Niobium in dual phase steel


  • DP steel offers higher strength than HSLA steel and simultaneously provides comparably good formability. In practice, however, DP steels showed tendency to cracking in roll forming and die bending especially for tighter bending radii. Furthermore, DP steel showed to be sensitive to edge cracking during flanging operations. These problems are becoming more severe with increasing martensite share, i.e. for higher strength variants.
  • The origin of cracking is caused by the hard-soft contrast between the phase constituents within the DP microstructure. Plastic deformation is localized in the soft ferrite grain leading to delamination from the hard martensite phase or inducing cracking in the martensite. This can result in macroscopic cracking when martensite islands are large in size or clustered.
  • Niobium microalloying was shown to alleviate this problem. The simple addition of niobium to an existing DP steel results in a much-refined microstructure in the ferritic-pearlitic hot-rolled strip that is subsequently inherited to the cold-rolled annealed (galvanized) strip. This refinement naturally results in smaller martensite island size and a more homogeneous dispersion of martensite in the ferrite matrix. Consequently crack starters are smaller in size and crack propagation is obstructed. By such approach, the bending behaviour (critical bending angle / radius) could be improved by around 30%.
  • New demands from the automotive industry have triggered metallurgical re-design of the initial generation DP steels. The need for improved weldability and better hole expansion ratio (HER) can be fulfilled by reducing the carbon content from traditionally over-peritectic (0.14-0.16%) to under-peritectic (<0.10%). In such new alloy design niobium plays a manifold role. The presence of niobium generally results in a strength increase of 70-100 MPa via grain refinement and precipitation hardening. This means the specified minimum strength can be achieved with a lower share of martensite phase. Consequently elongation becomes better due to an equivalent larger share of ductile ferrite. Industrial experience has shown that for under-peritectic alloys elongation is typically 2 points higher than in the over-peritectic variant, whereas the HER increases by 70-100%.
  • Another major benefit of niobium microalloying in DP steels is processing robustness. These steels are in the majority of cases produced via hot dip galvanizing lines. The challenge is to adjust the required phase distribution of ferrite and austenite during intercritical annealing. The austenite phase fraction should transform into martensite during quenching, but not partially decay into bainite. Grain refinement in the original ferritic-pearlitic cold-rolled microstructure accelerates the transformation kinetics and leads to quicker achievement of the desired amount of austenite. The smaller grain sizes also result in shorter diffusion distances and thus more efficient partitioning of carbon. This leads to more stable austenite islands and less decay into bainite. Via these mechanisms Nb-microalloyed DP steel is more robust against variations in line speed and can be processed on galvanizing lines with vertical as well as horizontal furnace.
  • DP780 and DP980 grades including the recent DP-HD grades are nowadays standardly microalloyed with niobium.


Niobium in TRIP steel


  • The metallurgical mechanisms of niobium in DP steel function identically in the production of TRIP steel. The smaller austenite grains in this case can accumulate more easily a sufficient amount of carbon, stabilizing the austenite phase down to ambient temperature.
  • Furthermore, solute niobium contributes to preventing cementite precipitation during bainitic holding, thus preserving more carbon for austenite stabilization.
  • As a consequence, niobium alloyed TRIP steel is showing increased amounts of retained austenite in the final product. The fact that austenite grains are smaller in size causes them to transform into martensite only at higher applied strain. In this way two-fold work hardening behaviour is achieved, firstly by a DP-like effect at low strain and secondly by the TRIP effect at higher strain.


Niobium in complex phase steel


  • Complex phase steel has higher yield and tensile strength as compared to DP steel but less elongation. As the name suggests many different phases are represented in the microstructure of CP steel such as ferrite, bainite, martensite and small amounts of retained austenite. Due to the good bendability CP steel is excellently suited for roll forming.
  • CP steel has a very fine-grained microstructure, which is usually achieved by niobium microalloying. The fine grain contributes to strength and also homogenizes the microstructure resulting in good bendability.
  • Niobium can also contribute to precipitation strengthening of the ferrite phase in CP steel