Evolution of Microstructure and Mechanical Properties of HSLA Strip Steel after Cold Rolling and Annealing

A large testing programme of a strip steel HSLA grade, microalloyed by vanadium, titanium and niobium, was conducted. The experiment was based on combination of cold rolling, recrystallization annealing, mechanical testing, metallographic examinations and TEM analysis. Flat samples with thickness 3.9 mm were rolled in several passes with the total height reduction 5 to 75 %. Afterwards the laboratory mill products were annealed in the vacuum furnace with the protective gas atmosphere consisting of N2+H2. The annealed samples underwent the mechanical testing. The gained results – hardness, yield stress, tensile strength and their ratio, as well as elongation, were summarized in graph in dependence on relative height reduction before annealing. It was confirmed that by a suitable combination of size of previous cold deformation and parameters of the following recrystallization annealing it is possible to influence a complex of mechanical properties of particular strips. Particular trends of strength and plastic properties correspond to each other and they may be utilized for optimization of terms of heat treatment of the investigated HSLA steel in a cold rolling mill. These trends are caused by structure-forming processes (recrystallization, grain coarsening, changes of the state of precipitates) which were documented by micrographs.