2017 年 103 巻 8 号 p. 475-482
The mechanism behind the large elongation of Ti-added interstitial free (IF) steel has been investigated from the view point of voids’ nucleation, growth and coalescence in a local elongation region of tensile tests. Electron back scatter diffraction showed that in the case of IF steel, 50% of the voids nucleated at grain boundaries, 20% nucleated at Ti (C, N) on grain boundaries, 30% nucleated at Ti (C, N) in matrix. In the case of industrial pure iron, 70% of the voids nucleated at large angle grain boundaries while 30% nucleated at non-recrystallized grain boundaries. Synchrotron X-ray Laminograph observation showed that the voids in the IF steel grew, however, the coalescence was suppressed until a large plastic strain as compared with the pure industrial iron. Nano-indentation hardness (HIT) was measured when the tensile test stopped at a maximum load. This result showed that HIT at the grain boundaries of the industrial pure iron largely increased during tensile tests as compared with the IF steel. This finding was an indication that the industrial pure iron has heterogeneity of strain near grain boundaries previously in the region of uniform deformation. This finding also suggested that the large local elongation in the IF steel is caused by lowering heterogeneity of strain in the uniform deformation region followed by the suppression of voids’ growth and coalescence in the local elongation region.
