The ferrite/bainite dual-phase microstructure is obtained through controlling two-phase region temperature in the designed medium carbon steel. The influence of the two-phase region temperature on microstructure morphologies and volume fraction, as well as mechanical properties, is thoroughly studied. The microstructure at 795°C heat treatment process exhibits excellent strength and elongation. This is attributed to the increased proportion of large-angle misorientation, the better deformability between ferrite and bainite, and the TRIP effect of retained austenite. The strain hardening exponent curve of the sample subjected to the 795°C heat treatment process does not intersect with the straight-line n = e until the strain reaches the value of 0.1, which results in high tensile strength and high uniform elongation.
The effects of VC precipitation on phase transformation, microstructure, and mechanical properties were studied by controlling two-step isothermal treatment, i.e., austenization followed by intercritical transformation. The results show that the bainite transformation time of 950 °C–860 °C treatment and 950 °C–848 °C treatment is shorter than that of 950 °C single-step treatment. This is related to the isothermal ferrite transformation in the intercritical transformation range. The formation of ferrite nuclei increases the density of medium temperature bainite nucleation sites and decrease the bainite nucleation activation energy. At the same time, a large number of VC particles are precipitated. The additional VC particles provide numbers of preferential nucleation sites. The toughness of the specimen treated at 950~870 °C is improved, which is related to the large proportion of high angle grain boundaries. High angle grain boundaries can hinder crack propagation or change the direction of crack propagation. The specimen treated at 950 °C–848 °C exhibits large proportion of low angle grain boundaries, which is beneficial for the strength improvement.
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