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Previously reported low-Mn ferritic-based lightweight steels are potential candidates for industrial applications, however, they typically exhibit lower strength, with < 1 GPa and lower strength-ductility balance, than medium- and high-Mn austenitic lightweight steels. Herein, we introduce a low-temperature tempering-induced partitioning (LTP) treatment that avoids the strength-ductility dilemma of low-Mn ferriticbased steels. When the LTP process was performed at 330 oC for 665 s, the strength of typical ferritic base Fe-2.8Mn5.7Al0.3C (wt%) steel with heterogeneously sized metastable austenite grains embedded in a ferrite matrix, exceeded 1.1 GPa. Notably, the increased strength-ductility balance of the LTP-processed ferritic steel was comparable to that of the high-Mn based austenitic lightweight steel series. Using microscale to nearatomic scale characterization we found that the simultaneous improvement in strength and total elongation could be attributed to size-dependent dislocation movement, and controlled deformation-induced martensitic transformation.

The H contents and H embrittlement (HE) of Zn- and ZnNi-electrodeposited SCM435 bolts were quantitatively evaluated before and after baking. The Zn-coated bolt possessed higher H contents than the ZnNi-coated bolt in as-electrodeposited and baked states. H atoms were strongly trapped inside the coating layers. The Zn-coated bolt underwent flaking of coating layer during baking at temperatures above 200 °C due to H effusion through the brittle δ phase. The Zn-coated bolt exhibited no HE regardless of baking despite of its high H content due to no permeation of H into the steel substrate during electrodeposition and tensile deformation.

Tempering approach is designed for better understanding the effects of heat treatment induced by production process when manufacturing on the transformation-induced plasticity steels containing 1.0 wt.% Al. Specific attention is placed on the roles of tempering temperature and the holding time on the decomposition of retained austenite and the redistribution of alloying elements. Using transmission electron microscopy, we found the retained austenite was decomposed into ε-carbide and ferrite in the steels tempered at 300 °C for 9 h. An increase in the temperature of 400 °C and the holding time over 3 h accelerate the nucleation kinetics of cementite formation, leading to the deteriorated thermal stability of austenite. In addition, atom probe tomography studies confirmed the partitioning tendency of alloying elements across the ferrite/cementite interfaces as well as the compositional spikes of Mn at the interfaces during tempering over 400 °C for 9 h. Keywords: Transformation-induced plasticity steels, Retained austenite, Decomposition, Atom probe tomography

In the present study, the effects of microstructure, chemical composition on the transformation behavior and mechanical stability of individual retained austenite (RA) with the different sizes and morphology in TRIP steels were intensively investigated. In order to characterize the property of the individual RA, various analytical techniques including atom probe tomography (APT), step-wise straining EBSD and nano-indentation were applied. The blocky type RA as we categorized have many defects and lower carbon contents (~25%) compared to film type RA. And also, step-wise straining EBSD and nano-indentation results revealed that the mechanical stability of blocky type RA was lower than that of film type RA which means blocky type RA would be easily transformed into martensite. It is considered that many defects existing in blocky type RA could work as the nucleation site of martensite transformation. In contrast, high carbon contents and hard phases enclosing film type RA would increase the resistance to shear transformation resulting in the inhibition of transformation of RA to martensite.