FERRITE GRAIN REFINEMENT AND PRECIPITATION STRENGTHENING IN V-MICROALLOYED STEELS

摘要

The present work has concentrated on two strengthening mechanisms in V-microalloyed steels: (i) grain refinement by promoting the formation of intragranular ferrite, and (ii) the role of nitrogen and carbon in precipitation strengthening by interphase and random precipitation of V(C,N) in ferrite. The experimental results strongly indicate that vanadium can by effectively used not only for precipitation strengthening but also for ferrite grain refinement. It was shown that vanadium contributes to the formation of two types of intragranulary nucleated ferrite; polygonal (idiomorph) ferrite and acicular (sideplate) ferrite. Intragranular polygonal ferrite nucleates on VN particles which grow in austenite during isothermal holding or slow cooling throughout the austenite range. The intragranular polygonal ferrite forms in the narrow temperature range, between 650-600 deg C for the investigated compositions. Acicular ferrite microstructure forms in V-microalloyed steels during isothermal transformation at lower temperatures (approx 450 deg). The acicular ferrite microstructure was obtained in V-microalloyed steels containing high, medium or very low nitrogen levels. This suggests that vanadium on its own can promote the formation of the acicular ferrite microstructure. Vanadium is an effective and easy controllable precipitation strengthening element. The degree of precipitation strengthening of ferrite for a given vanadium content depends on the available quantities of carbon and nitrogen. It was confirmed that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 6 MPa for every 0.001 percent N, essentially independent of processing conditions. experimentally it is demonstrated that the V(C,N)-precipitation becomes denser and finer with increasing N-content. carbon content, on the other hand, has usually been considered not relevant to precipitation strengthening when the precipitation occurs in ferrite because of the very small carbon content in solution in ferrite at equilibrium. The present results have shown that the precipitation strengthening of V-microalloyed steels increases significantly with the total C-content, approx 5.5 MPa/0.01 percent C. The explanation is that the C-content of the steel delays the pearlite (ferrite + cementite) formation and thereby maintains the higher, metastable C-content in ferrite given by the austenite/ferrite equilibrium for a longer time. This effect of carbon is particularly significant for medium carbon steels typically used for hot rolled bars and sections.