This paper presents an overview of the recent works on dynamic strain aging (DSA) of Fe-Mn-C austenitic steels including Hadfield and twinning-induced plasticity (TWIP) steels. First, a model of the DSA mechanism and its controlling factors are briefly explained in terms of Mn-C coupling and dislocation separation. Then, we introduce the effects of DSA on mechanical properties such as work hardening capability, uniform elongation, post-uniform elongation, and fatigue strength. Specifically, we note the pinning effect on extended dislocation for the work hardening, the Poretvin-Le Chatelier banding effect on damage evolution for the elongation, and the crack tip hardening/softening effect on crack resistance for the fatigue strength. We believe that this overview will help in designing advanced high-strength steels with superior ductility and fatigue resistance.

To investigate the characteristics of dislocation evolution in ferritic and austenitic stainless steels under tensile deformation, neutron diffraction line-profile analysis was carried out. The austenitic steel exhibited higher work hardening than the ferritic steel. The difference in the work hardening ability between the two steels was explained with the dislocation density estimated by the line-profile analysis. The higher dislocation density of the austenitic steel would originate from its lower stacking fault energy. Dislocation arrangement parameters indicated that the strength of interaction between dislocations in the austenitic steel was stronger than that in the ferritic steel. This would mainly originate from the difference in dislocation substructures; while dislocation tangle, which can be prompted by the cross slip, was expected in the ferritic steels, highly dense dislocation walls induced by planar glide of dislocations as well as the tangle were expected in the austenitic steel. It was confirmed that the stronger interaction between dislocations in the austenitic steel resulted in the smaller strain field of dislocation. Consequently, the coefficient for the root square of dislocation density in the Bailey-Hirsh equation became smaller in the austenitic steel. X-ray diffraction line-profile analysis was also carried out for the tensile-deformed specimens. The dislocation arrangement parameter evaluated by X-ray diffraction was smaller than that evaluated by neutron diffraction. This would be caused by the difference in the relationship between the loading direction and the scattering vector. On the other hand, the dislocation density evaluated by both methods was almost identical.

The influence of Si content in steel on tool wear in turning of 0.8 mass% C hardened steels with TiAlN coated CBN cutting tools is investigated. The Si contents in the steels are varied between 0.05 and 0.6 mass%. Although these steels have similar microstructure, hardness and volume fraction of retained austenite, the width of the flank wear of the tool increases with increasing the Si content. Adhered oxides are formed on the flank face after cutting, and the amounts, compositions and crystal structures of these oxides are changed as the Si content is varied. The higher Si content results in large amounts of adhered oxides. The crystalline oxide containing a large amount of Fe is mainly formed when cutting the 0.2 mass% Si steel, while the amorphous oxide containing a large amount of Si is mainly formed when cutting the 0.6 mass% Si steel. At the interfaces between the tool coating and the adhered oxides, the Al element of the tool coating tends to diffuse more easily into the Si containing amorphous oxide than into the Fe containing crystal oxide. This indicates that the Si containing amorphous oxide, formed with the higher Si added steel, promotes diffusion wear, resulting in increased tool wear.

SEM observation was conducted on cross-sections of galvannealed (GA) boron-steel sheet specimens, subjected to direct hot-stamping tests (V-bending), to study liquid metal embrittlement (LME), caused by liquid zinc in the coating. Specimens were heated to 1173 K (900°C) in a combustion gas furnace, and subsequently hot stamped in a cooled, V-shaped die. The locus of intersections between the Fe-Zn ferrite grain-boundaries (expected to be filled with liquid zinc) of the coating layer, prior austenitic grain boundaries of the steel substrate, and the coating interface, were examined. Specimen cracking originated at the coating/steel interface, and propagated along prior austenitic grain boundaries, where liquid zinc directly contacted the steel substrate. These prior austenitic grain boundaries were considered to be “geometrically favored” sites for initiating LME cracking. Cracking did not occur at sites where direct contact with liquid zinc was not established. There were numerous sites where cracking did not occur despite contact between liquid zinc and prior austenitic grain boundaries, at the coating/steel interface. In heavily cracked specimens, there were 4.7 to 5.9 cracks per mm of coating interface. Cracking occurred in only 23 to 36% of the “geometrically favored” sites at the coating interface. At the bottom of large cracks, cracks were round-bottomed. Vickers hardness at the bottom was lower than that at the sidewall. Therefore, ferrite or bainite transformation, enhanced by plastic deformation, was indicated. This suggested an absence of zinc propagation at deeper austenitic grain boundaries, terminating crack propagation despite the plastic deformation.

Prior to electrodeposition, the steel sheets were polished with each method such as emery paper, buff and electrolytic polishing, and Zn deposition was performed galvanostatically at 1500 A/m² in an agitated sulfate solution at 40 °C to investigate the effect of surface textures of steel sheets on the crystal orientation of Zn. The strain of steel surface and the degree of decrease in grain size of Fe by the strain were largest with emery paper polishing, and were larger in the order with buff polishing, polishing-free, and were smallest with electrolytic polishing. The preferred orientation of the {0001} Zn basal plane of the hcp structure was largest with electrolytic polishing, and was larger in the order with polishing-free, buff polishing, and was smallest with emery paper polishing. That is, the preferred orientation of the {0001} Zn increased with decreasing the strain of steel sheets and increasing the grain size of Fe. With electrolytic polishing, since the strain of steel sheets decreased and the grain size of Fe increased, the epitaxial growth of deposited Zn was easy to occur. The initial Zn deposits seems to grow epitaxially according to the orientation relationship of {111}Fe//{0001}Zn because the preferred orientation of the steel substrate used in this study is {111} Fe. The preferred orientation of the {0001} Zn seems to increase under the conditions where the epitaxial growth of deposited Zn is easy to occur.

To investigate basic characteristics of the phase transformation that strongly affects mechanical properties in a high-strength β-rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, microstructures of the specimens continuously cooled from the β region at various cooling rates were examined.

Vickers hardness at room temperature sharply increased with increase of cooling rates in the specimens cooled at cooling rates lower than 20°C/s, reached the maximum value of 477 HV at 20°C/s and decreased with increase of cooling rates higher than 50°C/s.

At the cooling rates higher than 50°C/s, the α″ phase (orthorhombic martensite phase) laths of 0.1 to 0.5 μm in width were formed. Meanwhile, the grain boundary α phase and the side-plate α phase were formed in the specimens cooled to room temperature at 20°C/s. In the specimens cooled at 5 to 50°C/s, the black plate type of the α phase was formed. TEM observation revealed that the black plates have hcp crystalline structure and very fine (0.05 to 0.2 μm in width) compared with the α″ phase in the specimens cooled at 50°C/s or higher cooling rates. In addition, the extremely fine acicular hcp phase of 50 nm or less in width was also formed at the area where the black plates were not formed. It is considered that the above fine microstructures led to quite high Vickers hardness of around 477 HV.

The formulated continuous cooling transformation diagram indicated that Ti-5Al-2Fe-3Mo is excellent in hardenability and thermal processability, suggesting that the alloy has a lot of advantages industrially.