PSI - Issue 13

Tsubasa Kumamoto et al. / Procedia Structural Integrity 13 (2018) 710–715 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Ferrite/martensite dual-phase (DP) steels have been used for automotive body parts as they have a good combination of tensile strength and ductility (Senuma et al. 2001). In general, micro-damage, such as voids/cracks in ductile materials, results from inclusions. By contrast, the damage initiation of DP steels is associated with strain localization at a microstructural interface (Marteau et al . 2013, Yan et al. 2015). For instance, the ferrite near the martensite/ferrite interface is a strain localization site, resulting in micro-damage evolution at the interface and within martensite (Park et al. 2014, Avramovic-Cingara et al. 2009). Furthermore, the micro-damage evolution behavior can be classified as three regimes: damage incubation, damage arrest, and damage growth (Koyama et al . 2014). All regimes are associated with local deformability. Thus, the total elongation of DP steels is determined by a combination of plastic damage initiation resistance and damage growth arrestability. This fact implies that hydrogen must have a critical effect on the damage evolution because hydrogen enhances strain localization and lowers crack resistance. In fact, DP steels are susceptible to hydrogen embrittlement (Davies et al. 1981), which is related to the plasticity-based damage evolution (Koyama et al . 2014). In this context, the strain rate must be an important factor because it affects the time for microstructural hydrogen diffusion/segregation at specific microstructures or at the damage tip. In this study, tensile tests were carried out on a DP steel with different strain rates of 10 − 2 and 10 − 4 s − 1 . We performed damage quantification, microstructure characterization, and fractrography. Specifically, the quantitative data of the damage evolution was analyzed using the classification of the damage evolution regimes in order to separately elucidate the effects of hydrogen on damage initiation resistance and damage arrestability. A steel bar, of the chemical composition shown in Table 1, was solution-treated at 1250°C for 30 min. The bar was hot-rolled, and subsequently cold-rolled, to 1 mm. In order to obtain a ferrite/martensite DP microstructure, the sheet was intercritically annealed at 800°C for 3 min and subsequently water quenched. The optical micrograph of the intercritically annealed specimen on a transverse direction (TD) plane is shown in Fig. 1a. The specimen of this image was etched with a 3% natal solution, followed by color etching with a 5% Na 2 S 2 O 5 aqueous solution on a mechanically-polished specimen. After the color etching, ferrite and martensite appear as white and black, respectively. According to the optical microscopic observation on the color-etched surface, the fraction of martensite was measured to be 52%. Tensile specimens with a gauge section of 2.0 mm width × 1.0 mm thickness × 8.0 mm length were cut with electric discharge machining. Hydrogen pre-charging was performed for 3 h in a 3%NaCl aqueous solution containing 3 g l − 1 of NH 4 SCN at a cathodic current density of 10 A m − 1 . A platinum foil was used as the counter electrode. The diffusible hydrogen content was measured to be 0.145 mass ppm. The tensile tests were carried out along the rolling direction at initial strain rates of 10 − 2 s − 1 and 10 − 4 s − 1 at room temperature with and without hydrogen pre-charging. In the case with hydrogen pre-charging, the tensile tests began in air 30 minutes after the hydrogen pre-charging had finished. During the tensile tests, optical images were captured to measure the local strain on the specimens via the digital image correlation (DIC) method, as shown in Figs. 1b and c. A random pattern to apply the DIC method was formed using a black spray on a specimen undercoated by white enamel. The local strain data were used to correlate the quantified damage parameters with strain, as described in the following paragraphs. Damage quantification experiments were performed on TD cross-sections along the tensile direction of the fractured specimens. In order to observe the central region of a specimen, half the specimen width was removed using mechanical grinding. The final observation surfaces were prepared by etching with a 3% natal solution on mechanically-polished surfaces. The damage evolution behavior was quantified by measuring the damage area fraction, the damage number density, and the average damage size. The damage indexes for each location were plotted against the corresponding local strain in the fractured specimen. The local strain was obtained by the DIC analysis mentioned in the previous paragraph. The micro-damage and associated microstructures were observed by scanning electron microscopy (SEM). Further details of the damage quantification have been presented elsewhere (Tasan et al . 2012, Koyama et al . 2014, Uehata et al . 2017). 2. Experimental procedure