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Choi - 2011- Optimization ... Steel.Pdf Optimization of microstructure and properties of high strength spring steel 고강도 스프링강의 미세조직 및 물성 최적화 Ph.D. thesis January, 2011 Sangwoo Choi ii The research in this thesis has been financially supported by POSCO Korea. Optimization of microstructure and properties of high strength spring steel Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 31 januari 2011 om 15.00 uur door Sangwoo CHOI Master of Science in Materials Science and Engineering Korea Advanced Institute of Science and Technology, Korea born at Gwangju, KOREA iv Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. S. van der Zwaag Dit proefschrift is goedgekeurd door de promotor: Rector Magnificus, voorzitter Prof. dr. ir. S. van der Zwaag, TU Delft, promotor Prof. dr. ir. J. Sietsma, TU Delft Prof. dr. R. Boom, TU Delft Prof. dr. E.H. Brueck, TU Delft Prof. dr. Y. Houbaert, Ghent University, Belgium Prof. dr. ir. R. Benedictus, TU Delft Dr. P.E.J. Rivera Díaz del Castillo, University of Cambridge, United Kingdom The research in this thesis has been finacially supported by POSCO KOREA Keywords: Alloy design, Quenching and tempering, Spring steel, Ultra high strength, Reduction of area, Precipitation behavior, Step tempering, Dilatation, Modeling, Decarburization, Oxidation Cover designed by Inkyung You Copyright @2011 by Sangwoo Choi [email protected] All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission from the author. Printed in The Netherlands by Ipskamp Drukkers B.V. ISBN: To my parents and wife, daughters vi Contents 1 Introduction 1 1.1 Review of spring steel evolution (and justification of the research in an economic context) ...................................................................................................................... 1 1.2 Metallurgical background of spring steel ................................................................... 1 1.2.1 Effect of chemical composition on the tempering property............................ 1 1.2.1.1 Microstructural change of plain carbon steel during tempering .................. 2 1.2.1.2 Effect of Si in Spring steel ........................................................................... 3 1.2.1.3 Effect of V and Nb in spring steel................................................................ 4 1.2.2 Sag resistance.................................................................................................. 4 1.2.3 Fatigue ................................................................................................... 5 1.3 Development of suspension coil spring steel ............................................................. 6 1.3.1 1800MPa grade steel : SAE9254 .................................................................... 6 1.3.2 Steel grades higher than 2000MPa.................................................................. 8 1.4 Processing of Suspension coil spring ....................................................................... 10 1.4.1 Manufacturing process of wire rods.............................................................. 10 1.4.2 Manufacturing process of suspension coil spring ......................................... 11 1.5 Need for developing a new suspension coil spring steel.......................................... 12 1.6 Aim of the research .................................................................................................. 14 viii Contents 2 Modeling transformation kinetics from the dilatation curves during cooling 17 2.1 Combining thermochemical database with dilatation cooling kinetics.................... 17 2.1.1 Physical and mathematical model................................................................. 18 2.1.1.1 Length change of a sample during cooling................................... 19 2.1.1.2 Relative length change due to the proeutectoid ferrite transforma- tion ................................................................................................. 20 2.1.1.3 Relative length change due to the pearlite transformation ........... 21 2.1.2 Algorithm ...................................................................................................... 22 2.2 Unit volume and lattice parameter of phases ........................................................... 22 2.3 Experiments.............................................................................................................. 25 2.3.1 Experiments for the dilatation analysis during cooling................................. 25 2.3.2 Microstructural investigations....................................................................... 26 2.4 Results and discussion.............................................................................................. 26 2.5 Conclusion................................................................................................................ 30 3 Modeling transformation kinetics from the dilatation curves during heating 31 3.1 Physical and mathematical model ............................................................................ 32 3.1.1 Classical model ............................................................................................. 32 3.1.1.1 Mathematical model ..................................................................... 32 3.1.1.2 Relative length change due to the reaustenitization transformation from ferrite + cementite................................................................. 34 3.1.1.3 Relative length change due to the reaustenitization transformation from ferrite..................................................................................... 35 3.1.1.4 Algorithm...................................................................................... 36 3.1.2 New model .................................................................................................... 36 3.1.2.1 Mathematical model ..................................................................... 36 3.1.2.2 Relative length change due to the reaustenitization transformation from ferrite + cementite................................................................. 39 3.1.2.3 Algorithm...................................................................................... 40 3.2 Unit volume and lattice parameter of phases ........................................................... 42 3.3 Experiments.............................................................................................................. 42 3.4 Results and discussion.............................................................................................. 43 3.5 Conclusion................................................................................................................ 47 4 Development of new spring steels with optimized strength/ductility combinations 49 4.1 Design concept of new alloys................................................................................... 49 4.2 Experimental methods .............................................................................................. 51 4.2.1 Materials and processing conditions ............................................................. 51 Contents ix 4.2.2 Mechanical tests ............................................................................................ 52 4.2.3 Metallographic tests ...................................................................................... 52 4.3 Results and Discussions ........................................................................................... 53 4.3.1 Mechanical properties ................................................................................... 53 4.3.2 Microstructure of as-rolled state ................................................................... 56 4.3.3 Microstructure of oil quenching and tempering state ................................... 58 4.3.4 Microstructure of S1 and S3 steels with austenitization temperature and temepring temperature ................................................................................. 58 4.4 Conclusion................................................................................................................ 64 5 Microstructure studies to determine strengthening mechanisms 65 5.1 Introduction .............................................................................................................. 65 5.1.1 Microstructural change during tempering..................................................... 66 5.1.2 Strengthening mechanism of tempered martensite of alloy steels ................ 67 5.1.2.1 Solid solution strengthening ............................................................ 67 5.1.2.2 Dislocation strengthening ............................................................. 68 5.1.2.3 Grain refining................................................................................ 69 5.1.2.4 Precipitation strengthening ........................................................... 69 5.1.3 Changes of strength and reduction of area for the proposed steels............... 70 5.2 Experimental methods .............................................................................................. 70 5.2.1 Optical and SEM metallography..................................................................
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