Grain refinement of cold rolled microalloyed steels by rapid transformation annealing
Refinement of grain size improves strength and toughness. Under conventional processing of low carbon sheet steel products there is a practical limit to the refinement of the polygonal ferrite grain size. The current status is the production of hot rolled thermomechanically treated HSLA-steels with a minimum grain size of 4-5 mu m (ASTM G 12.5). The grain size of cold rolled and recrystallised material is mostly in the range of 10-20 mu m (ASTM G 8-10), whereas no transformation annealing is applied. The present project addresses how fine-grained microstructures can be achieved through rapid transformation annealing (hereinafter RTA) of cold rolled low carbon steel strip using the effect of transformation-induced grain refinement during heating and cooling. Furthermore, the mechanical properties in relation to the achieved grain sizes are determined and process windows for future industrial applications are developed. RTA cycles include the full or partial transformation of cold rolled non- or partly recrystallised a microstructure to g microstructure. In this way, a substantial grain refinement compared with conventional industrial annealing of cold rolled strip can be achieved. The target grain size of = 4 mu m was widely overcome with approximately 2 mu m in a microalloyed and approximately 2.5 mu m in a carbon manganese steel grade. Grain refinement results in a significant increase in strength and a pronounced strain hardening. Simultaneously, a slight loss of uniform and total elongation has to be accepted, which is the physical consequence of Considère's instability criterion. The applied temper rolling of 2 % is able to remove completely the yield point elongation. With a refinement from 4.2 to 2.3 mu m, the yield and tensile strength can be increased by about 130 MPa to 535 and 650 MPa respectively, while the total elongation is reduced by 4 % to 21 %, i.e. without a severe loss of formability.
Bibliographic Reference: EUR 22008 EN (2006), 239 pp. Euro: 35
Availability: Katalogue Number: KI-NA-22008-EN-S The paper version can be ordered online and the PDF version downloaded at: http://bookshop.europa.eu
ISBN: ISBN: 92-79-01101-4
Record Number: 200719290 / Last updated on: 2007-07-10
Original language: en
Available languages: en