Effect of Complex Carbide Precipitates on the Structure and Mechanical Properties of Cold-Rolled Ultra-Low Carbon Steels During Bell-Furnace Annealing

Features of the formation and evolution of nanosize carbonitride precipitates and their effect on the microstructure and mechanical properties of cold-rolled product of medium-carbon steels microalloyed (stabilizing) with titanium of an industrial melt with a different carbon content are studied. It is established that precipitates have a complex (Ti,Nb)C composition, there is almost no nitrogen within them, although Nb is present in spite of its low (impurity) content in the steel. Precipitate size increases with an increase in temperature, annealing duration, and carbon content in steel. It is shown that during annealing in bell furnaces the structure and properties of rolled product are determined by the action of three main factors: occurrence of primary and selective recrystallization, with a direct and indirect (by delaying recrystallization) effect of nanosize carbide precipitates, and also the degree of reduction during skin rolling. During annealing of test steel cold-rolled product in bell furnaces, it is possible to achieve a unique set of mechanical properties that are difficult to combine: a prescribed low value of yield and ultimate strength ratio (0.60-0.75) with relative reduction of area of 30%, which is important for manufacturing complex shape components by efficient forging methods and deep drawing. Optimum values of annealing and skin rolling parameters providing preparation of these mechanical properties are determined.