Study of the Effect of Recrystallization Annealing Regimes in Continuous Operating Units on the State of If-Steel Solid Solution and Mechanical Properties

Laboratory simulation of recrystallization annealing in continuously operating units is performed for rolled IF-steels of nine different compositions of laboratory and industrial melting. It is established that by controlling parameters of low-temperature processing regimes in a CAU and CHGU, it is possible to improve ductility and formability of rolled IF-steels significantly. During annealing rolled products in a CHGU it is possible to reduce the intensity of aging processes and prevent a reduction in ductility and formability using a strip temperature at the exit of the accelerated cooling section (P6) of at least 540°C, as well as a strip temperature at the exit from the cooling section after the galvanizing bath (P10) of not more than 180°C. This is especially relevant for IF-steels with a low sulfur content, when the possibility of stabilizing carbon in the form of titanium carbosulfide is limited. During annealing in a CAU rolled products from fully stabilized IF steels are slightly sensitive to overageing temperature regime. With incomplete stabilization of carbon in IF-steels, due to a low titanium concentration, its removal from solid solution in the form of cementite precipitates and the improvement of ductility and formability of rolled products is possible when using an overageing regime of 420–330°С, and especially at 390–300°С. Removal of carbon from the solid solution may be prevented by a high phosphorus concentration (≈ 0.011 wt.%) in steel. In this case, complete stabilization of IF steel is necessary by increasing titanium and/or niobium concentration.