Suppression of banded structure and refinement of the microstructure of high-strength cold-rolled steels by optimisation of the hot-rolling cooling path
The objective of this project is to study the effect of the hot-rolling scheme on cold-rolled high-strength steels. Two metallurgical points are addressed: banded structures and microstructure refinement. The detrimental effect of microstructural banding on in-use properties was demonstrated and quantified. The underlying cause of microstructural banding is segregation in the form of alternating high and low substitution element layers. For the typical C content of 600 MPa grades, the occurrence of microstructural banding is very low but it becomes of prime importance for 800 MPa grades. In the hot-rolled state, microstructural banding does not always appear in steel with compositional gradient. At high cooling rates and/or low coiling temperatures, the underlying pattern of segregation can be hidden behind a more uniform bainitic or martensitic microstructure. At the same time, a combination of low coiling temperature and high cooling rate induces an increase in the tensile strength of the materials, linked with a slight refinement of the microstructure and a change from pearlite to bainite and martensite as a second phase. Even if the accelerated cooling after hot rolling and a low coiling temperature are efficient in order to minimise the microstructural banding of the hot-rolled strip, this only hides the microchemical banding, which tends to reappear during the intercritical annealing. By applying a classical industrial continuous annealing or galvanising scheme, bands are present in the final state and the improvement made on the hot-rolled microstructure is lost. Only a slight increase of the mechanical properties is kept, due to microstructure refinement.
Bibliographic Reference: EUR 22830 EN (2007), 163 pp. Euro: 25
Availability: http://bookshop.europa.eu/is-bin/INTERSHOP.enfinity/WFS/EU-Bookshop-Site/en_GB/-/EUR/ViewPublication-Start?PublicationKey=KINA22830 (Catalogue Number: KI-NA-22830-EN-S)
ISBN: ISBN: 978-92-79-05478-5
Record Number: 200719509 / Last updated on: 2007-10-19
Original language: en
Available languages: en