Efeito dos parâmetros de recozimento contínuo na microestrutura e propriedades mecânicas de um aço revestido por imersão a quente da classe de resistência de 800 MPa

Abstract

Dual phase steels are characterized by a microstructure of martensite islands dispersed in a ferrite matrix; lower amounts of other constituents can be present. Their high tensile strength, low tensile ratio, high work hardening rate and high total and uniform elongation, enables the manufacture of lighter structural parts. In this work, the objective was to evaluate the effect of different thermal cycles on the microstructures and mechanical properties of a hot dip galvanized Dual Phase steel, class 800 MPa, from full hard samples of cold rolled manufactured in a pilot plant, considering two soaking temperatures in the intercritical annealing (740°C and 780°C) and three isothermal temperatures in the rapid cooling (610°C, 560°C and 510°C). A HDPS (Hot Dip Process Simulator) was utilized in order to investigate which process parameters have a higher probability of success in industrial scale, which was evaluated through tensile testing, bake hardening (BH2%) and hole expansion. The characterization of the microstructural constituents and determination of their volume fractions was performed via optical microscopy (OM) and scanning electron microscopy (SEM), as well as evaluation of their influence on the obtained mechanical characteristics. Microstructural characterization and dilatometric testing showed that the soaking temperature of 740°C was not sufficient for the complete dissolution of carbides, leading only to the full recrystallization of ferrite and nucleation of low fractions of austenite in the intercritical annealing, resulting in low fractions of constituents of the second constituent in all assessed microstructures. Thus, the minimum tensile strength of 800 MPa was not reached. Furthermore, the results of ductility, toughness, bake hardening and hole expansion fell short from those obtained by soaking at 780°C. In turn, the simulations with soaking temperature of 780°C and isothermal temperature of 510°C and 560°C in the rapid cooling showed satisfactory results, reaching the required minimum tensile strength, with good toughness and hole expansion slightly higher than that obtained in the simulations with soaking at 740°C, which was influenced by the higher fractions of constituents of the second constituent, in particular by the presence of the MA constituent (martensite-austenite).