To verify the effect of anisotropy of internal stress on the heterogeneous deformation behavior in martensitic steel, in-situ digital image correlation (DIC) method was conducted on as-quenched and pre-strained Fe–18%Ni–0.02%C alloys with lath martensitic structure. Uniaxial tensile loading and unloading were repeated in several steps to apply stepwise pre-strain, and it is considered that the pre-strain makes decreases in anisotropy of internal stress or alters the state of internal stress. In the 1st step of deformation, during the transition from elastic to plastic deformation, the heterogeneous deformation begins block-by-block. In the 2nd step (after pre-strain of the 1st step) and the later steps, a similar strain distribution to that in the 1st step was observed. If the effect of anisotropic internal stress generated during quenching appear in the initial stage of deformation and be reduced or altered by pre-strain, the strain distribution should gradually change with increasing pre-strain. The unchanged strain distribution seems to have demonstrated the effect of internal stress on the heterogeneous deformation is relatively small. Slip system analysis was conducted on almost all blocks in the DIC field of view. It was confirmed that in-lath plane slip systems and in-habit plane slip systems were activated in blocks with higher strain concentrations. Based on these results for the ultra-low-carbon martensitic steel, it is concluded that the preferential activation of in-lath plane slip systems could be a more dominant factor for the heterogeneous deformation of lath martensite, compared to the anisotropy of internal stress introduced during martensitic transformation.

In the JIS standard case-hardening steel, spheroidizing annealing can improve cold forgeability but inevitably results in uneven distribution of carbides (a mixture of lamellar carbides and spherical carbides, and uneven carbides distribution). This degrades cold forgeability and promotes grain coarsening during carburizing. To address this problem, new SA methods combined with suitable chemical compositions are required to achieve a uniform distribution of carbides in a hypoeutectoid steel.

This study found that an ideal structure with uniformly dispersed carbides can be obtained by using high-Cr case-hardening steel with ferrite - pearlite as the initial microstructure and holding it below the A₁ temperature by precipitation of carbides in ferrite in addition to the dissolution of cementite in pearlite. Microstructure analysis and thermodynamic calculations revealed that in high-Cr compositions, the chemical potential of C in pearlite is significantly higher than in ferrite, which drives C diffusion from pearlite to ferrite, and precipitation of spherical carbides in ferrite. Furthermore, it was found that AlN particles in ferrite act as nucleation site of carbides, promoting precipitation of carbides in ferrite.

The Fe–10%Mn–0.1%C alloy exhibits an ultra-fine microstructure containing α’-martensite (bcc), ε-martensite (hcp), and retained austenite (fcc). However, Mn causes grain boundary embrittlement and intergranular fracture at the prior austenite grain boundaries at low temperatures. In this study, <0.9 mass% Mo was added to Fe–10%Mn–0.1%C alloys to suppress intergranular fracture and improve their toughness in their as-quenched states. Electron backscatter and neutron diffraction revealed that Mo addition displayed little influence on the prior austenite grain size, martensitic block width, and phase fractions. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy indicated that Mo segregated at the prior austenite grain boundaries at approximately 3 mass% after austenitizing 0.9%Mo steel at 1173 K, whereas Mn segregation remained almost unchanged. Thermodynamic calculations using the Hillert–Ohtani model confirmed that Mo exhibited a stronger segregation tendency than that of Mn. Charpy impact tests indicated that the ductile-to-brittle transition temperature (DBTT) decreased from 275 K for the Mo-free steel to 220 K for the 0.9%Mo steel. Fractographic observations revealed that intergranular fracture, which was dominant in the Mo-free steel at 77 K, was significantly suppressed by Mo addition. In the 0.9%Mo steel, intragranular fracture occurred with a characteristic size corresponding to the block size of α’-martensite. Therefore, Mo segregation effectively strengthens the prior austenite grain boundaries and suppresses intergranular fracture within Fe–10%Mn–0.1%C steel, leading to an improved low-temperature toughness and a reduced DBTT.