Accelerated Cooling: A Physical Metallurgy Perspective

Abstract

In the 25 years that accelerated cooling has been used in the production of steel, its role has undergone drastic changes. In the early days of accelerated cooling, i.e. 1962–1970, it was used exclusively on hot strip mills, mainly to reduce the length of run-out tables otherwise required for low temperature coiling. The unexpected benefits derived from the initial uses of accelerated cooling spawned a large research and development effort in that general area, which has continued to the present. There have been significant advances made as a result of this interest. Among the first, and perhaps the most significant, was the appreciation of the importance of the condition of austenite prior to cooling. Later work on alloy design and process variations within the scope of accelerated cooling (cooling rate, water end temperature, final cooling rate, etc), led to improvements in ferrite-pearlite steels, and to the development of “multiphase” steels for both strip and plate applications. The dual-phase (as-rolled) strip steels and the ferritebainite plate steels are prime examples of important developments. These steels offer final properties which were simply not available in earlier times.Viewing accelerated cooling and the role it plays in the genesis of final microstructure is an interesting and informative pursuit. The evolution of the high and low temperature transformation products, the factors which govern the nature, amount and distribution of the various phases and microconstituents, and the control of microstructure for optimization of final properties are some of the interesting facets of the success story of accelerated cooling.The purpose of this paper is to discuss accelerated cooling from the viewpoint of how cooling rates and continuous cooling transformation (CCT) diagrams can jointly determine the sequence of phases which form, and how physical metallurgy principles govern their amounts and distributions. Of principal interest are (i) the factors which control the hot rolled microstructure of austenite, (ii) the influence of austenite microstructure and composition on the CCT diagram, and (iii) the influence of cooling path on microstructure evolution for a given CCT diagram.