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 ofTi 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. Hoherfeste vakuumentkohlte Stahlemit Mikrolegierung unter besonderer Berucksichtigung von IF·Stahlen. Hohertsster IF-Stahl weist im Vergleich mit anderen Hohertest-Konzepten ein besonders hohes Umformvermogen bei erhohtsr Festigkeit auf, wobei die Blechdickenreduzierung durch Umformen beim hohertesten IF-Stahl insgesamt am geringsten ist. Ais MechaniSmus der Festigkeitssteigerung eignet sich besonders eine Mischkristallhartung durch Si, Mn und P, die bei angehobenen Festigkeitswerten zu vergleichsweise geringen EinbuBen in der Umformbarkeit tuhrt. Ein zusatztlcher Festigkeitszuwachs ist unter Ausnutzung des Bake-Hardeninq-Effektes moglich. Dieser kann mit einer unvollstandlqen C-und N-Abbindung durch Ti und/oder Nb oder durch eine stochiometrische Mikrolegierung mit V erreicht werden. Der Effekt ist im Wesentlichen auf die verhaltnismaBig geringe thermodynamische Stabilltat von VC zurOckzufOhren. V-Iegierter, vakuumentkohlter Stahl kombiniert auf diese Weise ein gOnstiges Umformverhalten mit einem Festigkeitszuwachs durch Bake-Hardening. Eine Mischkristallverfestigung mit P kann durch eine steigende Segregation dieses Elementes an den Korngrenzen zu einer ungOnstigen VersprOdung bei hOherfestem IF-Stahl und zu interkristallinem Bruchverhalten fOhren. B-Zusatze in Massengehalten von ca. 0.0...

Further Results from Strip Casting with the Single-Belt Process.

“…Hence, a considerable range of elongation and tensile strength is covered. For the as-cast material the elongation values are quite poor (below 10%) and they fall below the established relationship between tensile strength and elongation 8) which is the effect of the porosity. As expected the ductility is raised considerably by in-line hot rolling which was with jϭln(d 0 /d 1 )ϭ0.4 by our single stand rolling facility, and the data points come close to the establiched relationship.…”

“…Relationship between tensile strength and elongation for carbon steels cast with the single belt process and hot rolled with different degrees of reduction, and comparison with the known relationship available in the literature. 8) volving high linear strand/strip velocities and complete coupling of the steps casting/rolling/coiling requires the strict control of the relevant operation parameters. The system used in Clausthal is rather elaborate for a university facility but certainly not sufficient for a production line which would need more sophisticated controls.…”

Influence of the Hot Rolling Process on the Mechanical Behaviour of Dual Phase Steel

“…Vanadium is a weaker carbide former than Ti or Nb [Dav78b]. Hence, it is expected to dissolve easily during annealing and, as a result, increases the amount of solute C, thus enhancing bake hardenability of the LC steels [Eng96].…”