Tensile stress-strain analysis of single-structure steels
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NTRODUCTION
SINCE there has been significant development in the process control of steel production, it is expected that a more precise control of processing conditions, such as chemical composition, rolling conditions, and cooling condition, would enable the production of steels with novel properties. The process control is now mainly based on computer simulation in combination with empirical rules and experimental measurements. Since the 1980s, significant efforts on property prediction have been made not only to express the mechanical properties of final products by the processing parameters, but to predict the properties through the knowledge of the formation and evolution of the microstructures during processing as well.[1,2] The aim of these studies is to predict the microstructure during each step of processing, to correlate it to mechanical properties, and, finally, to establish a universal model that can predict mechanical properties from chemical composition and processing conditions, especially the rolling and cooling conditions. Table I[3–12] summarizes the empirical equations used to calculate the mechanical properties, such as strength and elongation, of various single-structure steels. Their applicability and accuracy are limited by the composition ranges and the microstructural measurements. Additionally, limited information in the stress-strain curves has been considered, e.g., yield stress, ultimate tensile strength, and elongation. Other characteristics of the stress-strain curves, such as their
shape and work-hardening behavior, have not been incorporated in the analysis, although they are rather important to control the rolling conditions. Until now, considerable effort has been directed toward developing empirical laws that describe the work hardening of polycrystalline metals and alloys, as reflected by the derivation of the relationships proposed by Ludwik,[13] Hollomon,[14] Voce,[15] Swift,[16] etc. The parameters involved in these relationships, particularly the work-hardening exponent (n), have been correlated to the changes in the microstructure and deformation processes. The present investigation is devoted to elucidating the relationship between the microstructure and mechanical properties of steels. Numerous samples of steels with four kinds of structures, namely, ferrite, pearlite, bainite, and martensite, were tensile tested, and several empirical equations were examined to analyze the true stress–true strain data in order to better understand the relationship between the microstructure and mechanical properties of these singlestructure steels. II. ANALYTICAL A. The Differential Crussard–Jaoul Analysis The Crussard–Jaoul (C–J) analysis[17,18,19] of the tensile stress-strain curves has been applied to many kinds of alloys, and meaningful results have been obtained by Krauss and coworkers[20,21,22] and other groups.[23,24] It assumes the power Ludwik relation,
⫽ 0 ⫹ k n M. UMEMOTO, Professor, and K. TSUCHIYA, Associate Professor, are with the Department of Production Systems Engi
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