Development and processing of low carbon bainitic steels
1University of Oulu, Faculty of Technology, Department of Mechanical Engineering
|Online Access:||PDF Full Text (PDF, 23 MB)|
|Persistent link:|| http://urn.fi/urn:isbn:9789514292576
|Publish Date:|| 2009-10-20
|Thesis type:||Doctoral Dissertation
|Defence Note:||Academic dissertation to be presented with the assent of the Faculty of Technology of the University of Oulu for public defence in Raahensali (Auditorium L10), Linnanmaa, on 30 October 2009, at 12 noon
Doctor Sebastian Cobo
Professor Devesh Misra
The aim of this work was to study systematically the effects of composition and processing on austenite grain growth and static recrystallization (SRX) kinetics, austenite decomposition under controlled cooling as well as microstructures, mechanical properties and weldability of hot rolled low carbon bainitic (LCB) steels. The results showed that the coarsening of austenite grain structure is influenced by the chemical composition. Steels with Nb-Ti alloying exhibited fine and uniform austenite grain size up to 1125 °C, whereas higher temperatures led to formation of the bimodal grain structures. However, with Nb-Ti-B microalloying, the abnormal grain growth was already observed at 1050 °C. SRX rate at roughing temperatures, determined by the stress relaxation method, was found to be retarded markedly by Mo, Nb and B alloying. For the test conditions investigated, the decomposition of austenite started in the temperature range from 780 °C to below 550 °C. All alloying elements with the exception of Nb (0.04–0.10 wt-%) decreased the phase transformation temperatures and increased the hardness of dilatometric specimens. Detailed microstructural examinations enabled the identification of 4 different ferrite morphologies: polygonal ferrite, quasi-polygonal ferrite (QF), granular bainitic ferrite (GB) and bainitic ferrite (BF), generally as a mixed microstructure consisting of 2–3 types morphologies. Consistent with the microstructures detected in dilatometric experiments, the microstructures of rolled plates comprise various combinations of low C ferrite morphologies. These microstructure types provided the yield strengths from 500 MPa up to 850 MPa in hot rolled condition and from 500 MPa to 950 MPa in heat-treated condition (600 °C/1h). The yield strengths from 500 MPa to 570 MPa were mainly related to QF microstructures in as-rolled condition, while the steels with the yield strength from 570 to 700 MPa had GB-QF microstructures. Steels with the yield strengths above 700 MPa consisted of BF. The most effective alloying element regarding the strength properties is B. Also C, Mn, Cr, Mo and Ni have strong influences, but Nb in the range of 0.05–0.10 wt-% is ineffective. Strengthening with B and Mo was detrimental to toughness. Alloying with Ni and Mn is beneficial to good strength and toughness combination. Mn, Mo, Nb and B contents mainly dictate CGHAZ toughness.
Acta Universitatis Ouluensis. C, Technica
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