Papers by Keyword: Vanadium Microalloyed Steel

Abstract: The microstructure and precipitate of the two kinds of medium-carbon vanadium microalloyed steels whose nitrogen contents were 0.0035% and 0.012% respectively, were studied by image analysis and transmission electron microscope (TEM). The results show there are the large amount of 10~20nm dispersion distribution irregular flake VC precipitates within the ferrite, the part of clustered fibrous VC precipitates with the diameter of 4~13nm that grow toward to ferrite intracrystalline along the grain boundary with some angle in the local area, and only a very small amount of 20 ~ 50nm spherical particles V (C, N) in the low-nitrogen steel. However, in the high-nitrogen steel, the precipitates are divided into two stages: the first stage is the part of 30~80nm spherical particles V(C, N) which precipitation in austenite, the second stage is flakiness VC which precipitation in ferrite during the following γ → α phase transformation and cooling process. Compared with the low-nitrogen steel, the number of precipitates in decreased significantly and the size increased obviously in the high-nitrogen steel. The substantial increase of nitrogen content leads to the rapid increase of driving force that V (C, N) precipitation in austenite. A lot of V (C, N) that precipitation before phase transformation results in the significant increase of ferrite nucleation rate, which leads to the microstructure of high-nitrogen steel fined obviously.

Abstract: The continuous cooling transformation (CCT) behaviors of two vanadium (V) micro-alloyed steels with different Nitrogen (N) levels were investigated under the undeformed and deformed conditions, respectively. It is found that the austenite-bainite transformation is suppressed during dynamic CCT process due to hot deformation. For V-N steel, the transformation of austenite-proeutectoid ferrite and austenite-acicular ferrite (AF) occur under a wider range of cooling rates.

Abstract: Three 0.15% carbon steel samples containing small additions of vanadium and nitrogen singly or in combination have been carburized in a natural Titas gas atmosphere at a temperature of 9500C and a pressure of about 15 psia for time periods ranging from 1 to 5 hours and quenched in 10% brine from the carburizing temperature of 9500C after pre-cooling to 8600C in the furnace followed by tempering at a low temperature of 1600C. The structure and properties of the carburized and heat treated specimens were studied systematically by optical microscopy, surface hardness and microhardness measurements, X-ray diffractometry and impact tests. It was found that vanadium without nitrogen does not have any effect in the formation of retained austenite while vanadium with nitrogen is effective in promoting the formation of retained austenite in the case of carburized and hardened steels. It was also found that vanadium alone and vanadium with nitrogen refine the martensite platelets (needles) in the case of carburized and hardened steels, vanadium with nitrogen being more effective. Microhardness measurements have shown that vanadium improves the case hardness and the core hardness values; vanadium with nitrogen is more effective than vanadium alone in increasing the case hardness and the core hardness. The hardenability is found to increase with the increase of austenite grain size and with the extent of carbon penetration of the case of carburized steels. Vanadium as vanadium carbide, VC are detrimental to toughness and vanadium as vanadium carbonitride, V(C, N) are beneficial to toughness of the core of low carbon steels in carburized and hardened condition.

Abstract: The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V microalloyed steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation.