Effect of Composition and Processing Parameters on Microstructure and Mechanical Properties of If-Steel Cold-Rolled and Galvanized Rolled Products

The effect of composition, parameters of hot and cold rolling, and recrystallization annealing in continuous annealing units (CAU) on the microstructure and mechanical properties of cold-rolled and galvanized steel is studied for IF-steel rolled products from three laboratory melts. The possibility is established of changing the strength properties of cold-rolled and galvanized rolled products during annealing in a CAU, and treatment in a continuous hot galvanizing unit (CHGU) of IF-steels stabilized with Ti and Ti + Nb over a wide range, while obtaining good ductility properties. It is shown that in order to increase the level of steel strength properties it is advisable to obtain a ferrite grain size of 10–11 μm (preferably with a size of no more than No. 10 according to GOST 7639), C, P, Mn contents closer to the upper level, and S not more than 0.003%. Equally important is use of lower temperatures for the end of rolling at 860–890 °С and strip winding at 650–680 °С, increased degrees of deformation during hot rolling in the last stand of at least 12–15%, during cold rolling steel not less than 80%, low annealing temperatures of 740–780 °C and an increased strip speed of 120–160 m/min, and 1.0–1.4% elongation during tempering. It is shown for the first time that in order to resolve the extremely difficult problem of obtaining cold rolled and galvanized steel from IF-steels of strength class 180, it is necessary to use all or most of the methods indicated. For steels stabilized with titanium a significant excess of its concentration above the stoichiometric value is desirable.