SUS301, SUS304 and SUS316L steels with different carbon content, austenite stability and grain size were prepared, and their mechanical
properties were investigated by In–situ X–ray diffraction in tensile tests using Synchrotron radiation. In addition, the occurrence and growth of
necking immediately after yielding was investigated using a CCD camera. As is well known, the yield point increased with decreasing in grain size. Increasing rate of the yield point was dependent on carbon content, and the slope
(Hall–Petch coefficient) increased with increasing carbon content. In SUS304 and SUS316L ultrafine grained structural steels, necking occurred immediately after
yielding, the nominal stress decreased, and the necking progressed and fractured. On the other hand, in SUS301 ultrafine grained structure steel, after yielding at
1600 MPa (upper yield stress), a necking occurs and the stress decreases slightly (lower yield stress), but the progress of the necking stopped and propagated in
the longitudinal direction of the specimen. The nominal elongation was Lüders deformation of 20ÿ, followed by uniform deformation with a total elongation of 35ÿ.
This phenomenon is due to the large amount of strain induced martensitic transformation. In–situ X–ray diffraction results of tensile tests in SPring–8 showed that
65ÿ of austenite was transformed to martensite. On the other hand, its volume fraction for SUS304 and SUS316L ultrafine grained structural steels were 18ÿ at most.
From the point of true stress–true strain curve, the plastic instability condition is satisfied because their work hardening rate is extremely small. Therefore, after the
necking occurs, it breaks as it progresses. In SUS301 ultrafine structured steel, the work hardening rate is small just after yielding and the necking occurred
immediately. However, its work hardening rate became large immediately after necking started. When the carbon content is high, the grain size is ultrafine, and the
austenite stability is low, high yield point of 1600 MPa and large Lüders deformation can be obtained.