Effect of Strain Localization on the Mechanical Properties from Nonuniform Grain Size Distribution of Ultralow Carbon Steel

The effect of the bimodal grain size distribution in the range of several tens of micrometers is studied to investigate the mechanical properties of metallic materials with single‐phase ferritic microstructure. The previous studies on the bimodal distribution of grain size have been performed on materials with nanocrystalline, ultrafine, or multiphase microstructures, and have generally reported that the mixed structure of fine and coarse grains improves the ductility of the material, because the coarser grains preferentially accommodate deformation. However, herein, the coarse grains do not accept the deformation well during plastic deformation and cause nonuniform deformation, which lowers the work hardening coefficient and accelerates necking of sheet. The influence of the bimodal grain size distribution on the mechanical properties is evaluated using tensile and various formability tests. Microstructural changes before and after deformation are tracked by electron backscatter diffraction. Herein, the bimodal distribution is artificially generated by adding small amount of niobium in ultralow carbon steel. Coarse grains contribute less to the plastic deformation than small grain. As a result, the work hardening coefficient and uniform elongation dramatically decrease with the bimodal grain size distribution which is different from the previous studies on nanocrystalline and multiphase material.