Influência de microestrutura na aplicação de aços multiconstituídos da classe 980 MPa

Abstract

In the current scene of automotive modernization, factors such as energy efficiency, reduction of pollutant emissions and gains in safety are essential. These aspects drive the steel industry to develop new materials with emphasis on Advanced High Strength Steels. Over time these have been enhanced and as they become increasingly suitable for use in different parts, they are consolidated in the structural construction of vehicles. The selection stage is significant for the success of its application, in view of the increase in the failure rate by different mechanisms when compared to conventional steels. The microstructure is one of the main factors to consider, because for the same strength class, there may be divergent results in the drawing of the parts, depending on the geometry and the implicit forming requirements. The present work deals with the development of different designs of Multiphase steel of 980 MPa class, aimed at application in pieces with stretching flangeability, aiming a better hole expansion results, compared to traditionally traded Dual Phase steel, of microstructure typically constituted by ferrite and martensite. In order to reach this advance, tried to produce steels with more homogeneous, refined microstructure and with less difference of hardness between the constituents: one of balanced microstructure (ferrite, martensite and bainite), combining elongation and hole expansion ratio; another of majority bainitic microstructure, in order to maximize the hole expansion ratio. It was used a new chemical composition with changes in relation to conventional steel of the same grade (C-Mn-Si alloy), with reduction of carbon and addition of other elements to increase its hardenability. From annealed cold rolled coil samples of the new alloy, annealing cycles were simulated through Gleeble, varying soaking and overaging temperatures. From the pilot scale results, industrial tests were carried out aiming at commercial scale production and sampling was performed on products generated in continuous annealing. For a comparison between the new steels and the conventional steel, tests were made for hole expansion ratio and tensile tests at room temperature, besides microstructural characterization. The new design presented microstructural and tensile properties differences which are correlated to the best results of hole expansion ratio.