Round and spiral-shaped coil springs enable various peening angles that correspond to the surface location, and the directional shot angles may lead to a nonuniform residual stress on the coil spring surface. It is commonly known that a material under directional deformation exhibits a nonlinear 2θ-sin2ψ diagram (ψ split) due to the triaxial stress state. In this study, the residual stress distributions of spring materials deformed by shot peening at different angles were measured, and the microstructure for carbide precipitates was examined using a field-emission scanning electron microscope (FE-SEM). The nonlinearity in the 2θ-sin2ψ diagram for the shot peening samples was revealed. The extent of the ψ split increased with increasing shot peening angles, and was dependent not only on the mass fraction of carbide particles but also on the size distribution of the carbide particles.