MICROALLOYED, VACUUM DEGASSED HIGH-STRENGTH STEELS WITH SPECIAL EMPHASIS ON IF STEELS

A comparison with other high-strength concepts, high-strength IF steel has a particularly:high forming capacity with increased strength, whi le the sheet thickness reduction through forming is, on the whole, sma llest with high-strength IF steel. Solid-solution hardening through Si , Mn and P is particularly suitable as a mechanism for increasing stre ngth, as this leads to comparatively low formability losses with incre ased strength values. An additional increase in strength is possible u sing the bake-hardening effect. This can be achieved with an incomplet e C and N binding through Ti and/or Nb, or through stoichiometric micr oalloying with V. The effect is basically caused by the relatively low thermodynamic stability of VC. V-alloyed, vacuum degassed steel combi nes in this way good forming behaviour with an increase in strength th rough bake-hardening. Through an increasing segregation of P at grain boundaries, solid solution hardening with this element can lead to unf avourable embrittlement in higher strength IF steel and to intercrysta lline fracture. B-addition to the amount of 0.002% (mass content) grea tly reduces the tendency to become brittle without leading to any rema rkable 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 th ere 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 conti nuous annealing simulation sufficient to achieve a transformation of e xisting Ti-precipitates into FeTiP, which might occur according to lit erature.