Avaliação da influência das condições de superenvelhecimento na microestrutura e propriedades mecânicas de um aço bifásico produzido por recozimento contínuo

Abstract

Dual-phase steels have emerged as an alternative solution with great advantages to conventional steels, because they combine high mechanical strength and good formability. In this study, the influence of overaging temperature and residence time during continuous annealing on the microstructure and mechanical properties of a cold-rolled steel with 780 MPa of tensile strength was assessed, aiming to determine the best processing conditions for the material. The mechanical properties of these steels are directly related to its microstructural characteristics: type, quantity, morphology and distribution of the constituents, which, among other things, depend on the processing parameters used, particularly in continuous annealing. It was studied the influencce of overaging step at 310°C, 350°C and 390 ºC, and the time spent at this stage in the microstructure and mechanical properties. Tests were performed using a thermomechanical simulator and a dilatometer on cold-rolled steel specimens obtained from the commercial production line of Usiminas. The annealed steel had a complex structure composed of ferrite (F), bainite and undissolved carbides (B + C), martensite (M) and martensite/retained austenite (MA), the second constituent being predominantly bainitic. The microstructure change for longer times and higher temperatures during the overaging step it was an increase in the percentage of bainite and a reduction in the same proportion in the amount of ferrite. The mechanical behavior in tension and microhardness varied according to variables used in the study. We can observe a fall in the yield strength, tensile strength and hardness with increasing overaging time and temperature. There were also an increase in the strain hardening exponent, improving the ability to evenly distribute the deformation. In addition, the temperature rise of OA was accompanied by a tendency to increase the ratio of decrease in elastic index BH. Finally, it was observed that the temperatures OA at 350°C or 390°C fulfilled the requirements of the mechanical properties required for a DP steel grade of 780 MPa tensile strength.