Niobium in interstitial-free (IF) steel


  • Ultra-low carbon steel provides the best formability and is excellently suited for deep-drawing and stretch forming. It is produced by vacuum degassing bringing the amount of interstitial carbon and nitrogen to a level below approximately 30 ppm. The low level or complete absence of interstitials provides low yield strength and high elongation.
  • Strong carbide and nitride formers such as niobium or titanium bind any remaining carbon and nitrogen after degassing of liquid steel.
  • If the purpose of alloying is only to fix solute N and C atoms, it is sufficient to add only Ti to steel. However, addition of Nb is necessary to obtain a higher r-value. The reason why the r-value increases further with Nb addition can be explained by the grain refinement effect of Nb. Nb addition is effective in increasing the r-value firstly because the finer grains produce higher strain in rolling regardless of the rolling conditions, and secondly because recrystallization at deformation sites with the higher strain begins due to the formation of larger numbers of nuclei with desirable orientations. As a result, during annealing, a more pronounced {111} texture can develop in the Nb-bearing strip than in strips containing only Ti. Accordingly, the r-value increases in steels containing Nb.
  • Ti-alloyed IF steels always display a lower yield strength and higher elongation than Nb-alloyed IF steels.
  • The higher yield strength level in Nb-alloyed steels, which is explained partly by its finer grain, yet also partly by finer carbide precipitates, naturally is the better starting platform for high strength IF steels.
  • Compared to Ti-alloyed IF steels, Nb-alloyed IF steels require higher annealing temperatures for complete recrystallization, except for extremely low carbon contents.
  • In Ti-alloyed steels the Ti/C + N mass ratio has only a small influence at the start of recrystallization, while it exerts a strong influence at the end of recrystallization. The particularly slow recrystallization of more or less stoichiometrically composed alloys is attributed to the, in this case, extremely fine precipitates. The accelerating effect exerted on recrystallization by over-stoichiometric titanium additions can be explained by a scavenging effect.
  • In Nb-alloyed IF steels the start of recrystallization is unaffected by the C content; it shifts to higher temperatures if the content of dissolved Nb increases, i. e. when there is a hyper-stoichiometric Nb/C ratio of > 7.75.Nitrogen in these steels is fixed in the form of AIN precipitates. In over-stoichiometric alloyed steel, the end of recrystallization shifts towards lower temperatures. Solute niobium atoms, on the other hand, retard the start of recrystallization.
  • The recrystallization of austenite is known to be retarded by solute Nb. It is also well recognized that the recrystallized state of the austenite strongly influences the resulting transformation texture in the as coiled hot band. The {100} component of the transformation texture results from the recrystallization of austenite. Hence, if solute Nb suppresses austenite recrystallization, the resulting hot band texture will have both a lower {100} component and higher ratio of {111}/{100} resulting in better deep drawability.
  • An important aspect regarding the practical application in the production line is that Nb-alloyed IF steels require temperatures some 50 °CK higher as compared to Ti-alloyed steels in order to achieve complete recrystallization.
  • Ti-alloyed IF steels display strong planar anisotropy of mechanical properties responsible for ear forming after deep drawing. Addition of niobium to Ti-alloyed IF steels reduces planar anisotropy

High strength IF steel


  • High strength IF steels with yield strength >200 MPa (after temper rolling) and tensile strength >350 MPa can be produced by adding phosphorus. An attractive combination of properties is obtained by Nb + P alloyed IF steels in particular. Phophorous is added to high strength IF steels for solid solution strengthening. However, in presence of Ti, P forms FeTiP type of precipitates leading to the deterioration of both drawability and loss of solution strengthening.
  • The very strong binding reaction of Ti with C and N renders very clean grain boundaries. This phenomenon is responsible for grain boundary embrittlement by P as it segregates to ferrite grain boundaries.
  • The presence of solute niobium, especially at the grain boundaries, can largely offset the embrittling effect of P.

Bake-hardening (BH) steel


  • Bake-hardening steel is a particular variant of higher strength IF steel where carbon is not completely stabilized but a small amount is left in interstitial solution. This free carbon is diffusing to dislocations generated during forming and locking them by the so-called Cottrell effect.
  • The BH effect delivers a yield strength increase of 30-40 MPa, yet too much free carbon leads to premature ageing already at ambient temperature. The difficulty for the steelmaker is to accurately adjust the amount of free carbon in the range of 5-10 ppm.
  • The free-carbon adjustment is most efficiently done in Nb-Ti dual stabilized steel. In this concept Ti is added only for fixing nitrogen. Carbon is partially fixed by sub-stoichiometric addition of niobium. Since niobium does not react with other elements present in these steels and its addition can be done very accurately due to high recovery, it is comparably much easier for the steelmaker to hit the targeted free carbon range.

Zinc coated IF steel


  • A very beneficial use of solute Nb can be found in galvannealed IF and IFHS steels. Nb reduces outburst (visible defect at coating surface) and powdering (flaking off) of Zn layers.
  • In Ti-alloyed IF steels, the outburst structure with brittle 𝜞1-phase grows deeply at each grain boundary location due to the absence of segregated carbon at the grain boundaries after scavenging of carbon by Ti. On the other hand, in Nb–Ti-alloyed IF steels, where Nb and Ti scavenge C and N, respectively, a small amount of carbon segregates at the grain boundaries. Consequently, outburst is not as deep as in the Ti–IF steels.
  • Solute Nb present on the free surface can reduce the brittle gamma layer formation, leading to better coating adherence and hence less powdering.