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Modern high and ultra-high strength bainitic steels derive their strength from fine and ultra-fine bainitic ferrite grains in which the contribution from precipitation strengthening is of paramount importance. In the present work the strengthening potential of V, Nb, Ti and Cu has been evaluated for ultra-low, low and medium carbon bainitic steels. It was found that titanium provides a very strong and reliable precipitation strengthening of about 200 MPa at 0.2% Ti, directly after coiling at temperatures from 550°C to 350°C. The strengthening effect of niobium is weaker, with 150 MPa at 0.1% Nb, and requires coiling at temperatures of around 550°C. In plate rolling Nb can be fully utilised after post-TMCP heat treatment at 550-600°C. Vanadium shows a weak effect both in as-hot rolled products (strip and plate) and in isothermally heat-treated engineering steels, even at high V+N contents. This result is in sharp contrast with the well-established V-strengthening effect of polygonal ferrite, and is explained in the present work by the sluggish precipitation of V(C,N) in bainitic ferrite and low modulus of the weak, shearable (V,X)(C,N) particles. The full effect of V+N can only be obtained after heat treatment of the direct quenched material. The strengthening effect of copper was found to be also weak, with 50 MPa at 1 % Cu. Models were developed for predicting both the precipitation behaviour and the contribution from precipitates to yield strength. The strength of particles is expressed as a function of particle size which reaches the maximum level when the critical size of non-shearable particles is obtained.

Additional information

Authors: ZAJAC S, KIMAB, Stockholm (SE);PERRARD F, Ascometal, Hagondange (FR);KUZIAK R, Instytut Metalurgii Zelaza, Gliwice (PL);IUNG T, Arcelor Research, Maizières-lès-Metz (FR);ANTONISSEN J, OCAS, Zelzate (BE);KÖMI J, Rautaruukki Oyj, Raahe (FI);PIZARRO R, SIDENOR I+D, Basauri (ES)
Bibliographic Reference: EUR 23722 EN (2009), 179 pp. Free of charge
Availability: Katalogue Number: KI-NA-23722-EN-S The PDF version can be downloaded from:
ISBN: ISBN: 978-92-79-10621-7
Record Number: 200910240 / Last updated on: 2009-04-10
Original language: en
Available languages: en