Thermomechanical Processing of Steels,Thermomechanical Processing of Steels,Thermomechanical Processing of Steels
ECONOMIC AND TECHNICAL DEMANDS of the last three decades have forced the global steel industry to undergo a
series of important changes. These changes were made imperative by the need of the steel industry to maintain a
competitive and leadership position in relation to other engineering materials. Significant advances have been made in
both process metallurgy and product metallurgy. These advances have transferred the steel industry into a modern, costeffective,
and high-quality manufacturing force in the world marketplace. The improvements in steelmaking, ladle
refining, and continuous casting practices have been matched with other advances in the science and technology of
microstructural control in the final product. It is this ability to control microstructure during processing that has allowed
significant and cost-effective improvements in the final properties of steel to be achieved. Central to the concept of
controlled processing is thermomechanical processing (TMP).
It is now possible to produce as-rolled steels with final properties tailored to the requirements of the final application. The
concept of tailored final properties is possible primarily through the ability of the heat treater to control the final
microstructure in a predictable manner. This control of the final microstructure is based on an understanding of the way
that steels respond to hot processing and how that response can be altered through alloying.

It is well known that the control (that is, refinement) of final microstructure begins during solidification and proceeds
during reheating, hot rolling, and final transformation. Because the final transformed microstructure reflects the
microstructure and composition of the austenite prior to transformation, it is obvious that the refinement of this final
austenite is critical to obtaining the optimum final microstructure and properties. One of the key processing elements used
to obtain the proper austenite microstructure is known as thermomechanical processing, which, as will be shown later, is a
special type of hot deformation process (for example, rolling, forging, piercing, and so on). The most common form of
thermomechanical processing in use today is called controlled rolling (Ref 1). During controlled rolling, it is the as-rolled
austenite microstructure that is being controlled.
Because the goal of thermomechanical processing is the refinement of the austenite grain structure, the control of
recrystallization and/or grain coarsening during processing are among the metallurgical techniques available. The
presence of minute quantities of elements such as niobium, titanium, and vanadium have been shown to be particularly
useful during thermomechanical processing because of the change in the solubilities of their
carbides/carbonitrides/nitrides in austenite as a function of temperature. These elements are known as microalloying
elements (MAE) because they are generally present at levels at or below 0.1 wt%. Hence, the use of these microalloying
elements enables the goal of thermomechanical processing to be easily achieved because these elements permit forces
retarding recrystallization and grain coarsening to be governed by controlled precipitation during processing. It is,
therefore, normal to have microalloyed steels mentioned in discussions of thermomechanical processing. The discussion
that follows is a review of various aspects of thermomechanical processing, including its justification, history, underlying
physical metallurgy, methods of application, beneficial effects on properties, and current and future applications.

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