Microstructural characterization and strengthening behavior of nanometer sized carbides in Ti–Mo microalloyed steels during continuous cooling process

Abstract

Nanometer-sized carbides that precipitated in a Ti–Mo bearing steel after interrupted continuous cooling in a temperature range of 620–700°C with or without hot deformation were investigated by field-emission-gun transmission electron microscopy. The nanometer-sized carbides were identified as randomly homogeneous precipitation carbides and interphase precipitation carbides coexisting in the ferrite matrix. It is found that this dual precipitation morphology of carbides in the steel leads to the non-uniform mechanical properties of individual ferrite grains.Vickers hardness data mainly revealed that, in the specimens cooled at a rate of 0.5°C/s without hot deformation, the range of Vickers hardness distribution was 230–340HV 0.1 when cooling was interrupted at 680°C, and 220–360HV 0.1 when cooling was interrupted at 650°C. For the specimens cooled at a rate of 0.5°C/s with hot deformation, the range of Vickers hardness distribution was 290–360HV 0.1 when cooling was interrupted at 680°C, and 280–340HV 0.1 when cooling was interrupted at 650°C. Therefore, a narrower range of hardness distribution occurred in the specimens that underwent hot deformation and were then cooled with a lower interrupted cooling temperature. The uniform precipitation status in each ferrite grain can lead to ferrite grains with a narrower Vickers hardness distribution.On the other hand, interrupted cooling produced a maximum Vickers hardness of 320–330HV 0.1 for the hot deformed specimens and 290–310HV 0.1 for the non-deformed specimens with cooling interrupted in the temperature range of 660–670°C. The maximum Vickers hardness obtained in such a temperature range can be ascribed to the full precipitation of the microalloying elements in the supersaturated ferrite matrix with a tiny size (~4–7nm).