In recent years, strong demands for improved automobile fuel economy through the reduction of car body weight have increased the need for the development of high strength steels. However, there is a concern of "delayed fracture (hydrogen embrittlement)" which is considered to absorb hydrogen into steels while using these steels in applications that require high strength steels exceeding 1180 MPa.Corrosion of steel under an atmospheric corrosion condition which is assumed to be the environment in which vehicles are used, is considered to be progress in accordance with the Evans model and hydrogen are absorbed into steels.As shown in weathering steels, it is thought that the characterization and properties of rust have an important influence on the generation and absorption of hydrogen. In this study, we investigated whether the hydrogen absorption into steels can be reduced by controlling the characteristics of rust formed on the surface of the steels using outdoor exposure testing. In addition, to determine whether to delay fractures occur in steels when using them in the above mentioned environment, it has been proposed that delayed fracture occurs when the amount of hydrogen absorbing from the environment (H_E) exceeds the amount of hydrogen at which fractures form (H_C). Thus, in steel sheets with high strength (1180-1470 MPa), we examined the effect of strength level and the structure of the delayed fracture. Steel A(1470TBF), Steel B (1470DP), and Steel C (1180DP) were used this experiment. To test the amount of H_E, U-bend test pieces were prepared and directly exposed to an atmospheric corrosion condition. Approximately a 10 mm × 10 mm steel piece was cut from the head area of the U-bend the test piece to measure the amount of hydrogen, and the rust on the test piece surface was removed by shot blasting before measuring the amount of hydrogen. The amount of hydrogen was measured using the atmospheric pressure ionization mass spectrometer (API-MS). To confirm the long-term behavior of the fracture that occurred. U-bend test pieces were exposed for about 3-10 years. The amount of hydrogen absorbed was changed by changing the density of the applied current to the bent part of the U-bend a test piece in the solution. Observation in the head area of U-bend the test piece was carried out while an electric current had been applied. The amount of diffusible hydrogen at the time of occurring fracture was evaluated as Hc. The amount of hydrogen absorption into the bent portion of the steels with a high mechanical strength of 1180 to 1470 MPa, which was a corrosive atmospheric environment, was approximately 0.2 ppm The addition of elements that improve corrosion resistance and the prevention of rust formation on the steel's surface allowed for the reduction in the amount of hydrogen absorption into the steels. TBF structure is better delayed fracture properties than DP structure just as 1470 MPa grade.
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