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In this project a new microstructure, carbide free bainite, is explored in a wide range of steels. Cementite has been responsible for the limited application of conventional bainitic steels, however cementite precipitation during bainite formation can be suppressed using controlled rolling followed by multi-stage cooling processes in low carbon steels, and by the judicious use of silicon in medium carbon steels. Thus, it has been found that the optimum TMCP processing for obtaining fine carbide-free bainite with a yield strength of 950 MPa in a 0.2 % Ti steel is based on high reheating temperature of 1290°C to dissolve titanium, finishing rolling at low temperature of 820°C for obtaining fine pancake austenite grains and accelerated cooling at 35°C to 300°C for austenite transformation to fine carbide free bainite. Moreover, higher carbon contents were also tried in the alloy design to reach higher strength levels. In this case, thermodynamic and kinetic models were used to design steels with a mixture of bainitic ferrite and carbon-enriched retained austenite. All the designed grades present the desired microstructure after hot rolling and a two-steps cooling. Their tensile strengths range from 1600 MPa to 1950 MPa while keeping a uniform elongation equal to 4 % and a total elongation over 10 %. Regarding toughness at room temperature, they match quenched and tempered martensitic steels. However, for some applications, weldability has to be ensured. We also succeeded in simulating the target microstructure following classical hot-rolling schedule including a coiling stage comparable to existing Si-TRIP steels.

Additional information

Authors: CABALLERO F G, CENIM-CSIC, Madrid (ES);SANTOFIMIA M J, CENIM-CSIC, Madrid (ES);CAPDEVILA C, CENIM-CSIC, Madrid (ES);GARCÍA DE ANDRÉS C, CENIM-CSIC, Madrid (ES);ZAJAC S, KIMAB, Stockholm (SE);ALLAIN S, Arcelor Research, Maizières-lès-Metz (FR);IUNG T, Arcelor Research, Maizières-lès-Metz (FR);COURTURIER A, Arcelor Research, Maizières-lès-Metz (FR);DRILLET J, Arcelor Research, Maizières-lès-Metz (FR);QUIDORT D, Arcelor Research, Maizières-lès-Metz (FR);COLIN C, Arcelor Research, Maizières-lès-Metz (FR);GARAT X, Arcelor Research, Maizières-lès-Metz (FR);SPINDLER H, Voest Alpine, Linz (AT)
Bibliographic Reference: EUR 22450 EN (2007), 142 pp. Euro: 25
Availability: Katalogue Number: KI-NA-22450-EN-S The paper version can be ordered online and the PDF version downloaded at:
ISBN: ISBN: 978-92-79-04610-0
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