Continuous cooling transformations and microstructures in a low-carbon, high-strength low-alloy plate steel
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INTRODUCTION
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class of plate steels which provides good combinations of strength, toughness, and weldability is alloyed with nickel, chromium, manganese, molybdenum, and copperJ ~-7] Most of the elements provide hardenability for the transformation of austenite to fine, ferritic microstructures in heavy sections. In addition, strength is provided by the precipitation of copper-rich particles during aging, t8"9"~~ Thus, strength is achieved without carbidecontaining microconstituents, and toughness and weldability are enhanced by the low carbon content of the alloys. Despite relatively high alloy content, these steels are frequently referred to as high-strength low-alloy (HSLA) steels, tl-Tj The more highly alloyed HSLA plate steels develop microstructures which are bainite-like but differ in several aspects from the classical bainites which form in medium-carbon steels. [11,12,13] Aaronson and co-workers have presented and discussed three definitions of bainite. t~4,15'~61A generalized microstructural definition identities bainite in steel as a nonlamellar, two-phase eutectoid decomposition product of austenite. Crystals of the two phases form by diffusional nucleation and growth in a
S.W. THOMPSON, Assistant Professor, and G. KRAUSS, Professor and Director, Advanced Steel Processing and Products Research Center, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. D.J. COLVIN, formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines, is Research and Development Coordinator, BenEt Laboratories, Watervliet Arsenal, Watervliet, NY 12189. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
noncooperative manner and may have any morphology. Another definition identifies the bainite reaction as having a separate C-curve on an isothermal transformation diagram. The bainite C-curve is intermediate to transformation temperature ranges for polygonal fetrite/pearlite and martensite, and in alloy steels, a well-defined upper temperature limit, termed the bainite start (Bs) temperature, exists. A third definition considers bainite to be a product of phase transformation characterized by surface relief produced in association with shear. Diffusion may be coupled with or may occur subsequent to ferrite formation by shear. Aaronson and co-workers strongly support the generalized microstructural definition of bainite, t14,~s'16Jwhile arguments in support of the surface relief definition have been presented by Christian and Edmonds t17] and Olson e t al. t~Sj As the balance of this paper shows, the microstructures of low-carbon, HSLA steels, continuously cooled at rates typically associated with bainite formation, do not readily fit availa
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