Microalloyed, vacuum degassed high-strength steels with special emphasis on IF steels

Abstract

In comparison with other high-strength concepts, high-strength IF steel has a particularly high forming capacity with increased strength, while the sheet thickness reduction through forming is, on the whole, smallest with high-strength IF steel. Solid-solution hardening through Si, Mn and P is particularly suitable as a mechanism for increasing strength, as this leads to comparatively low formability losses with increased strength values. An additional increase in strength is possible using the bake-hardening effect. This can be achieved with an incomplete C and N binding through Ti and/or Nb, or through stoichiometric microalloying with V. The effect is basically caused by the relatively low thermodynamic stability of VC. V-alloyed, vacuum degassed steel combines in this way good forming behaviour with an increase in strength through bake-hardening. Through an increasing segregation of P at grain boundaries, solid solution hardening with this element can lead to unfavourable embrittlement in higher strength IF steel and to intercrystalline fracture. B-addition to the amount of 0.002% (mass content) greatly reduces the tendency to become brittle without leading to any remarkable losses with regard to formability. Examinations concerning the precipitation behaviour of Ti-IF steel with P show that a formation of iron titanium phosphate (FeTiP) in the hot strip only occurs where there is an excess of Ti and high coiling temperatures are used. For a P-alloyed IF steel with stoichiometrical Ti-content no precipitation of FeTiP takes place in hot band, nor is the thermal activation in continuous annealing simulation sufficient to achieve a transformation of existing Ti-precipitates into FeTiP, which might occur according to literature.