, resulting in the increase of yield strength, tensile strength and impact energy. In contrast, while the decrease in ferrite content led to a significant decrease in uniform elongation. At constant intercritical annealing temperature, the tensile strength decreased slightly, and the impact property improved after austempering above Ms.