Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Structure and mechanical properties of dual phase steels: An experimental and theoretical analysis
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Mechanical Metallurgy.
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 [en]
Dual phase steels, continuous annealing, dilatometry, microstructure, mechanical properties, process parameters, dislocation model, plastic deformation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-24869OAI: oai:DiVA.org:kth-24869DiVA: diva2:353680
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
List of papers
1. Influence of niobium and coiling temperature on the mechanical properties of a cold rolled dual phase steel
Open this publication in new window or tab >>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, 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.

Keyword
Metallurgy
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24973 (URN)000247042700005 ()
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2010-10-04Bibliographically approved
2. Effects of process parameters prior to annealing on the formability of two cold rolled dual phase steels
Open this publication in new window or tab >>Effects of process parameters prior to annealing on the formability of two cold rolled dual phase steels
2008 (English)In: STEEL RES INT, ISSN 1611-3683, Vol. 79, no 4, 297-305 p.Article in journal (Refereed) Published
Abstract [en]

This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability-related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DIP-steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.

Keyword
cold rolled dual phase steels, coiling temperature, cold rolling reduction, mechanical properties, formability, microstructure, banding, niobium, manganese
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24972 (URN)10.1002/srin.200806354 (DOI)000256177900007 ()
Note

QC 20101004

Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2017-06-08Bibliographically approved
3. Simulation of the Soaking and Gas Jet Cooling in a Continuous Annealing Line using Dilatometry
Open this publication in new window or tab >>Simulation of the Soaking and Gas Jet Cooling in a Continuous Annealing Line using Dilatometry
2010 (English)In: Steel Research International, ISSN 1611-3683, Vol. 81, no 2, 158-167 p.Article in journal (Refereed) Published
Abstract [en]

The present study concerns the simulation of a continuous annealing line (CAL), using dilatometry. Simulations of CAL have been performed on four commercial steel grades with different chemical compositions in order to investigate how the alloying elements C, Mn, Si and B affect the microstructure and hardness of dual phase (DP) and martensitic steels. Three annealing cycles corresponding to those used in a CAL have been applied. When annealing intercritically, as is the case in DP-steel production, the materials do not reach equilibrium during soaking. Mn and C increase the austenite content and consequently the hardness of the materials. Higher levels of Si (0.4 wt %) are required to retard the formation of new ferrite during cooling in the gas jet section, prior to quenching. 6 increases hardenability effectively when annealing in the austenite region but is not as efficient during intercritical annealing, which implies that boron restrains ferrite nucleation rather than impeding ferrite growth. Results from DICTRA calculations show that it is possible to simulate the phase transformations during soaking, gasjet cooling and quenching.

Keyword
dilatometry, simulation, continuous annealing, high strength steels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24971 (URN)10.1002/srin.200900097 (DOI)000275142400009 ()2-s2.0-77949483612 (Scopus ID)
Note
QC 2010104Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2011-03-16Bibliographically approved
4. A dislocation model for the stress-strain behavior of dual phase steels
Open this publication in new window or tab >>A dislocation model for the stress-strain behavior of dual phase steels
2008 (English)In: IDDRG 2008 International Conference, 2008Conference paper, Published paper (Refereed)
Abstract [en]

The Bergström dislocation model is used to study the influence of coiling temperature after hot rolling on the mechanical properties of the cold rolled SSAB-grade Docol 800DP. Further, the anisotropy-effect for this type of steel, caused for instance by banding, is investigated. The physical dislocation model is based on the assumption that the work hardening behaviour is controlled by the creation, the immobilisation and the remobilisation of dislocations and it is demonstrated that all these processes are strongly influenced by the amount of hard martensite particles in DP-steels. The true strain to necking is a widely used tool for estimating the stretch formability of DP-steels. A detailed discussion regarding the influence of for instance friction stress, rate of dislocation remobilisation and mean free path of dislocation motion is presented. It is also indicated that the best stretch formability of DP-steel is obtained by minimising the quotient between the rate of work hardening at smaller strains with that at larger strains, close to the strain at necking.

Keyword
Strain hardening, Dislocation model, Dual-phase steel, Formability
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24978 (URN)
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2010-10-04Bibliographically approved
5. A dislocation based theory for the deformation hardening behavior of DP-steels: Impact of martensite content and ferrite grain size
Open this publication in new window or tab >>A dislocation based theory for the deformation hardening behavior of DP-steels: Impact of martensite content and ferrite grain size
2010 (English)In: Journal of Metallurgy, ISSN 1687-9465, E-ISSN 1687-9473, Vol. 2010, no 647198Article in journal (Refereed) Published
Abstract [en]

A dislocation model, accurately describing the uniaxial plastic stress-strain behavior of dual phase (DP) steels, is proposed and the impact of martensite content and ferrite grain size in four commercially produced DP-steels is analyzed. It is assumed that the plastic deformation process is localized to the ferrite. This is taken into account by introducing a non-homogeneity parameter, f(ε), that specifies the volume fraction of ferrite taking active part in the plastic deformation process. It is found that the larger the martensite content the smaller the initial volume fraction of active ferrite which yields a higher initial deformation hardening rate. This explains the high energy absorbing capacity of DP-steels with high volume fractions of martensite. Further, the effect of ferrite grain size strengthening in DP-steels is important. The flow stress grain size sensitivity for DP-steels is observed to be 7 times larger than that for single phase ferrite.

Keyword
DP-steels, dislocation model, inhomogeneous deformation hardening, martensite content, ferrite grain size
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24980 (URN)10.1155/2010/647198 (DOI)
Note

QC 20101004. Updated from submitted to Epub ahead of print, 20120315. Updated from Epub ahead of print to print 20131126

Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2013-11-26Bibliographically approved

Open Access in DiVA

Summary(19355 kB)20724 downloads
File information
File name FULLTEXT10.pdfFile size 19355 kBChecksum SHA-512
61d11d60e6bfb23e43dbdd9c598537bd2fcef930951d865d862ef736e893dd8df7b914b58f43acec597072b16d71f1edb48b51d74730a8aa1d7a2640f8ad98fc
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Granbom, Ylva
By organisation
Mechanical Metallurgy
Metallurgy and Metallic Materials

Search outside of DiVA

GoogleGoogle Scholar
Total: 20724 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

urn-nbn

Altmetric score

urn-nbn
Total: 1269 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf