Micro-processes of brittle fracture initiation in bainite steel manufactured by ausforming

In recent years, the high tensile steels have got much attention in steel structural fields. Especially, the steel plate which has bainitic structure manufactured by TMCP (thermo-mechanical control process) technology becomes widely used because it has good balance of strength, ductility and toughness. Microstructure of those material is basically bainite structures, which are consisted of bainitic ferrite, cementite and MA (Martensite-Austenite constituent). Furthermore bainite microstructure manufactured by modem TMCP has extremely fine structures due to its transformation process from heavily deformed band structure including sub-grain structures which were generated during large amount of rolling in unrecrystallized temperature range called ausforming. However, the theoretical mechanism of brittle fracture initiation of bainite is still not clear because of its complexity of its structure. According to previous research including the observation of the fracture surfaces or cross sections of fractured specimens, the prior phenomenon of brittle fracture may occur at either the boundary area between MA and matrix or inside of MA. However, the details of the prior event for the macroscopic fracture mechanism are still unknown. The purpose of this study is to clarify the precursor phenomenon and estimate the quantitative criterion of brittle fracture initiation of bainite steels, which have fine microstructures, by observing the process of it directly by the in-situ observation methods. Copyright (C) 2016 The Authors. Published by Elsevier B.V.