The properties of a microalloyed steel, with Nb and V in its composition, were studied, after different intercritical thermal treatments and at different austenitizing and tempering temperatures. The mechanical properties of the specimens were measured in a Vickers hardness tester, and their microstructures were analyzed by optical microscopy, with the aid of a digital image processor. After austenitizing at 1100 °C and tempering at 625 °C, the samples showed significantly higher tempering resistance, reflected by their retention of high hardness, which may be associated with a secondary hardening precipitation of Nb carbon nitrides. In the sample with dual-phase microstructure, the martensite volume fraction varied from 18.2 to 26.3% and the ferrite grain size remained unchanged, upon the variation of the time length of the intercritical treatments. Tempered samples showed Vickers hardness (HVN) varying from 327 to 399, and dual-phase samples showed HVN from 362 to 429.

O efeito da temperatura de austenitização sobre a cinética de transformação da austenita em um aço microligado com Nb foi estudado em resfriamento contínuo através da análise térmica diferencial. Verifica-se uma forte dependência da temperatura de início de formação da ferrita alotriomórfica com a temperatura de austenitização devido ao seu efeito sobre o tamanho de grão austenítico. O aumento da temperatura de austenitização implica um aumento no tamanho de grão austenítico e uma diminuição na temperatura de início de transformação da austenita em ferrita. A presença de Nb em solução durante a transformação aumenta o tempo de transformação da austenita em ferrita alotriomórfica, reduz a taxa efetiva de transformação, levando à formação de bainita granular para temperatura de austenitização de 1100ºC e resfriamento no ar.

The effect of austenitization temperature on austenite transformation for a low-carbon steel and microalloyed with Nb was investigated under continuous cooling conditions by means of differential thermal analysis. The starting temperature for ferrite formation depends strongly on austenitization temperature due to its effect on the austenite grain size. Increase in austenitization temperature implies increase in austenite grain size and decrease in temperature starts ferrite formation. The presence of Nb in solution during the transformation increases the transformation time of austenite to ferrite and reduces its effective transformation rate, leading to the formation of granular bainite, at the austenitization temperature of 1100ºC and air cooling.