PSI - Issue 13

Atsuhisa Kitade et al. / Procedia Structural Integrity 13 (2018) 1845–1854 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Outline of research

1.1. Research background

Recently the production and consumption of natural gas has increased worldwide. Even in Japan, since the nuclear power plant stopped due to the Great East Japan Earthquake of 3.11, the amount of electricity generated by natural gas has increased as a result of diversification aiming at a stable supply of energy. At present, two means of transportation are mainly used: continuous transportation by pipeline or ship transport by LNG tanker vessel. Globally, the ratio of the transportation of natural gas by pipeline is 70 percent. Moreover, it is reported that the demand of natural gas will grow steadily for this 20 years [Kobayashi, 2018] In order to balance the safety design of steel for pipeline and its financial viewpoint, thick plate manufacturing technology including TMCP (Thermo-Mechanical Control Process) technology has been developed and put to practical use. TMCP technology is aiming at improving the strength and toughness of steel by effectively utilizing temperature control during hot rolling and cooling process of steel and refining the base microstructure. While development of TMCP technology has achieved fruitful results, it is thought that further safety in severe environments such as cold district and deep sea floor will be strictly required in near future. Further development of high toughness steel, or dramatic cost reduction is demanded socially. In order to improve the toughness by controlling the crystal structure, it is first necessary to specify the micro mechanism itself of brittle fracture. Attempts to predict fracture characteristics from microstructural information have been conducted by many researchers, but the concept used in many of them has been "Multi barrier model". [Martin Meizoso et al, 1994] applied "Multi barrier model" to the bainite structure and tried to describe the process of brittle fracture at three stages. Stage 1: carbide crack Stage 2: Propagation of carbide cracks into effective bainite packets Stage 3: Propagation of bainite packet cracks to adjacent tissues Fig.1 shows an image of "Multi barrier model". However, for the TMCP steel discussed in this paper, it is not known at present which stage is the controlling factor. Yoshie et al tried to construct a model to predict toughness from the rolling conditions in TMCP steel and the like, but all of them were limited to statistical analysis. In this research, we try to elucidate the physical mechanism of fracture occurrence and to construct a physical model. 1.2. Historical review of research of micromechanism of brittle crack initiation

Stage 1

Stage 2

Stage 3

Fig.1 Multi barrier model

1.3. Purpose of the study The aim of this research is to construct a prediction model that enables to estimate the toughness (85% SATT value) of the DWTT test (Drop Weight Tear Test)[ASTM, 2014], for example, 85% SATT value which is regularly required in major specifications, merely by inputting the process parameters which are alloy content and rolling conditions of the steel. The DWTT test is used as a quality assurance test for simplified evaluation of the brittle crack propagation arrest property as a pipeline. Once this model is completed, it will be an important tool for planning new materials