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This study aims to elucidate the behavior of diffusible hydrogen and delayed fracture in martensitic steel with 1500 MPa strength during automotive painting process and under vehicle service conditions. A sequential process of automotive pretreatment line and vehicle service environment is simulated to evaluate the hydrogen pick up in each process. In case of the automotive painting line, the absorption of hydrogen is within the common range in the process of phosphating treatment and electrodeposition. The baking process plays an effective role for desorbing the diffusible hydrogen absorbed during the automotive pre-treatment such as zinc-phosphating, and electrodeposition process. In case of the corrosion environment under the automotive driving conditions, hydrogen induced delayed fracture is accelerated as the exposure time increases. Further, it is clarified that severe plastic deformation are the significant factors for hydrogen induced delayed fracture under with low pH value and present of chloride ion in a chemical solution parameter. In summary, hydrogen is transported constantly during electrodeposition sequential line process of automobile manufacturing below the hydrogen content of 0.5 ppm, which is not critical value for leading to hydrogen delayed fracture based on results of slow strain rate tensile tests. However, exposure to extreme conditions under service environment of vehicle, such as acidic solution and chloride chemistry solution that result in high level of hydrogen absorption, severe plastic deformation in the sheared edge, and constantly applied internal or external stresses, can cause the hydrogen induced delayed fracture in the fully martensitic steels.

• Hydrogen diffusivity and trapping behaviour in relation to microstructure of Q&P steel were investigated. • Hydrogen trapped after plastic deformation in Q&P steel was mainly reversible. • The hydrogen trapping sites by dislocations can be related to the strain induced plasticity of Q&P steel. • The mechanism of the hydrogen trapping behavior under different levels of plastic deformation was proposed. The mechanism of hydrogen trap behaviors in the Q&P steel for different strain levels [Display omitted] The relationship between plastic deformation and hydrogen transport behaviour in the quenching and partitioning (Q&P) steel is studied. The evolution of microstructure after different levels of plastic deformation is characterized using the electron backscatter diffraction (EBSD) and X-ray diffraction technique. Also, the hydrogen diffusion and trapping of the plastically deformed Q&P steel are investigated using the hydrogen permeation electrochemical method and thermal desorption spectroscopy (TDS) on pre-strained specimens up to 14% engineering strain. The major findings through the experimental approach can be summarized as follows: (1) retained austenite fraction in Q&P steel decreased after plastic deformation; (2) the geometrically necessary dislocation (GND) density measured by KAM analysis increased; (3) hydrogen diffusivity decreased, while solubility increased as the plastic deformation increased. The increase in hydrogen trapping in the Q&P steel with plastic deformation can be attributed to the enhanced hydrogen trapping environment by increased dislocation density in ferrite and martensite transformed from metastable austenitic phase. The TDS analysis could support the hydrogen-trapping characteristics of Q&P steel, in which two major peaks were measured in different temperature ranges. Moreover, the EBSD observation and scanning transmission electron microscope (STEM) using transmission Kikuchi diffraction (TKD) analysis provided good correlation with the result of TDS analysis. [ABSTRACT FROM AUTHOR]