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Recently developed ultra-strong GIGA STEEL exhibites superior mechanical properties, with a high tensile strength of more than 1.5 GPa and good impact toughness. Nevertheless, its application to auto parts has been severely restricted, mainly due to a substantial reduction in the resistance to hydrogen embrittlement (HE) induced by aqueous corrosion. The susceptibility to this HE is closely associated with the carbon content, which leads essentially to the precipitation of iron carbides (Fe 2.4 C / Fe 3 C) with a low hydrogen overvoltage. This study focuses mainly on the effect of carbon content on the aqueous corrosion and hydrogen diffusion characteristics of ultra-strong steels. The hydrogen reduction reaction on the steel surface, and its diffusion kinetics in the steel matrix were evaluated using electrochemical polarization and hydrogen permeation tests, respectively. Furthermore, the HE indices of steels with different carbon contents were determined in a weakly acidic solution using the slow strain rate test (SSRT). This study demonstrates clearly that the hydrogen reduction rates were increased and its diffusion kinetics were decreased significantly, with increasing carbon content. Based on the present results, it is concluded that the critical technical issue for the development of ultra-strong automotive steels is the effective control of the carbide fractions in the microstructure.

With the stricter international regulations on CO 2 emissions, fuel economy, and auto-safety, the application of novel materials with both higher strength and lower weight is becoming a major technical issue in automotive industries. Among the various lightweight concepts, ultra-strong GIGA STEEL with a tensile strength of more than 2 GPa is a major breakthrough in light of the remarkable weight reduction of vehicle without a decrease in auto-safety. Despite the outstanding mechanical performance, hydrogen embrittlement induced by aqueous and/or atmospheric corrosion is a serious problem that has restricted the application of steel to auto-parts. This study reports that such a critical challenge can be overcome by Ni-alloying, which leads to a lower cathodic reduction rate on the steel surface and slower H-infusion kinetics in the steel matrix. In contrast to the beneficial effects of Ni-alloying, conflicting results can be obtained when steel with a higher Ni content (≥1 wt.%) is exposed to neutral-corrosive environments, but the results have not been verified using conventional metallurgical approaches. This paper proposes a mechanism for these conflicting results, and provides a new and economic strategy for superior resistance to corrosion-induced hydrogen embrittlement, by making optimal use of Ni-alloying of ultra-strong steel.

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