Morphology and properties of low-carbon bainite
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I.
INTRODUCTION
T H E morphology of upper bainite is very similar to that of lath martensite elongated in a [i01]f direction with a habit plane between (1 11)f and (232)f. 11-6! A l t h o u g h upper bainitic ferrite forms without partitioning the substitutional alloying elements, it contains much fewer carbon atoms than the austenite from which it forms. The carbide particles in upper bainite are exclusively cementite, and alloy carbides have never been recognized. These facts show the possibilities that the upper bainite may form via a displacive mechanism as far as the substitutional elements are concerned though it may accompany the diffusion of interstitial elements, t3,5,7-14j The bainite of low-carbon steel always has lathlike ferrite. In this sense, all the bainites formed in low-carbon steels should be classified into upper bainite. But there exist three types of cementite morphologies: t31 Type I is carbide-free, Type II has the cementite particles in the shapes of layers between them, t151and Type III has fine platelets lying parallel to a specific ferrite plane in the interior. The latter morphology of cementite is especially predominant in the bainite at relatively lower temperatures in the vicinity of Ms temperature and is often thought o f as evidence of lower bainite, t161as depicted in Table I. The production of high-tensile-strength steel, on the H. OHTANI, General Manager of Tube and Pipe Department, S. OKAGUCHI, Research Engineer, and Y. OHMORI, Principal Research Manager, are with the Research and Development Division, Sumitomo Metal Industries, Ltd., Amagasaki 660, Japan. Y. FUJISHIRO, Assistant Manager, is with Osaka Steel Works, Sumitomo Metal Industries, Ltd., Osaka 554, Japan. 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
other hand, is performed by a continuous cooling process such as quenching, moderate accelerated cooling, and normalizing. The microstructure, therefore, is composed of a mixture of various kinds of transformation products such as martensite, bainite, and ferrite. Above all, the bainite plays an important role in obtaining strength and toughness. For example, the toughness of the transformation products is not always monotonically dependent on cooling rate. t~7]There is an optimum cooling rate for high toughness, and the microstructure to obtain the lowest brittle-ductile transition temperature was confirmed to be bainite/martensite duplex structure. This is probably due to the refinement of the effective grain size by the bainite formation prior to that of martensite. It is also reasonable that the bainite has tempering characteristics different from those of martensite because of its different cementite precipitation behavior. In addition to the conventional heat treatment, the th
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