A lecture given by Toshihiro Tsuchiyama, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. How particles such as copper, which can be penetrated by dislocations in the ferrite, influence the properties. The second phases in steel, such as carbide, oxide, martensite, and so on, are usually used for enhancing work hardening to prevent the plastic instability during deformation. These hard structures are effective for increasing uniform elongation, but conversely, it tends to deteriorate the local elongation and reduction of area. To improve both uniform and local deformabilities, it would be desirable that the work hardening is enhanced by dispersed second phase in the initial stage of deformation, and then it disappears or becomes invalid in the higher strain region, leading to work softening. Authors believe that soft particles is one of possibilities to exhibit such a functional change and call it hetero-to-homo structural transition. In this report, the effect of soft Cu precipitates on tensile deformation behaviour of ferritic steel will be compared with that of hard VC carbide precipitates. In addition, the plastic deformation and mechanical dissolution behaviour of Cu particles by severe cold rolling will be demonstrated.