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Relation Between Microstructure and Mechanical Properties of a Low-Alloyed TRIP Steel

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Relation Between Microstructure and Mechanical Properties of a Low-Alloyed TRIP Steel Relation between Microstructure and Mechanical Properties of a Low-alloyed TRIP steel Von der Fakultat¨ fur¨ Georessourcen und Materialtechnik der Rheinisch-Westfalischen¨ Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Ingenieurwissenschaften genehmigte Dissertation vorgelegt von M.Sc. Kemal DAVUT aus Kadikoy,¨ Turkei¨ Berichter: PD Dr.-Ing. Stefan Zaefferer Professor Dr.-Ing. Dierk Raabe Univ.-Prof. Dr.-Ing. Wolfgang Bleck Tag der mundlichen¨ Prufung:¨ 12. Februar 2013 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfugbar.¨ WICHTIG: D 82 überprüfen !!! Berichte aus der Materialwissenschaft Kemal Davut Relation between Microstructure and Mechanical Properties of a Low-alloyed TRIP steel Shaker Verlag Aachen 2013 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de. Zugl.: D 82 (Diss. RWTH Aachen University, 2013) Copyright Shaker Verlag 2013 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publishers. Printed in Germany. ISBN 978-3-8440-1782-3 ISSN 1618-5722 Shaker Verlag GmbH • P.O. BOX 101818 • D-52018 Aachen Phone: 0049/2407/9596-0 • Telefax: 0049/2407/9596-9 Internet: www.shaker.de • e-mail: [email protected] Preface This thesis is submitted in partial fulfillment of a Dr.-Ing degree from the RWTH Aachen University. The work presented in the thesis was carried out during the pe- riod April 2008 to September 2012 at the Max-Planck-Institut fuer Eisenforschung (MPIE), in the Department of Microstructure Physics and Alloy Design, under the supervisions of Associate Professor Dr.-Ing Stefan Zaefferer and Professor Dr.-Ing Dierk Raabe. The work was a part of the joint research project between MPIE and RWTH Aachen University, having the title ”The relation between microstructure and damage mechanisms in multi-phase steels (Zusammenhang zwischen Gefuge¨ und Schadigungsmechanismen¨ bei mehrphasigen Stahlen)”¨ and financially supported by the German Research Foundation (DFG - Deutsche Forschungsgemeinschaft) under reference numbers BL 402/19-1 and ZA 287/5-1 Acknowledgements This doctoral thesis come into its final form with the help and support of kind people around me, to only some of whom it is possible to give particular mention here. First and foremost, I gratefully thank Prof.Dr.-Ing. D. Raabe, the head of the depart- ment and the director of the institute, for giving me the opportunity to carry out this project. Valuable and inspiring discussions together with the given scientific freedom have made the results possible. This work has reached this stage with the strong support, constant encouragement and day-to-day supervision of P.D.Dr.-Ing. Stefan Zaefferer. His comments have often set me on the right track towards the solution. Besides introducing me the world of EBSD and texture analysis, our stimulating discussions over the years have always inspired and helped me to improve my scientific formation. Thank you for sparing time for me, for the really great work together, and for your friendship. I am grateful to the defense committee members, Prof.Dr.-Ing. D. Raabe and Prof.Dr.- Ing. W. Bleck for their valuable comments on this thesis. My working days (and nights and also conference trips) have become enjoyable due to the friendly and cheerful colleagues in the institute, particularly to mention Tom, David, Dr. A. Khorashadizadeh, Pradeep, Darius, Dr. O. Dmitrieva, Dr. C. Tas¸an, Igor, Xia, Farangis, Dr. Duancheng Ma, Helene, Dr. P. Romano, Dr. D. Tjahjanto, Dr. H. Fabritius and also Dr. I. Gutierrez. Eralp, Mutlu, Bekir, Ceylan and Onur have formed a small Turkey inside the Institude and they all deserve a separate thank. Special thanks to Claudio for sharing his Schmidt factor calculation code and LATEX header files. I also thank Dr.-Ing. D. Ponge and Dr.-Ing. P. Eisenlohr for the valuable discussions. The technical assistance and support for the experimental set-ups by M. Nellessen, K. Angenendt, H.Bogershausen,¨ M. Adamek, H. Faul, F. Schluter,¨ G. Bialkowski and B. Breitbach are gratefully acknowledged. C. Regner and P. Siegmund are acknowledged for the fast and reliable library services. A. Kuhl and B. Beckschafer¨ are thanked for their continuous support in the areas of computational hardware and software. I greatly acknowledge the financial support of the project by German Research Founda- tion (DFG). Department of Ferrous Metallurgy (IEHK, Institut fur¨ Eisenhuttenkunde)¨ in RWTH Aachen University is specifically thanked for providing the steel sheets used in this study. Prof. A.D. Rollet is thanked for a very helpful discussion during a GLADD meeting at UniGhent. This specific discussion brought the idea of applying a logistic regression vi model to austenite stability. I have appreciated the expertise of Peggy Schmidt from Thyssen & Krupp Stahl AG on determining the phase fractions of TRIP steels via XRD. Last, but by no means least, I express my deepest gratitude to my family, especially to my parents Lale and Haluk for their endless love and long-distance support. My aunt Oya Araslı needs to be mentioned separately, as she helped me to make the deci- sion of proceeding an academic carreer. I also thank my friends in my home-country, especially Serhat, Anıl, C¸agrı,ˇ Barıs¸, Yudum, Aslı and Eylem for keeping the social enviroment warm during my visits. Special thanks to Dr. Caner S¸ims¸ir for the friend- ship, stimulating discussions and also for showing me an example life-cycle between Germany and Turkey. My final thanks go to Ozlem¨ who not only helped me with the statistical analysis but more important than that she brought more joy and happiness to my life. A single word thanks is not enough for the care and support I received. Contents Preface iii Acknowledgements v 1 Introduction 1 1.1 Background: Low-alloyed TRIP steels ................. 1 1.1.1 How are the TRIP steels produced ? .............. 4 1.1.2 Alloying concepts ....................... 6 1.1.3 Martensitic transformation in TRIP steels ........... 7 1.2 Objectives and Scope .......................... 9 1.3 Thesis Outline .............................. 10 2 Experimental 13 2.1 Material ................................. 13 2.2 Microstructure characterization ..................... 13 2.3 Mechanical tests ............................. 15 2.4 FEM model ............................... 15 3 The reliability of EBSD-based microstructure description 19 3.1 Introduction and motivation ....................... 19 3.2 EBSD-based phase fraction determination ............... 20 3.2.1 Estimation of the required number of measurement points for statistical reliability ....................... 20 3.2.2 The effect of step-size and observed area size on reliability . 22 3.2.3 Determination of the size of a representative area ....... 23 3.2.4 Error estimation ........................ 28 3.2.5 EBSD vs. XRD ......................... 30 viii CONTENTS 3.3 EBSD-based macro-texture analysis .................. 32 3.3.1 Measuring the statistical reliability of a texture analysis .... 33 3.3.2 Comparing the results of conventional EBSD mapping tech- niques .............................. 34 3.4 A new mapping technique ....................... 36 3.4.1 The need for a new technique ................. 36 3.4.2 The application of the technique ................ 36 3.4.3 Comparing the results of the new and conventional EBSD mapping techniques ...................... 38 3.4.4 EBSD vs. XRD based texture analysis ............. 43 3.5 Conclusions ............................... 44 4 The effect of microstructure on macroscopic work-hardening rate and stress-strain partitioning 49 4.1 Introduction and motivation ....................... 49 4.2 Martensitic transformation and the work hardening rate ........ 49 4.3 The effects of grain size and morphology on the deformation of austenite 53 4.4 The effect of grain shape on stress partitioning ............. 55 4.5 Conclusions ............................... 61 5 Comparative study of the texture evolution of TRIP microstructure con- stituents during room temperature deformation 63 5.1 Introduction and motivation ....................... 63 5.2 Texture development during bending .................. 63 5.2.1 Changes in BCC texture components ............. 64 5.2.2 Changes in FCC texture components .............. 64 5.3 Relation between BCC and FCC shear ................. 67 5.4 Comparing the changes in FCC and BCC texture components ..... 68 5.5 Conclusions ............................... 69 6 Microstructural parameters influencing the stability of retained austenite 71 6.1 Introduction and motivation ....................... 71 6.2 The detection of deformation and transformation of meta-stable austenite 72 6.3 Identification of stable and un-stable austenite grains ......... 80 6.4 Factors influencing the stability of austenite .............. 82 CONTENTS ix 6.5 Relative contributions of the microstructural parameters ........ 84 6.5.1 Grain size and composition ................... 84 6.5.2 Crystallographic orientation .................. 86 6.5.3 Grain morphology ....................... 93 6.5.4 Statistical analysis of the contribution of the microstructural parameters ........................... 93 6.6 Why the transformation
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