Functions of Microalloy Elements Nb, V and Ti in Steel

Overview

Microalloy element refers to those elements with less than 0.1% mass fraction in the steel yet having significant or special influence on steel properties and microstructure. Among all, Nb (Niobium), V (Vanadium) and Ti (Titanium) are the most important microalloy elements. Adding a small amount of Nb, V and Ti to the steel allows to precipitate the solid solution of Nb, V and Ti through the diffusion of their carbon and nitride particles (size less than 5 nm) when the carbon equivalent is low. The grain is thus refined, and the strength and toughness of the steel, especially the low temperature toughness, are greatly improved, enabling the steel to have good weldability and usability.

Functions of Nb, V and Ti in steel

Refining grains

Refining grains will give enhanced steel strength and toughness; it is one of the most economical and effective ways to improve steel properties.

Preventing Austenite Grain Coarsening

As the heating temperature increases and the holding time prolongs, the austenite grains will become coarser. The coarse austenite is detrimental to steel workability and ferrite refinement; adding Nb, V, and Ti can prevent the austenite grain growth and increase the steel roughening temperature. This is because their carbon and nitride-dispersed particles can stabilize the austenite grain boundary and hinder the austenite grain boundary migration, i.e., prevent the austenite grain growth.

Refining Ferrite Grains

With the ongoing rolling, the temperature gradually goes down, the solubility product of Nb, V, and Ti carbon and nitride decreases in austenite, plus the deformation induces the precipitation, the carbon and nitride are precipitated before transformation from austenite to ferrite. The smaller the size of particles precipitated, the better. The particle size should be smaller than 5 nm. If greater than 100 nm, they will become inclusions, detrimental to the steel performance. These particles will become the nucleating agent for ferrite, allowing for large formation of ferrite under a small degree of subcooling; they are not easy to grow; therefore, the ferrite grains are refined. The effect of refining grains varies with the content of microalloy elements, among which, Nb is the most obvious, Ti is the second, and V is the worst.

Increasing Austenite Recrystallization Temperature

Nb, V, Ti, carbon and nitride have relatively high solubility in austenite. When the temperature is low, austenite is supersaturated solid solution. At this time, deformation induces precipitation of supersaturated austenite; these fine precipitates are largely distributed in the deformed austenite grain boundary or near the sub-crystal, which inhibits grain boundary migration and has a significant delay effect on recrystallization. The reason why the grain boundary migration can be suppressed is listed as below:

1) The dragging effect of solute atoms caused by Nb, V, Ti segregation at the grain boundary;
2) The pinning effect caused by the dispersion of their carbon and nitride at the grain boundary. Increasing the austenite recrystallization temperature can expand the non-recrystallization zone, which is very useful for controlled rolling in the non-recrystallization zone.

Improving the Steel Welding Performance

Steel often needs welding during use. The welded joint is the weakness, especially the heat affected zone which is most vulnerable to damage, while adding a small amount of Nb, V, Ti to the steel can improve the steel welding performance: 1) It can increase the steel strength while reducing the carbon equivalent content to improve the weldability; 2) it can fix impurities such as oxygen, nitrogen, sulfur, etc. to improve the weldability; 3) the effect of their micro particles, such as the insolubility of TiN at high temperatures, can prevent the coarsening of grains in the heat-affected zone, improving the toughness of the heat-affected zone to enhance the steel welding performance.

Conclusion

To conclude, the functions of Nb, V, Ti in steel are as below:

1) Preventing austenite grain coarsening;
2) Refining the ferrite grains;
3) Increasing the austenite recrystallization temperature;
4) Improve the welding performance of steel.

In addition, there are also many other important functions, such as solid solution strengthening, precipitation strengthening, control of inclusion forms, etc. With deeper understanding, improved control of rolling and cooling level and higher demand for high-performance steel, we believe Nb, V, Ti will play more important role in steels.

Contact

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2018.10.12

CIVMATS