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The microstructures and chemical composition of nano-precipitates in vanadium (V) steels were investigated by the alloy contrast variation method (ACV) using small-angle X-ray scattering (SAXS) coupled with small-angle neutron scattering (SANS) at holding temperatures ranging between 600 and 700°C. Both the SAXS and SANS profiles exhibited clear scattering, depending on the holding temperature, due to the presence of nano-precipitates. The scattering profiles of the precipitates are characteristic of spherical or disc-like particles. The average diameters of these precipitates increased from 0.5 nm at 600°C to 23 nm at 700°C, whereas the number density of the precipitates decreases with increased holding temperature. Therefore, the increasing holding temperature results in an increase in the growth rate of the precipitates. ACV analysis revealed that the chemical composition of the precipitates corresponds to NaCl-type vanadium carbide (VC) at 675 and 700°C, and as VC0.9 at 625 and 650°C. The formation of a different heterogeneity, non-NaCl type, was found in the sample at a holding temperature of 600°C. This probably corresponds to a precursor of the NaCl phase in the initial process of precipitation.

The interphase boundary precipitation behavior of vanadium carbide during isothermal ferrite transformation, which is the important phenomena for the hot forged medium carbon steels, was investigated and modeled. It was found that the intersheet spacing of interphase boundary precipitation of VC decreased with a decrease of ferrite growth rate during isothermal transformation. As a major factor affecting such an interphase boundary precipitation behavior, it was deduced that the vanadium segregation on migrating austenite/ferrite interphse boundary is quite important to understand the repeated precipitation of VC. In numerical simulation of VC precipitation, the influence of the vanadium concentration on the precipitation behavior at austenite/ferrite interface during isothermal transformation was examined based on a time-dependent solute drag model incorporated with parabolic growth rate. It was found that the change of the intersheet spacing during ferrite growth can be simulated by the newly proposed model for interphase boundary precipitation of alloy carbide.KEY WORDS: interphase boundary precipitation; vanadium carbide; medium carbon steel.

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