Vanadium in Steels
Vanadium in Steels
Vanadium (V) is the 17th most commonly occurring element in the earths’ crust and finds wide use as an alloying element in steels. The atomic number of vanadium is 23 and its atomic weight is 50.94. It has a density of 6.11 gm/cc. Its melting point is 1910 deg C and boiling point is 3407 deg C. The density of liquid at melting point of vanadium is 5.5 gm/cc. It has a body centred cubic structure. The phase diagram of the iron – vanadium binary system and is given at Fig 1.
Fig 1 Iron-vanadium phase diagram
Around 85 % of the vanadium produced is used as ferro-vanadium or as a steel additive. The considerable increase of strength in steel containing small amounts of vanadium has been discovered in the early 20th century.
Vanadium is a preferred alloying element during steelmaking because of (i) its ease of use during steelmaking, (ii) high recovery of the element during the additions, (iii) good castability of the vanadium alloyed steel, (iv) high solubility of vanadium during the reheating of the cast steel, (v) no additional roll forces are required with the vanadium alloyed steel, and (vi) there is a robust response during the tempering of the steel.
Application of vanadium steels
Vanadium steel is being used for applications in axles, bicycle frames, crankshafts, gears, and other critical components. There are two groups of vanadium alloyed steels. Vanadium high carbon steel alloys containing 0.15 % to 0.25 % V, and high-speed tool steels (HSS) For high-speed tool steels, hardness above HRC 60 (Rockwell hardness) can be achieved. HSS steel is used in surgical instruments and tools.
The high content of vanadium carbides in the alloy steels increases the wear resistance significantly. Vanadium alloyed steels with the vanadium content upto 18 % are available. One application for vanadium alloyed steels is tools and knives. For specialty tool steels, vanadium is used as an alloy, forming massive vanadium carbides for very high hardness and wear resistance.
Vanadium is by far the most used element to improve wear resistance in tool steels. There are several vanadium alloyed tool steels available. In these steels, the wear decreasing vanadium rich carbides are either formed eutectically at the solidiﬁcation of the residual melt or in isolation at approximately the same time as the eutectic formation. Though the formation of primary vanadium carbides, which make the melt viscous, is possible, there is no information available about the difﬁculties of pouring or gas atomizing melts with higher vanadium content.
For heat treated steels, vanadium can provide grain refinement and temper resistance. Fine grains provide toughness and strength, temper resistance provides higher strength and hardness after tempering. Vanadium contents in these steels can range from 0.03 % to 0.30 %.
For as-rolled and as-forged steels most common in construction steels, OCTG (oil country tubular goods) steels and steels for power generation and transmission, vanadium is used for its precipitation strengthening properties in as-rolled or as-formed applications. Vanadium contents are typically in the 0.03 % to 0.10 % range in these steels, but can go higher.
For automotive steels, there are many different applications which demand a variety of contributions from vanadium, all depending on the final micro-structures of the finished part.
Effect of vanadium on properties
In steel, vanadium forms stable compounds with both carbon and nitrogen and the way in which these elements interact with vanadium determines many of the properties of vanadium containing steels.
Important metallurgical properties of vanadium in the steel includes (i) high solubility of vanadium carbo-nitrides of vanadium in the austenite, (ii) low ‘solute drag coefficient’ of vanadium in the austenite, (iii) nitrogen becomes the preferred element in the precipitation of vanadium carbo-nitrides, and (iv) since vanadium carbide is in solution at normal heat treating temperatures, vanadium is in solution during the tempering of martensite provides temper resistance.
Vanadium increases the yield strength and the tensile strength of carbon steels. Small amount of vanadium increases the strength of steels significantly. It promotes fine grains and elevates coarsening temperature of austenite. It increases hardenability when dissolved. It resists tempering and causes marked secondary hardening.
Vanadium is one of the primary contributors to precipitation strengthening in micro alloyed steels. When thermo-mechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness. Vanadium causes effective precipitation strengthening at all levels of carbon. Predictable strengthening is achieved upto 0.15 % of vanadium content in the steel. Available nitrogen improves the strengthening contribution of vanadium, by converting nitrogen from an unwanted residual into a useful alloying element (Fig 2).
Fig 2 Precipitation strengthening of steels with vanadium and nitrogen
The impact transition temperature increases when vanadium is added. Vanadium, niobium, and titanium combine preferentially with carbon and/or nitrogen to form a fine dispersion of precipitated particles in the micro alloyed steel matrix. Vanadium forms stable nitrides and carbides, resulting in a significant increase in the strength of steel.
Vanadium carbides, vanadium nitrides, and vanadium carbo-nitrides show solubility both in the austenite and in the ferrite. In austenite, the solubility of vanadium carbides is the highest as compared to the niobium and titanium carbides. While the solubility of vanadium nitride is lower than that of vanadium carbide and is more equal to that of niobium carbide, titanium carbide, and niobium nitride. It is also interesting to notice that titanium nitride has the lowest solubility in austenite. Vanadium carbide also shows higher solubility in ferrite than that of the other micro-alloy carbides and nitrides. It is important to note that the solubility of all the carbides and nitrides reduce as the temperature falls and that the solubility in ferrite is significantly lower than that in austenite.
The reduction in solubility with decreasing temperature and on transforming from austenite to ferrite leads to the possibility of precipitation of vanadium compounds in steel. However, because of their relatively high solubilities, vanadium compounds tend not to precipitate in austenite until relatively low temperatures are reached, usually in the presence of high levels of vanadium and nitrogen (or carbon) and normally in the presence of deformation. It has been seen that the start of such precipitation can be enhanced by the presence of a suitable substrate. Example is precipitation of vanadium nitride as a cap on existing manganese sulphide.
Rolling of vanadium containing steels
The solubility of vanadium carbide in austenite is significantly higher than that of vanadium nitride. However, even in the case of the nitride the solubility in austenite is quite high. Fig 3 depicts the equilibrium solubility temperature for steels containing different vanadium and nitrogen contents.
Fig 3 Equilibrium solubility temperature for steels containing vanadium and nitrogen
From Fig 3, it can be seen that for steel containing 0.1 % vanadium and 0.02 % nitrogen, a relatively high combination, the equilibrium solution temperature is only 1098 deg C. This relatively low solution temperature permits the use of energy efficient low soaking temperatures with little or no loss of precipitation strengthening capability in vanadium containing steels. This has proved to be particularly important in the new process of thin slab casting and rolling, where the temperature in the equalization furnace is typically in the range 1050 deg C to 1150 deg C, This is also be important in the rolling of the reinforcement bars where high furnace pushing rates are desirable.
One potential drawback of these relatively low solution temperatures, especially if soaking temperatures are not reduced, is that austenite grain coarsening can occur during reheating. As has been widely demonstrated, such coarsening can be controlled by an addition of around 0.01 % of titanium, although it is a fact that this addition of titanium also uses nitrogen which otherwise would have been used for the precipitation strengthening. Also, the possibility that the presence of titanium nitride as a substrate encourages precipitation of vanadium carbo-nitride at high temperatures is also needed to be taken into account.
Since vanadium compounds tend to remain in solution during rolling and vanadium only shows a small solute drag effect, vanadium steels recrystallize during rolling, even down to relatively low temperatures. Hence, by the process of deformation, recovery and recrystallization on a falling temperature scale, it is possible to produce austenite grains, of high surface area/volume ratio, which transform to fine ferrite on subsequent cooling. Also, at high surface area/volume ratio the grain sizes obtained for the vanadium containing steels (45 micro meters) are similar to those obtained for niobium steels transforming from deformed austenite. A further effect of the recrystallization behaviour shown by vanadium micro-alloyed steels is that their rolling loads are similar to those for carbon-manganese steels when measured in the same temperature range.
Further, since the recrystallized austenite grain size of vanadium-containing steels appears to show little variation over a fairly wide range of temperature, the properties of such steels are relatively insensitive to changes in finish rolling temperature. It is seen that as the finish rolling temperature increases from 870 deg C to 1050 deg C, in steel with vanadium content of 0.05 % to 0.23 % and nitrogen content of 0.009 % to 0.014 %, there is no significant change in the yield strength and a relatively small effect on impact transition temperature. It is also seen that the absence of Widmanstatten ferrite in vanadium-containing steels, finished rolled at high temperature, assists in maintaining the impact properties. Thus, in the rolling mill, vanadium micro-alloyed steels are relatively user friendly and their properties tend to be relatively insensitive to changes in rolling conditions.
However, in most modern clean steels unless special rolling schedules and/or chemical compositions are used, such a process is unlikely to make a significant contribution to precipitation of vanadium nitride in austenite. Even when such special processes are adopted, it is likely that less than 10 % of the available vanadium precipitates, during rolling, in austenite. This is advantageous as it means that, in the majority of steels, most, if not all, of the vanadium added to the steel is likely to remain in solution up to the start of transformation from austenite to ferrite. Thus, by far the majority of precipitation in vanadium containing steels takes place during and after transformation, giving rise to precipitation strengthening. The precipitates, which form during transformation, tend to form in rows, while those, which form afterwards, tend to be more randomly dispersed and to have both a smaller particle diameter and inter-particle spacing than the row precipitates.
Vanadium and HSLA steels
Vanadium has been used in HSLA structural steels since a long time, and indeed it was the first of the so called micro-alloying elements (excluding aluminum) to be used. Vanadium alone, and together with the other micro-alloying elements aluminum, niobium, and titanium, contributes extra strength to carbon-manganese structural steels through the formation of carbides and nitrides, acting directly to strengthen the steel by precipitation, or indirectly by mechanisms leading to refinement of the ferrite grain size.
Grain refinement also increases the low-temperature toughness of HSLA steels. In some cases, as when added with aluminum, the vanadium precipitates and precipitates of other micro- alloying elements reinforce each other in their effects on the steel. In other cases, such as control-rolled vanadium niobium steel plates, the precipitates of the two micro-alloys are used to produce different effects on the structure and properties. For example, the niobium carbide enables a fine grain size to be obtained, and the vanadium carbo-nitride contributes precipitation strengthening.
In vanadium containing HSLA steels, the two main factors affecting strength and toughness are ferrite grain size and precipitation strengthening. There is a clear relationship between vanadium (and nitrogen) level and ferrite grain size and the ferrite grain size remains reasonably constant over a fairly wide range of finish rolling temperatures. Increasing vanadium and nitrogen levels also increases both yield strength and ultimate tensile strength, via precipitation strengthening. While the strength can be affected by parameters such as cooling rate and transformation temperature, it is broadly correct for steels with a wide range of carbon content.
Refining the ferrite grain size tends to improve toughness in HSLA steels. On the other hand, it is seen that increasing precipitation strengthening increases the impact transition temperature by about 3 deg C to 4 deg C for every 100 kg/sq cm increase in yield strength. It is also interesting to note that, in vanadium containing steels, even with high nitrogen content, no strain ageing due to nitrogen is observed, provided the vanadium to nitrogen ratio is maintained at or above the stoichiometric ratio of 4:1.
Effect of vanadium on high alloyed chromium-molybdenum steels
The specific roles of the various carbides and nitrides in any steel are controlled primarily by the relation of the temperature range over which they form to the Ar3 temperature. When more than one alloy is present, the alloy forming a carbide or nitride at the highest temperature dominates the structure and its effect on properties since (i) the compounds present at the high temperatures have a prior effect on grain structure over those forming at lower temperatures, and (ii) it has first choice of the carbon and nitrogen in the steel.
The presence of vanadium, even in small concentrations, has a positive effect on high alloyed chromium- molybdenum steels. Vanadium affects the solidification process of these alloy steels by narrowing of the temperature interval of crystallization. Namely, the crystals of V6C5 carbides are formed during the separation of primary austenite from the solution, blocking further growth of austenite dendrites and support the production of fine grained structures. Besides forming V6C5 carbides, similar to the molybdenum, vanadium, is partly distributed between present phases in the steel namely carbide (Cr,Fe)7C3 and austenite. The presence of vanadium enables the formation of (Cr,Fe)23C6 carbide and its precipitation in austenite during the cooling process. In local areas around fine carbide particles, austenite is transformed into martensite. In other words, vanadium reduces remained austenite and improves steel air-hardening. Vanadium concentration above 2.5 % significantly improves the impact toughness.