Grain refinement by intragranular ferrite nucleation on precipitates in microalloyed steels
Grain refinement through the formation of intragranular ferrite on VN precipitated in the centre of austenite grains has been systematically investigated and applied for development of TMCP for thick plate and heavy beam. A large amount of work has been carried out to determine the kinetics of strain induced precipitation of VN in austenite and elucidate the mechanism of intragranular ferrite formation on VN. The RPTT (recrystallisation-precipitation-time-temperature) diagrams were produced for a range of low, medium and high V+N steels. This was followed by interrupted TMCP simulations for determining the effect of austenite structure and size distribution of VN on the ferrite grain refinement. It was found that high nitrogen steels precipitation of VN starts within 10 second after deformation and a reduction by 40 % is sufficient for producing a high density of 20-80nm large VN particles in 0.1 % V steels. The ferrite crystals nucleate on the faces of VN cube-like particles due to the low atomic mismatch between ferrite and VN. The ferrite crystals growing from the opposite VN faces adopt the same orientation forming four-arm rosettes on the planar section. Grain refinement through intragranular ferrite formation was adopted for TMCP rolling of thick plate and heavy beam. For thick plate it was necessary to modify the chemical composition as aluminium in Al-killed steels inhibits the formation of VN. For heavy beam a new �hold� rolling practice was applied in which the cooling rate after final deformation was slowed down by an isolating blanket. The yield strength of V-N beams was increased by ~ 130MPa at good toughness properties. Ferrite was refined by 1/3.
Bibliographic Reference: EUR 22451 EN (2007), 149 pp. Euro: 25
Availability: Katalogue Number: KI-NA-22451-EN-S The paper version can be ordered online and the PDF version downloaded at: http://bookshop.europa.eu
ISBN: ISBN: 978-92-79-04814-2
Record Number: 200719221 / Last updated on: 2007-07-05
Original language: en
Available languages: en