Work hardening of ferritic steels containing fine carbides varied from 3 nm to 15 nm was investigated and compared to Ashby's model, which is well known as a work hardening theory of metals containing hard particles. A specific work hardening behavior was observed in the steels strengthened by the nanometer-sized carbides; work hardening proceeded in two stages within a few plastic strains. In the former step, the matrix deformed without the geometrically necessary dislocation since a misfit strain between the carbides and matrix is close to the Burgers vector. So the Ashby's model cannot explain this phenomenon. Yet in the later stage, the amount of work hardening was close to predicted value based on the Ashby's model. The plastic strain at which the later stage started decreased with the increase in the diameter of carbides since the geometrically necessary dislocation is easier to be generated by the larger carbides. A new model which can be applied to steels containing the nanometer-sized carbides by focusing generating dislocation into the matrix around carbides was established.

Synopsis : Work hardening of ferritic steels containing fine carbides varied from 3 nm to 15 nm was investigated and compared to Ashby's model, which is well known as a work hardening theory of metals containing hard particles. A specific work hardening behavior was observed in the steels strengthened by the nanometer-sized carbides; work hardening proceeded in two stages within a few plastic strains. In the former step, the matrix deformed without the geometrically necessary dislocation since a misfit strain between the carbides and matrix is close to the Burgers vector. So the Ashby's model cannot explain this phenomenon. Yet in the later stage, the amount of work hardening was close to predicted value based on the Ashby's model. The plastic strain at which the later stage started decreased with the increase in the diameter of carbides since the geometrically necessary dislocation is easier to be generated by the larger carbides. A new model which can be applied to steels containing the nanometer-sized carbides by focusing generating dislocation into the matrix around carbides was established.

The interface structure of semicoherent titanium carbide (TiC) nanoparticles and incoherent micron-sized TiC precipitates in Fe–C–Ti alloy hot-rolled steel sheets was studied by transmission electron microscopy (TEM) and positron annihilation spectrometry (PAS). The TiC nanoparticles were formed by interphase precipitation during the γ-to-α transformation, and the coarse micron-sized TiC precipitates remained in the matrix as undissolved carbides in the slab reheating process before hot rolling. The semicoherent TiC nanoparticles have coherent planes with the Baker–Nutting orientation relationship and incoherent surface almost parallel to {001}α planes. The present investigation revealed that positrons are not annihilated inside TiC particles but rather are trapped at open volume misfit defects located at the incoherent interface between TiC particles and the matrix. The positron lifetime related to misfit defects associated with the semicoherent TiC nanoparticles and incoherent micron-sized TiC precipitates was around 240 ps and 500 ps, respectively. Considering the results of TEM observations and the theoretical positron lifetime, it can be concluded that the positron annihilation site related to TiC nanoparticles is located at the incoherent interface and consists of some vacancies.