Lattice Strain and Strength Evaluation on V Microalloyed Pearlite Steel

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Strengthening effect by microalloyed vanadium (V) on eutectoid pearlite steel has been investigated from the perspective of nano-precipitation and lattice strain. 0.2% proof stress of specimens, isothermally transformed at 873 K, increases around 160-170 MPa with 0.1% V addition. However, interphase precipitation of vanadium carbide (VC), regarded as the principal strengthening factor, has not been detected by transmission electron microscopy or 3D atom probe microscopy (3D-AP). On the other hand, lattice strain in lamellar ferrite, analyzed by broadening of X-ray diffraction peak, has clear correlation with proof stress. The lattice strain data of 0.1% V added pearlite specimens are plotted on the same correlation line as of V free ones. In addition, elemental map by 3D-AP shows that vanadium atoms concentrate in lamellar cementite rather than ferrite, which could change cementite lattice parameters and gain ferrite/cementite misfit causing lattice strain increment. These results reveal that microalloyed V influences not only precipitation of VC in lamellar ferrite, but also the lattice strain increment in pearlite lamellar. As far as pearlite steels containing at most 0.1% V, lattice strain is considered to be the major factor of their yield behaviors. Furthermore, 0.1% V addition has not enhanced work-hardening behavior as notably as estimated by Ashby’s work-hardening theory of dispersion-hardened crystals. Therefore, VC precipitation should not necessary for V strengthening effect on pearlite steel.