Influência das temperaturas de recozimento contínuo na microestrutura e nas propriedades mecânicas de aço bifásico galvanizado da classe de resistência de 980 MPa

Abstract

Dual phase steels constitutes a family of high strength steels characterized by comprising a hard second phase, generally martensite dispersed in a ferrite matrix. Dual phase steels are strongly dependent on the microstructure obtained to meet the needs in its application and ultimately the required mechanical properties. Besides the parameters associated with hot rolling and cold rolling, together with the chemical composition, the process of continuous intercritical annealing directly affects the microstructure formation and consequently the mechanical properties. In this study, was evaluated via Gleeble thermo-mechanical simulator, the effect of varying two parameters intercritical annealing on the microstructure and mechanical properties of a dual phase steel of 980 MPa class (DP980) as follows: soaking temperature and slow cooling temperature. Five soaking temperatures, 720°C, 750°C, 780°C, 810°C and 840°C were evaluated by setting the slow cooling temperature at 670°C. Subsequently, using soaking temperature of 780°C five slow cooling temperatures, 610°C, 640°C, 670°C, 700°C and 730°C were tested. Aiming to evaluate the microstructural evolution during the stages of heating, soaking and slow cooling, interrupted tests were performed by the use of high cooling rates of the specimens. The temperatures tested were in the region of heating 500°C, 600°C, 620°C, 640°C, 660°C, 680°C and 700°C. For soaking temperatures of 750°C, 780°C and 810°C, the tests were stopped at the beginning and at the end of the soaking for microstructural evaluation. Analyzes by optical and scanning electron microscopy (SEM) revealed that the first microstructural variation in the region of heating was the spheroidization of carbides and later recrystallization of ferrite, which began approximately at the temperature of 660°C. At the temperature of 720°C throughout the ferrite was already recrystallized and the first nucleus of the austenite were observed on the boundaries of ferritic grains. Was observed increase in yield strength and tensile strength with increasing soaking temperature. The total elongation, uniform elongation and strain hardening exponent tended to decrease with increasing soaking temperature. The increased mechanical resistance and reduced ductility were due to the increased fraction of the constituents of the second phase (bainite, martensite and MA), ranging from 14% to 54% for soaking temperatures of 720°C and 840°C, respectively. The various slow cooling temperatures showed no great influence on the microstructure and mechanical properties, fixing the soaking temperature at 780°C. The microstructure obtained is basically composed of ferrite, bainite, martensite and MA constituent, as in other tests. At all slow cooling temperatures tested the tensile strength was over 980 MPa and the volume fraction of the second phase was 44% on average.