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cover next page > title : Physical Metallurgy of Thermomechanical Treatment of Structural Steels author : Mazanec, Karel.; Mazancova, E. publisher : Cambridge International Science Publishing isbn10 | asin : 1898326436 print isbn13 : 9781898326434 ebook isbn13 : 9780585237466 language : English subject Physical metallurgy, Steel--Metallurgy. publication date : 1997 lcc : TN693P49 1997eb ddc : 669/.96142 subject : Physical metallurgy, Steel--Metallurgy. cover next page > < previous page page_iii next page > Page iii Physical Metallurgy of Thermomechanical Treatment of Structural Steels Karel Mazanec and Eva Mazancová < previous page page_iii next page > < previous page page_iv next page > Page iv Published by Cambridge International Science Publishing 7 Meadow Walk, Great Abington, Cambridge CB1 6AZ, UK http://www.demon.co.uk/cambsci/homepage.htm First published December 1997 ©K Mazanec and E Mazancová ©Cambridge International Science Publishing Conditions of sale All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 1 898326436 Translated by V E Riecansky Production Irina Stupak Printed by Transtech Printers Ltd, London, England < previous page page_iv next page > < previous page page_v next page > Page v About the Authors Prof Ing Karel Mazanec, DrSc graduated from the Technical University in Ostrava in 1949. Since then, he has worked in a number of leading posts, including the Vitkovice Iron Works Research Institute in the area of the development of new types of steel for power engineering, research into the thermomechanical treatment of structural and high-strength steels, their weldability and brittle fracture resistance. In 1962, he defended his DrSc dissertation on 'Delayed fracture in martensite'. Prof Mazanec has published more than 350 studies in many leading journals and conference proceedings. In 1965 he became a professor at the Faculty of Metallurgy of the Technical University in Ostrava and later Dean of the Faculty. In 1983 he was elected a member of the Czechoslovak Academy of Sciences. He is also a honorary member of the French Metallurgical Society. At present, is a Professor Emeritus at the Technical University of Ostrava where he lectures and works in the research of unconventional methods of heat treatment of steel and physical-metallurgical problems of strengthening and increase of fracture resistance. Recently, he has been paying attention to physical-metallurgical problems of thermoelastic martensite in shape memory TiNi alloys. Ing Eva Mazancová, CSc, graduated from the Technical University of Ostrava in 1975 and started her career at the Research and Testing Institute of the New Iron Works in Ostrava where she now works in the structural and microfractographic analysis of structural steels, microanalysis of nonmetallic inclusions and research into segregation phenomena, including their effect of the properties of steel. In 1981, Eva Mazancová defended her PhD dissertation on 'Thermomechanical treatment of structural steels' in which she examined the application of thermomechanical treatment of microalloy steels in seamless tube production. She has also worked on problems of hydrogen and sulphide brittleness of materials for oil industry, the effect of chemical constitution and microstructure on corrosion cracking; problems associated with optimization of the heat treatment of oil pipes with higher strength and their physical-metallurgical characteristics also attracted her attention. Her current interests include problems of microfractographic analysis of structural steels and evaluation of relationships between microstructure, fracture parameters and cleanness of steel. She is also involved in evaluating the microcleanness of continually cast steels and its effect on the properties of structural steels after various heat treatments. During her scientific and research activity, Eva Mazancová has published 105 studies in leading journals and conferences, both home and abroad. < previous page page_v next page > < previous page page_vii next page > Page vii Contents 1 1 Introduction 2 3 Main Physical Metallurgy Characteristics of Thermomechanical Treatment 5 2.1 Sources of Hardening the Metallic Matrix 20 2.2 Analysis of Physical Metallurgy Characteristics of the Initial Austenitic Matrix Taking into Account the Subsequent Heat Treatment 34 2.3 Influence of the State of the Initial Austenitic Matrix on Phase Transitions in Thermomechanical Treatment 39 2.4 Physical Metallurgy of the Ferritic Phase Transformation in Work-Hardened Austenite 3 56 Thermomechanical Treatment of High-Strength Martensitic Steels 61 3.1 Effect of Thermomechanical Treatment on Properties 75 3.2 Effect of Thermomechanical Treatment on the Modification of Microstructural Characteristics 81 3.3 Evaluation of the Mechanical and Metallugical Properties of High-Strength Martensitic Steels after HTMT 82 3.4 Summary of the Achieved Results and Their Physical Metallurgy Analysis 4 93 Thermomechanical Treatment and Controlled Rolling of Carbon and Low-Alloy Steels 93 4.1 Application of HTMT in Strengthening the Matrix of Steels for Wider Technical Application and Determination of the Relationships between the Initial Structure and the Resultant Physical Metallurgy Properties of Products of Austenite Decomposition 101 4.2 Technical and Technological Characteristics of Controlled Rolling 103 4.2.1 Deformation in the Austenite Range Associated with Recrystallization 104 4.2.2 Deformation in the Region of Restricted Recrystallization 108 4.2.3 Deformation in the AusteniteFerrite (Two-Phase) Region 110 4.3 Application of Various Variants of Accelerated Cooling after Controlled Rolling 119 4.4 A Model of Predicting Changes in the Microstructure in Controlled Rolling and Accelerated Cooling of Structural Steels for Wider Technical Application 120 4.4.1 Perspective Technical and Technological Solutions 122 4.5 Effect of Controlled Rolling and Accelerated Cooling on Reducing the Susceptibility to Hydrogen Embrittlement 131 4.6 Technical and Technological Comments Regarding Application of Controlled Rolling and Accelerated Cooling 5 134 Conclusions References 137 Index 141 < previous page page_vii next page > < previous page page_1 next page > Page 1 1 Introduction The development of science and technology imposes new requirements on obtaining higher applied properties of metallic materials, especially higher strength and ductility properties. The main methods based on complex or higher alloying or on the development of metallic materials with new constitutions usually enable higher material and technical properties to be obtained, but this is achieved with higher or more extensive requirements on alloying elements. This increases the production cost. Despite the fact that in many cases this is the only possible approach that can be used, on a wider scale higher applied material and technical properties can be achieved in various types of metallic systems by other methods usually based on the application of new, unconventional heat treatment technologies. These solutions form suitable conditions for the maximum utilization of the processing properties of appropriate types of metallic materials. This has resulted in the development of a large number of unconventional heat treatment methods of metallic materials, some of which are based on using highly complicated technical and processing variants, as discussed in, for example, Ref.1. According to the currently available experience, different variants of thermomechanical treatment (TMT) represent one of the most efficient methods of unconventional heat treatment. This method of unconventional heat treatment can be used widely not only for steels but also in heat treatment of, for example, alloys of aluminium, titanium, nickel, etc.2 In this book, the authors pay special attention to analysis of the physical metallurgy parameters and structuralmaterial characteristics obtained in structural steels or iron-based alloys using various types of TMT. This method makes it possible to obtain higher applied properties of these materials, without any large changes in their chemical constitution. From the viewpoint of developing this heat treatment method, the proposed solution will include not only an analysis of basic variants of TMT in the region of stable and unstable austenite in high-strength martensitic steels and alloys but also the problems of application of TMT in optimizing the properties of low-alloy or carbon structural steels in which the austenite < previous page page_1 next page > < previous page page_2 next page > Page 2 to martensite transition does not take place during the phase transformation. Consequently, for these steel types we shall also discuss selective problems of the physical metallurgy of controlled rolling or the characteristics of accelerated cooling, which are closely linked with the problem of the processes of TMT of structural steels. The main feature of all these variants of TMT is that the plastic deformation of the initial austenitic matrix and the formation of its controlled structural and mechanical state with subsequent heat treatment, phase transformation, are linked. This link makes it possible to utilize the hardening or refining of the grains of the initial austenitic matrix and also the structural modification of the products of the phase transformation of austenite associated with the modification of structural and substructural characteristics of the austenitic matrix leading to a higher level of the set of the mechanical and metallurgical properties of various types of structural steel processed by this treatment. The book summarizes the results obtained in extensive research of the hardening characteristics of metallic materials (structural steels), with special attention given to the problems of the physical metallurgy nature of hardening of martensitic steels and control-rolled structural steels or structural steels subjected to subsequent accelerated cooling and used for wider technical applications. Taking this analysis into account, selected problems, whose solution leads to obtaining optimized technical and technological conditions of processing these types of steel by the selected unconventional heat treatment method, will also be discussed. The aim of this book is to present a detailed analysis of the main parameters of unconventional treatment of structural steels resulting in higher strength and ductility properties of these materials, including higher utility properties. This makes it possible to create suitable conditions for obtaining a higher level of safety and reliability of structures or structural components produced from materials treated in this manner. The main technical and technological characteristics of TMT of structural steels of different chemical composition and the conditions of their optimum use will also be determined. < previous page page_2 next page > < previous page page_3 next page > Page 3 2 Main Physical Metallurgy Characteristics of Thermomechanical Treatment Information on the controlling role of the structure and the conditions of influencing the so-called structure-sensitive characteristics of metallic materials has resulted in conclusions according to which the considered structure-sensitive properties are strongly affected by the defectiveness of the matrix. It is well known that lattice defects, present in the initial matrix, also strongly influence by both the mechanism and kinetics of phase transformations in heat treatment, i.e. the characteristics of the resultant structure and the final mechanicalmetallurgical properties of this type of metallic material. This shows that the aim of the solution must be to obtain both the favourable density of structural defects, namely dislocations, and their arrangement in the metallic matrix in various stages of TMT in order to obtain, in controlling the appropriate processes, the required structure and, consequently, the optimum level of the final mechanical and metallurgical characteristics. Plastic deformation and subsequent recovery processes in the parent metallic matrix are the main metallurgical processes enabling control of the dislocation density in the metallic matrix. These considerations then lead logically to a simple conclusion, according to which the most efficient method is to combine efficiently the appropriate physical metallurgy characteristics of phase transformations and plastic deformation (including control of the subsequent or simultaneous development of recovery and recrystallization processes) and the formation of a single technological unit. TMT is then a process resulting in the detailed coupling of controlled modification of the structure in deformation of the initial austenitic matrix and its decomposition in heat treatment. As indicated by previous studies concerned with the analysis of TMT conditions of various types of steels, this process must be regarded as a set of partial processes taking place during heating, deformation and cooling of metallic materials. This results in the formation of a structure enabling properties to be obtained similar to those which can be achieved in, for example, materials at a higher density and specific arrangement of lattice defects gen- < previous page page_3 next page > < previous page page_4 next page > Page 4 erated in plastic deformation in the metallic matrix prior to the final phase transformation.1,2 Initial studies in the area of research into the physical metallurgy and technical and technological characteristics of TMT in, for example, structural steels, were aimed at determining the conditions for obtaining the maximum level of strength whilst maintaining the required toughness level.1,3 Therefore, special attention was given to two main TMT variants: evaluation of the technical and technological parameters of TMT in the stable austenite range (high-temperature thermomechanical treatment, HTMT), and thermomechanical treatment in the unstable austenite range (low-temperature thermomechanical treatment, LTMT). In the former case, plastic deformation is carried out in stable austenite, whereas in the latter case plastic deformation takes place in unstable austenite, usually connected with subsequent quenching to martensite1 leading to a large increase of the mechanical and metallurgical properties of alloyed high-strength martensitic steels. During the subsequent development of TMT technologies, the coupling of plastic deformation with subsequent heat treatment was also extended to carbon and low-alloy steels, where the product of final heat treatment is usually the formation of a ferriticbainitic, bainitic or bainiticmartensitic structure.4 In all these cases, this is achieved by the rapid cooling of appropriate types of structural steels with the defined parameters of the initial austenitic structure, e. g. in a quenching press.1 In addition, in the further development of unconventional methods of heat treatment of structural steels, as a part of a wider concept of technical and technological variants of TMT, work is being carried out on technical and technological variants of controlled rolling or the superposition effect of accelerated cooling. This method is used mainly in low- carbon or microalloy steels for wider technical applications alloyed with, for example, niobium, vanadium, titanium, or with several microalloying additions. Usually, the phase transformation of the austenitic matrix is accompanied by the formation of non-martensitic products, with a special position in optimizing the resultant properties played by the optimum form of the resultant ferrite grains. The addition of microalloying elements, together with the modification of the reduction schedule in forming and the selection of holding time between passes (control of the temperature of completion of forming and the development of recovery processes in the parent metallic matrix austenite) has a beneficial effect on the resultant austenite grain size and the development of recrystallization processes (static and dynamic recrystallization) and the extent of strain-induced precipitation of carbides or carbonitrides of microalloying additions.5 < previous page page_4 next page >