Estudo e desenvolvimento de método de seleção de variantes cristalográficas na transformação ϒ-α’ e ϒ-ε de aços com elevado teor de Mn

Abstract

Polycrystalline materials exhibit a property known as crystallographic texture. It is defined as the preferred orientation of the crystals of a material. Textured materials display other properties, such as mechanical, electrical and magnetic qualities, that may vary along with the orientation. The orientation relationship between phases is also an important property to be investigated, which can be done through the analyses of the texture of the mother and product phases. Mn steels can form several important phases, including martensites α’ and ε. The texture and the orientation relationship between the product phase and the mother phase (austenite) can still be better understood. This work aims to analyze the variant selection on orientation relationship in steels with Mn additions (alloy 1 with 29Mn-0.15C, alloy 2 with 20Mn-0.2C, alloy 3 with 11Mn-0.2C, and alloy 4 with 5Mn-0.15C). This study was divided into two sections. The first section includes the material characterization, a hot compression test to induce the variant selection, and the analysis of the orientation relationship, all to describe the behavior of martensites. In this section, the results from the characterization showed that alloy 1, initially with austenite and ε-martensite induced by deformation, presented only austenite after compression. Both alloy 2 and alloy 3 had ε-martensite before and after compression, whereas alloy 3 also presented α’-martensite. Alloy 4 presented a microstructure composed entirely of α’-martensite. We characterized the orientation relationship studied in these materials as Kurdjumov-Sachs between austenite and α’-martensite, Shoji-Nishiyama between austenite and ε-martensite, and Burgers between α’-martensite and ε-martensite. The second section comprises the development, comparative evaluation and application of variant selection programs, in which it was possible to simulate the variant selection of materials subject to external uniaxial stress. The analyzed materials had gradual Mn values, which enabled the categorization of steels in presenting only α’-martensite, only ε-martensite, and both martensitic transformations. In this thesis, we used the models of Patel-Cohen and Humbert for the martensitic transformations α 0 and ε, respectively. Comparative analyses with already published articles showed that it was possible to improve the code of the Patel-Cohen model, correcting an error in the calculation of the variants. For the application of the variant selection models in high-Mn steels, the results showed that the developed programs yield good results, which made it possible to predict the locations of the variants with the highest intensity for the two martensites.