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Influence of niobium and coiling temperature on the mechanical properties of a cold rolled dual phase steel
2007 (English)In: Revue de métallurgie (Imprimé), ISSN 0035-1563, E-ISSN 1156-3141, Vol. 104, no 4, 191-197 p.Article in journal (Refereed) Published
Abstract [en]

Dual-phase (ferrite-martensite) low-carbon sheet steels are produced by annealing in the intercritical temperature range to produce ferrite-austenite mixtures, followed by accelerated cooling to transform the austenite phase into martensite. Dual-phase steels have gained considerable importance over the conventional HSLA steels due to the high strength and formability achieved. A good formability is achieved by high deformation hardening, due to a fine distribution of martensite. A cold rolled DP-grade (TS 600 Wa), coiled at 550 degrees C, was compared with a steel of approximately the same analysis but with the addition of 0.015 wt% Nb, coiled at 520 and 600 degrees C respectively. The Nb-addition refines the ferrite grain size substantially and a more uniform grain shape is achieved. The smaller grain size of the ferrite and martensite phases, results in a material with better formability. It appears that coiling at 520 degrees C results in a slightly better formability than coiling at 600 degrees C. The material shows a lower (alpha ->gamma start temperature, probably due to easier dissolution of thinner carbides, compared to the strip coiled at 600 degrees C. The recrystallization and phase transformation behavior was studied using the laser ultrasonic technique (LUS). Developing the LUS-technique for use in-line during annealing would open new ways of optimizing and controlling the mechanical properties of the final strip.

Place, publisher, year, edition, pages
2007. Vol. 104, no 4, 191-197 p.
Keyword [en]
Metallurgy
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-24973ISI: 000247042700005OAI: oai:DiVA.org:kth-24973DiVA: diva2:354752
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Structure and mechanical properties of dual phase steels: An experimental and theoretical analysis
Open this publication in new window or tab >>Structure and mechanical properties of dual phase steels: An experimental and theoretical analysis
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The key to the understanding of the mechanical behavior of dual phase (DP) steels is to a large extent to be found in the microstructure. The microstructure is in its turn a result of the chemical composition and the process parameters during its production. In this thesis the connection between microstructure and mechanical properties is studied, with focus on the microstructure development during annealing in a continuous annealing line. In-line trials as well as the lab simulations have been carried out in order to investigate the impact of alloying elements and process parameters on the microstructure. Further, a dislocation model has been developed in order to analyze the work hardening behavior of DP steels during plastic deformation.

From the in-line trials it was concluded that there is an inheritance from the hot rolling process both on the microstructure and properties of the cold rolled and annealed product. Despite large cold rolling reductions, recrystallization and phase transformations, the final dual phase steel is still effected by process parameters far back in the production chain, such as the coiling temperature following the hot rolling.

Lab simulations showed that the microstructure and consequently the mechanical properties are impacted not only by the chemical composition of the steel but also by a large number of process parameters such as soaking temperature, cooling rate prior to quenching, quench and temper annealing temperature.

 

Place, publisher, year, edition, pages
Stockholm: KTH, 2010
Keyword
Dual phase steels, continuous annealing, dilatometry, microstructure, mechanical properties, process parameters, dislocation model, plastic deformation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24869 (URN)
Public defence
2010-10-27, Nya konferensrummet, Brinellvägen 23, Stockholm, 16:20 (English)
Opponent
Supervisors
Note
QC 20101004Available from: 2010-10-04 Created: 2010-09-28 Last updated: 2010-10-06Bibliographically approved

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