ABSTRACT
The primary reason for rolling steels at elevated temperatures is to keep the required force and work as small as possible. Besides, forming at higher temperatures leads to a greater possible degree of deformation. But increasing the temperature causes many reactions between the various microstructural events during the process. Materials scientists have then tried to ana lyze these microstructural reactions and determine the optimum process conditions to obtain fine-grained steels with desirable combinations of micro constituents. Between the 1950s and the 1960s, materials scientists in Europe have devoted significant
efforts towards enhancing the mechanical properties of steel by heat treatment, and they have established a technology called thermomechanical treatment (Tamura et al., 1988). Since, in many cases, the microstructure created during cooling after hot deformation follows the same behavior as those formed during heat treatment, this technology has been applied to the hot rolling process and has evolved into what we call thermo mechanical processing technology, based on simple temperature control during rolling. Mill designers together with materials scientists had basically been interested in modeling the micro structure evolution during the rolling and cooling of steels, with such things as recovery, recrystallization, and grain growth. Through extensive experiments, they have made continuous efforts to set up models for describing the recovery, recrystalliza tion, and grain growth behavior, in terms of temperature, strain, strain rate at a pass, and holding time after deformation, and they have developed a number of equations based mainly on experiments for various steels. The thermomechanical proces sing techniques have been further developed into thermo mechanical controlled processing technology, which focuses on accelerated cooling during rolling with some change of roll pass schedule. Figure 7-1 shows a brief chronicle of thermo mechanical (controlled) processing techniques development.
