WHY USE VCN?
STRENGTHENING MECHANISMS
In order to achieve higher strength in steel, there are four options for steel producer:
· Work hardening
· Precipitation hardening
· Alloying
· Transformation hardening
Two main paths for all of above methods producers to increase strength: process related refinements and ferroalloy additives. Vanadium is the principal ferroalloy additive for increasing steel strength.
Vanadium additivities to iron products is increasing strength through precipitation hardening as vanadium forms stable carbides and nitrites. However, it contributes limited degree of grain refinement strengthening.
High strength to weight ratio of vanadium allows for harder ending alloys with lower weights than can be achieved in the absence of vanadium.
More importantly, adding vanadium to steel does not require any process related changes to melting, rolling or finishing procedures as opposed to unalloyed steel.
Main vanadium additivities to iron products are Ferrovanadium (FeV) and VCN. VCN has variety of advantages over FeV.
The high strengths provided by small vanadium additions allow steelmakers to produce the value-added high-strength, low-alloy (HSLA) steels that are widely used engineering applications. By replacing ferrovanadium with VCN, steelmakers can achieve these high strengths more efficiently and at lower costs. VCN optimizes the strengthening mechanisms in high-strength, low-alloy steels, allowing steelmakers to use less vanadium to reach desired strength levels. This significantly reduces vanadium costs. That is why VCN is the preferred addition worldwide for microalloyed high-strength steels.
For example, a 0.053% VCN yielded the same 425MPa (60 ksi) yield strength as 0.070% vanadium added as ferrovanadium. This reduction is possible because VCN strengthens steel more efficiently than ferrovanadium. Depending on desired strength levels, steel producers using VCN can reduce vanadium additions by 25-40% compared to ferrovanadium.
HOW NITROGEN IN VCN REDUCES VANADIUM ADDITIONS
Reducing vanadium additions yields major cost savings. In the example below, equivalent yield strengths are obtained when either 0.10% vanadium is added as ferrovanadium or 0.06% vanadium is added as VCN. Using VCN reduces vanadium additions by 0.40 kg (0.90 lbs.) per metric ton of steel.
A NITROGEN BONUS
Hot-rolled steel containing 100 parts per million of nitrogen needs only 0.04% vanadium to obtain a strength increase of 110 MPa (16 ksi). In contrast, a steel containing about 50 parts per million of nitrogen requires 0.07% vanadium to obtain a similar increase in strength.
NITROGEN: AN EFFICIENT STRENGTHENER
In the presence of vanadium, nitrogen is converted from an impurity into a cost-effective alloying element. The vanadium nitrides formed by vanadium and nitrogen are more stable and more finely dispersed than vanadium carbides. For that reason, vanadium strengthening is more efficient in the presence of nitrogen.
SAME STRENGTH WITH LESS VANADIUM
By increasing strength, nitrogen allows steelmakers to use less vanadium, as shown in the graph above. Here, a 0.07% vanadium addition is needed to obtain a 110 MPa (16 ksi) increase in yield strength in a steel containing only 50 parts per million of nitrogen. If the nitrogen content is increased to 100 parts per million, only 0.04% vanadium is needed to obtain the same yield strength.
SUBSTANTIAL SAVINGS
Reducing vanadium additions yields major cost savings. In the example below, equivalent yield strengths are obtained when either 0.10% vanadium is added as ferrovanadium or 0.06% vanadium is added as VCN. Using VCN reduces vanadium additions by 0.40 kg (0.90 lbs.) per metric ton of steel.
OBTAINING EQUIVALENT YIELD STRENGTHS WITH LESS VANADIUM
HOW VCN PROVIDES MORE EFFICIENT STRENGTHENING THROUGH
TWO STRENGTHENING MECHANISMS
The two key strengthening mechanisms accounting for high yield strengths in low-alloy steels are:
• Precipitation Strengthening
• Grain Refinement
VCN enhances precipitation strengthening and grain refinement – two mechanisms that provide up to 70% of the yield strength of a typical high-strength, low-alloy steel. Grain refinement is the only mechanism that improves both strength and toughness while reducing the embrittling effects of precipitation. By balancing grain refinement and precipitation hardening, good toughness is obtained in high-strength steels.
THROUGH PRECIPITATION
MORE EFFICIENT PRECIPITATES BUILD STRENGTH
When VCN is added to steel, vanadium preferentially combines with nitrogen to form nitrogen-rich vanadium-carbonitride precipitates. The nucleation rate of these precipitates increases at higher nitrogen contents and produces a large number of small particles, as shown below. For example, increasing the nitrogen content from 80 to 160 parts per million reduces the particle diameter by half but increases the number of particles eight times. Increasing the effectiveness of precipitation strengthening depends on reducing the distance between particles. The greater number of smaller vanadium-carbonitride precipitates formed by VCN strengthens steel more efficiently than the coarser vanadium-carbide precipitates formed by ferrovanadium.
MORE EFFICIENT STRENGTHENING FROM SMALLER PRECIPITATES
Increasing the nitrogen content of steel reduces the size of vanadium-carbonitride precipitates. These smaller precipitates provide more effective strengthening, reducing vanadium additions.
Increasing the nitrogen content of steel reduces the size of vanadium-carbonitride precipitates provide more effective strengthening, reducing vanadium additions.
Reducing the particle diameter of precipitates from 4 to 2 nm gives eight times the number of precipitates in a given volume of steel. The larger number of small precipitates gives more efficient strengthening by reducing interparticle spacing.
THROUGH GRAIN REFINEMENT
SMALLER GRAINS FORMED BY VANADIUM AND NITROGEN
Austenitic grain refinement in high-strength steels is achieved by hot rolling in the recrystallization temperature range. The coarse grains found at the beginning of the rolling process are refined by repeated deformation and recrystallization during hot rolling. Vanadium allows this multiple recrystallization to take place at normal rolling temperatures, yielding a very-fine austenitic-grain structure after the thermal-mechanical-controlled process (TMCP) has been completed.
Optimum properties are obtained when the fine-austenite grains are transformed to a very-fine ferritic structure. This fine ferrite is achieved by proper combination of accelerated cooling after rolling as well as through alloy additions (like manganese) that lower the transformation temperature. Such a practice enhances ferrite nucleation and slows grain growth, providing the desirable balance of strength and toughness that is characteristic of high-strength, vanadium-nitrogen steels.
INTERGRANULAR FERRITE NUCLEATION IN HEAVY SECTIONS
In heavy sections where accelerated cooling cannot be achieved, the vanadium-nitride precipitates produced during deformation promote the nucleation of ferrite grains within the grain boundaries. The combined nucleating effect within the grains and the grain boundaries produces a fine ferritic structure in the finished steel.
With proper processing, the smallest grains were formed in the vanadium-nitrogen steel at the far right. Therefore, this steel has the best combination of strength and toughness.
HOW VANADIUM AFFECTS PROCESSING
Vanadium does not raise the “recrystallization‐stop temperature” – the temperature below which the austenite grains do not recrystallize between passes in the rolling mill. Therefore, steelmakers can easily exceed this temperature during rolling, allowing austenite to be refined through repeated recrystallization. This process is therefore known as recrystallization‐controlled rolling (RCR).
Fine recrystallized austenite grains are desirable because they insure the formation of small ferrite grains on cooling. A fine ferritic structure gives high strength and good ductility. Rapid cooling on the run‐out table lowers the austenite‐to‐ferrite transformation temperature and prevents the growth of ferrite, yielding a ferrite grain size of 4μm or less in strip products.
In addition, vanadium carbides and nitrides that provide precipitation strengthening are more soluble in austenite at normal rolling temperatures than similar compounds of other microalloys. These vanadium compounds do not precipitate until after the ferrite is formed during cooling, optimizing precipitation strengthening.
CONCERNED ABOUT STAIN AGING NOT WHEN VANADIUM IS PRESENT
For some steelmakers, adding nitrogen would seem to be the fastest way to send a heat to the scrap yard. So-called “free” nitrogen causes strain aging in carbon steels, increasing yield strength and brittleness after cold working. Strain aging is particularly detrimental in sheet products where it reduces formability.
However, when a nitride former such as vanadium is present, nitrogen becomes an extremely useful element. In high-strength, low alloy steels, nitrogen combines with vanadium to become a very cost-effective strengthener.
Of the three nitride-forming elements-vanadium, aluminum, and titanium - vanadium is the only element that effectively strengthens steel by combining with nitrogen.
FOR OPTIMUM STRENGTHENING
Slow cooling after austenite is transformed into ferrite will optimize strengthening from vanadium and nitrogen. This practice maximizes the precipitation of the vanadium nitrides that provide strengthening while eliminating strain aging. For strip steels, coiling temperatures of 600 to 630 deg. C (1,100 to 1,150 deg. F.) followed by slow cooling in the coil optimizes vanadium-nitride precipitation.
Plain-carbon steels containing as little as 0.006% nitrogen showed significant strain aging after simulated coil cooling. On the other hand, strain aging was virtually eliminated in vanadium strengthened HSLA steels containing as much as 0.020% nitrogen.
EXCELLENT WELDABILITY
Extensive research and technological studies have shown that the toughness of the heat-affected zone (HAZ) in vanadium-nitrogen steels depends on the transformation products and not on the nitrogen content. Excellent toughness can be obtained in these steels at heat inputs up to 4 kJ per mm (100kJ per in.) used in a majority of welding processes.
VCN IMPROVES STEELMAKING OPERATIONS
TAILORING NITROGEN TO YOUR PRACTICE
