Mecanismos de aumento de resistência mecânica em aços de baixa liga e alta resistência micro-ligados ao Nb e Ti : efeito da adição de boro e das variáveis de processamento TMCP

Abstract

The chemical element boron, atomic number 5 in the periodic table, has the potential to improve the mechanical properties of high-strength, low-alloy steel (HSLA), niobium (Nb) micro-alloyed, although there are controversies about its effects on toughness. This fact prevents the addition of this element to shipbuilding steels and petroleum pipes. Two heats of HSLA steel, niobium microalloyed, were produced, one of the heats with boron and titanium addition, and the slabs were rolled into coils. Critical temperatures were determined in a hot torsion test. Variations in the TMCP (Thermo-Mechanical Control Process) process parameters were performed, and the effects on microstructure, tensile, toughness and hardness properties were analyzed. The predominant acicular microstructure of the sample with a high toughness result is related to a greater amount of boron segregated to the grain boundaries in non-equilibrium conditions, inducing a suppression in the nucleation of polygonal ferrite. Differences in reduction by rolling pass and cooling after rolling are related to the distance between the niobium carbide (NbC) and titanium nitride (TiN) particles, which allows the diffusional growth of the TiN-NbC composite at the austenite grain boundaries, thus motivating the nucleation of acicular ferrite. The high-angle boundaries, typical of acicular ferrite, are efficient in blocking microcracks before they grow to larger sizes, thus justifying the high toughness found in a sample at low test temperatures. Boron can be used to achieve high values of tensile properties, keeping toughness at high levels and with lower production costs, once as boron can replace more expensive elements such as molybdenum and chromium.