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I.

INTRODUCTION

T H E use of high-strength, low-alloy (HSLA) steels has been stimulated over the years by economic considerations. The same reasons have hastened the need for upgrading the properties of low-carbon structural steels which are subjected to a continuous increase in copper content, due to the growing concentration of this element in pricecompetitive available scraps and because these improved steels may offer an optimum balance of properties per unit cost. Indeed, the fabrication of steels, by routes involving the use of large amounts of scrap as the starting material, inexorably leads to an increased concentration of residual elements in the final composition. In this context, the development of low-carbon, copper-rich structural steels with increased strength levels, coupled with good ductility and toughness, has become a major necessity. Furthermore, the development of these steels for general structural applications may include their use in stringent conditions, such as those prevailing in regions of rigorous climate, for which a low brittle-ductile transition temperature is a paramount requirement. One of the most significant advances in metallurgy over the past three decades has been the development of quantitative relationships between the parameters which can be used to describe the different microstructures and their properties in terms of strength, to some extent ductility and work-hardening characteristics, and toughness. The understanding of structure-property relationships has JUAN J. AROZTEGUI, Technical Manager, is with Jos6 Maria Aristrain, S.A., Olaberrfa, 20212 Guiptizcoa, Spain. JOSE J. URCOLA, Research Fellow, and MANUEL FUENTES, Director of Research, are with the Centro de Estudios e Investigaciones Trcnicas de Guipfizcoa, Escuela Superior de Ingenieros Industriales, Barrio de Ibaeta s/n, 20009 San Sebastian, Spain. Manuscript submitted July 6, 1988. METALLURGICAL TRANSACTIONS A

enabled effective use to be made of the various strengthening methods available, and while some degree of empiricism has been used in establishing quantitative structure-property relationships, the most successful ones are based on sound theoretical principles, tl-71 Improved yield stresses are achieved, both in ferrite and ferritepearlite matrices, by grain refinement of the ferrite resuiting from the allotropic transformation of the parent austenite. The beneficial effect of this grain refinement on the ductile-to-brittle transition temperature is also well documented. The dependence o f both properties on grain size follows a Hall-Petch-type relationship. Leaving aside compositional parameters (which in the low-carbon steels studied in this research, remain practically constant), precipitation and dislocation strengthening are the other two microstructural parameters that affect the yield stress. Often, the rolling mills used in the fabrication of these steels are not versatile enough to allow the attainment of a significant variation in grain size, which leaves precipitation strengthening as the only