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Effect of Temperature Reversion on Hot Ductility and Flow Stress-Strain Curves of C-Mn Continuously Cast Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Chongqing University, China.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
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2015 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 46, no 4, 1885-1894 p.Article in journal (Refereed) Published
Abstract [en]

The influence of temperature reversion in secondary cooling and its reversion rate on hot ductility and flow stress-strain curve of C-Mn steel has been investigated. Tensile specimens were cooled at various regimes. One cooling regime involved cooling at a constant rate of 100 degrees C min(-1) to the test temperature, while the others involved temperature reversion processes at three different reversion rates before deformation. After hot tensile test, the evolution of mechanical properties of steel was analyzed at various scales by means of microstructure observation, ab initio prediction, and thermodynamic calculation. Results indicated that the temperature reversion in secondary cooling led to hot ductility trough occurring at higher temperature with greater depth. With increasing temperature reversion rate, the low temperature end of ductility trough extended toward lower temperature, leading to wider hot ductility trough with slightly reducing depth. Microstructure examinations indicated that the intergranular fracture related to the thin film-like ferrite and (Fe, Mn)S particles did not changed with varying cooling regimes; however, the Widmanstatten ferrite surrounding austenite grains resulted from the temperature reversion process seriously deteriorated the ductility. In addition, after the temperature reversion in secondary cooling, the peak stress on the flow curve slightly declined and the peak of strain to peak stress occurred at higher temperature. With increasing temperature reversion rate, the strain to peak stress slightly increased, while the peak stress showed little variation. The evolution of plastic modulus and strain to peak stress of austenite with varying temperature was in line with the theoretical prediction on Fe.

Place, publisher, year, edition, pages
2015. Vol. 46, no 4, 1885-1894 p.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-173155DOI: 10.1007/s11663-015-0349-3ISI: 000359022000035Scopus ID: 2-s2.0-84938747239OAI: oai:DiVA.org:kth-173155DiVA: diva2:854767
Funder
Swedish Research Council
Note

QC 20150917

Available from: 2015-09-17 Created: 2015-09-07 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Temperature dependent mechanical properties of as-cast steels: Experimental and theoretical studies
Open this publication in new window or tab >>Temperature dependent mechanical properties of as-cast steels: Experimental and theoretical studies
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The temperature-dependent mechanical properties of steels are important to avoid processing defects, to understand and to improve the high-temperature performance. At the same time, having access to thermal properties gives us opportunity to assess the first-principles theoretical predictions at elevated temperatures. These properties are directly bound up with the performance of individual phase and also the evolving microstructure states at different thermalmechanical processes. In the present thesis, the temperature-dependent mechanical properties of continuously cast steels and iron are investigated using experimental and theoretical methods. Experimental studies are performed centering on the influence of thermal cycles occurring in secondary cooling.

The temperature reversion in secondary cooling makes the hot ductility trough occurring at higher temperatures with greater depth. Increasing the reversion rate, the low temperature end of the ductility trough slightly extends to lower temperatures. As indicated by microstructure examinations, the intergranular fracture contributed from the thin film-like ferrite and (Fe,Mn)S particles slightly changes with the varying thermal cycles; however, the widmanstatten ferrite observed in the temperature reversion process seriously deteriorates the ductility. Due to the temperature reversion process, the peak stress slightly declines and the peak of strain to peak stress moves to higher temperatures. On the other hand, the sequential formations of ferrite and pearlite in the austenite transformation are indicated by two distinct peaks on the thermal expansion coefficient. By applying the developed concise model, the volume fractions of ferrite, pearlite, and austenite are quantitatively monitored in the phase transformation. Either increasing the cooling rate or the content of austenite stabilizing atoms Ni and Cu, the austenite transformation occurs at relatively low temperatures and indicates a greater phase transformation rate for both ferrite and pearlite. In addition, the final fraction of ferrite/pearlite increases/decreases with increasing the cooling rate, increasing the alloying atoms like Ni, Cr and Cu or lowering the carbon content.

The temperature dependence of the polycrystalline Young’s modulus and the tetragonal shear modulus c0 of iron is predicted using ab initio calculations within the exact muffin-tin orbitals formalism. The dependence exhibits a good consistency with that of the peak stress observed in the experiments for the commercial steel. Despite the significant effects of magnetic sate and crystal structure on the elastic property of iron, the magneto-volume coupling primarily determines the temperature dependence for the single phase. In contrast, the dominant role of the volume expansion is observed for both the paramagnetic (PM) face centered cubic (fcc) and body centered cubic (bcc) Fe, although they show different magneto-elastic behaviors. Based on the theoretically predicted thermal expansion for PM bcc Fe, both the lattice vibrations and the magnetic evolution contribute to the thermal expansion, and the former is dominant.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. viii, 40 p.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-177907 (URN)978-91-7595-777-7 (ISBN)
Presentation
2015-12-04, Sal N111, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20151130

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2015-11-30Bibliographically approved

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