Cooling processes in the thermal hardening of reinforcement

In the production of periodic steel reinforcement for ferroconcrete, the latest development is the production reinforcement of diameter up to 40 mm (inclusive) and of strength class A500C or higher (according to the STO ASChM7-93 standard) from regular steel (such as 3 OO and 3 EOO steel) thermally hardened by the rolling heat. Current bar-rolling mills in Russia, Ukraine, and Belarus permit the production of thermally hardened reinforcement of diameter 10–25 mm. Discontinuous quenching with self-tempering ensures the greatest thermal hardening. This technology may be implemented in two stages [1, 2]. Stage 1 consists of accelerated controllable water cooling, until the specified mean-mass self-tempering temperature is obtained. At this stage, in controllable cooling (including cyclic cooling), the disintegration of the supercooled austenite in the surface layers, to form stable martensite, occurs in two steps. In the intermediate layers, a mixture of bainite and nondisintegrating austenite is formed; in the central section, austenite is formed at temperatures above A r 3 . In the first step, organization of optimal cooling in the thermal hardening cycle calls for rapid cooling with sharp supercooling of the surface layers of the reinforcing rod; the cooling rate must be maintained close or equal to the critical rate for the given steel. This entails rapid water cooling. In many cases, on account of the design of the thermal-hardening units in existing mill lines, sections of water and air cooling alternate. The duration of the air-cooling sections is determining by the configuration of the mill equipment. It is expedient to end the first step, characterized by intense accelerated controllable water cooling, when the surface temperature of the rod is 200 ° C. In the second step, water cooling is considerably slower (mild cooling). The rate of heat extraction in mild cooling is chosen to be somewhat higher than the heat flux supplied to the surface from the central layers of the rod by conduction. In mild cooling, heat passes by conduction from the center of the reinforcing rod to its surface layers. As a result, the temperature difference over the cross section is reduced; the conversion of austenite to bainite in the intermediate layers continues; and the deposition of ferrite in the central layers begins. Mild cooling must end on reaching the specified meanmass temperature of subsequent self-tempering. This cooling system prevents extreme heat stress and permits thermocycling of the austenite in the surface layers of the reinforcing rod, which, in turn, improves the performance of the final product. In this case, the surface layers are those in which the heat-transfer conditions at the rod surface are determining, rather than the conditions of internal thermal resistance of the metal. Stage 2 is natural cooling in air. In this stage, the metal is cooled to the ambient temperature. The final temperature equalization over the cross section is ensured, which facilitates self-tempering. Thus, the surface martensite layer is tempered on heating on account of the heat from the center of the rod; the conversion of austenite to bainite and ferrite–pearlite mixture ceases here. The degree of completion of these processes determines the final strength of the product. Of course, the strength depends on the development of self-tempering and the ratio of the structures formed in this process. The following factors have a significant influence on the choice of the self-tempering temperature and the ratio of structures that appear: the chemical composition of the steel; the rod diameter; the temperature of the metal beyond the finishing cell in the rolling mill; and the rate and duration of accelerated controllable (intense and mild) cooling in stage 1.