The yielding behavior of interstitial-free steels and low-carbon steels with varying amounts of C and N were investigated in connection with the HallPetch relation. The HallPetch coefficient is as small as 150 MPa•μm1/2 in interstitial-free steels but it increases to 600 MPa•μm1/2 by adding solute carbon up to 60 ppm. Nitrogen does not have a significant effect on the HallPetch coefficient. The results of three-dimensional (3D) atom probe analysis indicated that carbon has 34 times greater segregation potential in comparison with nitrogen. The small effect of nitrogen on the HallPetch coefficient in steel is probably due to the small segregation potential of nitrogen. It was also confirmed that discontinuous yielding occurs when the difference between the yield stress and friction stress is increased by grain-refinement strengthening and that yielding occurs by dislocation emission from grain boundaries where primary dislocations have piled up. Carbon atoms segregated at grain boundaries seem to play a role in stabilizing dislocation emission sites at the grain boundaries, which enhances the HallPetch coefficient of iron. These results support the dislocation pile-up model of explaining yielding in polycrystalline metals.
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