Abstract: 316L stainless steel—a common austenitic stainless steel—exhibits excellent corrosion resistance and mechanical properties and has been widely used in pressure-vessel, petrochemical, medical, and nuclear-power industries. Its Chinese standard designation is 022Cr17Ni12Mo2, and its American standard code is S31603. Additionally, its Ni mass fraction ranges from 10% to 14%. Ni, as the main austenite-stabilizing element in 316L stainless steel, can enhance the corrosion resistance and cold working performance. China is low on Ni resources, and the demand for Ni from electroplating and battery chemistry has surged, thus causing prices to increase to the point where Ni constitutes a large portion of the raw-material cost of Ni-containing stainless steel. Under normal order conditions, enterprises tend to maintain the Ni mass fraction at the lower limit of the standard, for example 10.0%–10.3%, to reduce smelting costs. However, the low Ni mass fraction of 316L austenitic stainless steel increases residual ferrite in the continuous casting billet, and the residual ferrite and precipitated phases directly affect the hot workability and surface quality of austenitic stainless steel. In this study, 316L austenitic stainless-steel billets with a Ni mass fraction of approximately 10% are investigated, and the distribution of ferrite and phases in the billets is analyzed. The features of ferrite and precipitated phases in the 316L billets are examined using optical microscopy(OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and electron probe microanalysis (EPMA). Both the equilibrium solidification and Scheil solidification processes of the 316L billets are analyzed based on Thermo-Calc thermodynamic calculations. The results show that the ferrite area fraction of the 316L billets is distributed in an “M” shape along the width direction, with the ferrite area fraction increasing from 4.2% to 10.6% at the surface to 85 mm and then decreasing to 7.8% toward the center. As the distance from the surface increases, the secondary dendrite arm spacing gradually increases and stabilizes at 63 mm. Meanwhile, the cooling rate gradually decreases from the surface to the center of the billet and decreases more rapidly at locations closer to the surface. The solidification mode of the billet is the FA(Ferrite–Austenite) mode, in which ferrite solidifies and precipitates first. The ferrite morphology changes from granular and skeletal to lath and short rod shapes from the surface to the center in the thickness direction of the billet. The ferrite on the billet surface does not transform into other phases. The high-temperature δ-ferrite inside the billet decomposes into χ and σ phases during solid-state phase transformation after solidification. From the surface to the center of the billet, the transformation ratio of ferrite to the χ and σ phases increases: the transformation ratio of the χ phase increases from 7.9% at a distance of 75 mm from the surface to 13.4% at the center, whereas that of the σ phase at the center is 51%. EPMA experimental results show the microsegregation behavior of the elements during the formation and decomposition of ferrite: Ni is enriched in austenite, Cr is enriched in ferrite, and Mo and P are enriched in the χ phase. These results reveal the distribution pattern of δ-ferrite and its decomposition products (χ and σ phases) in low-Ni 316L billets.