Predicting the onset of transformation under noncontinuous cooling conditions: Part I. Theory
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I.
INTRODUCTION
SIGNIFICANT improvement in the final properties of steel products can be accomplished through the incorporation of accelerated cooling schedules into the conventional steel processing practices.tl-SJ Improvement in mechanical properties resulting from the application of alternative processing strategies enables steel products of the same strength to be produced with lower alloy additions. This not only reduces the cost of producing steels, but also allows steels of lower carbon levels to be used, resulting in an improvement in weldability.ES,6,7] Applications of process modeling to mathematically describe metallurgical changes relating to the accelerated cooling have allowed systematic approaches for detailed evaluation of the process. However, they have been limited to the continuous cooling situations. Accelerated cooling strategies as applied in modem steel processing, and many applications of the practical heat-treatment operations, consist of cooling rates which are altered discontinuously during the cooling. For example, in the Stelmor~,81process and the cooling of hot strip products on the run-out table, E91the temperature in the region near the surface is drastically lowered as the steel product is cooled by water. Upon exit from the water-cooling region, the surface of the steel product is rapidly heated by the conduction of heat from the center. Under these circumstances, determination of the transformation start temperature becomes increasingly difficult, even when using empirical methods.
T.T. PHAM, Research Scientist, is with BHP Research, Melbourne Laboratories, Mulgrave, Victoria, Australia. E.B. HAWBOLT, Professor, and J.K. BRIMACOMBE, Alcan Chair in Materials Process Engineering and Director, are with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T IZ4. Manuscript submitted November 21, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
To maximize the potential of numerical models, it is necessary to have a method for predicting the start of transformation; this method must be capable of handling the noncontinuous behavior during cooling, especially when a model is used to examine new processing routes. A collaborative study between BHP Research and the Centre for Metallurgical Process Engineering, the University of British Columbia, was conducted to establish a method for predicting the onset of the austenite decomposition during continuous and noncontinuous cooling conditions. The proposed method of analysis is fundamental and can be applied to any cooling situation, regardless of how complex the thermal path may be. While the method was derived for predicting the decomposition of austenite on cooling, it is also applicable to other thermally activated processes, such as precipitation and recrystallization, that occur during the processing of the high-strength low-alloy steelsY ~ 13]In the first article of this two-part series, details of the derived methodology are described. In the second article, experimental resul
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