To meet the requirement of low magnetic permeability, which, in turn, lowers the ferrite content of castings, of special interest is 316 stainless steel, whose low ferrite content renders it suitable also for nuclear power applications. Therefore, the effects of the composition and cooling rate of 316 stainless steel castings on the ferrite content are investigated. Three 316 stainless steel continuous casting samples with different compositions (primarily differing in the Ni content) are studied, i.e., low-alloy type (L-316), medium-alloy type (M-316), and high-alloy type (H-316). The austenite-forming element nickel of three different industrial samples is 10%, 12%, and 14%, respectively. The effect of the cooling rate on the ferrite content and precipitation phases of the high Ni content of the 316 stainless steel casting (H-316) is studied by remelting experiments and different methods of quenching of liquid steel. In both cases, the ferrite content and the precipitate phases in the microstructure are analyzed using SEM and EBSD. The results indicate that compositional changes within the 316 stainless steel range lead to changes in the solidification mode. In the L-316 casting, solidified by the FA mode (ferrite–austenite mode), ferrite precipitates first from the liquid phase, followed by the formation of austenite, and the ferrite content is 11.2%. In contrast, the ferrite content in the M-316 and H-316 castings, solidified by the AF mode (austenite–ferrite mode), is 2.88% and 2.45%, respectively. The effect of the solidification mode on the ferrite content is more obvious than that of the composition. The microstructure of the L-316 casting is mainly composed of the austenitic phase and the ferritic phase. The microstructure of the M-316 casting is composed of austenite, ferrite, and a small amount of sigma phase, with a small amount of ferrite transformed into the sigma phase. The microstructure of the H-316 casting is basically composed of austenite and the sigma phase, with the ferrite has been completely transformed into sigma phase. Changes in composition have a greater influence on the precipitate phases, while the solidification mode has a lesser impact. In the remelting experiments, the ferrite content in the H-316 ingot obtained through furnace cooling and air cooling is 1.49% and 1.94%, respectively, and the cooling rates are 0.1 °C/s and 3.5 °C/s, respectively. Under oil- and water-cooling conditions, with cooling rates of 11.5 °C/s and 25.1 °C/s, respectively, the ferrite content in the ingot is controlled to below 1%. The effect of the cooling rate on the precipitation phase of the H-316L ingot is that the amount of precipitated phase in the ingot decreases with an increase in cooling rate, but, when the cooling rate exceeds a certain value (air cooling 3.5 °C/s), the change in cooling rate has little effect on the amount of the precipitated phase.

This study investigates the distribution characteristics of residual ferrite in 304L austenitic stainless steel continuous casting slab and the impact of heat treatment processes on its content. Through optical microscopy (OM), thermodynamic calculation software (Thermo–Calc) and heat treatment experiments, it is found that the residual ferrite content along the thickness direction at the width center of the slab exhibits an “M”-shaped distribution—lowest at the edges (approximately 3%) and highest near the center (approximately 13%). Within the triangular zone of the slab, the residual ferrite content varies between 1.8% and 12.2%, with its average along the thickness direction also showing an “M”-shaped distribution; along the width direction, the average residual ferrite content is lower at the edge positions, while within the internal triangular zone, it ranges between 8% and 10%. The ferrite morphology changes significantly across solidification zones: elongated in the surface fine-grain zone, lath-like and skeletal in the columnar grain zone and network-like in the central equiaxed grain zone. Thermodynamic calculations indicate that the solidification mode of the 304L continuous casting slab follows the FA mode. Heat treatment experiments conducted across the entire slab thickness demonstrate effective reduction in residual ferrite content; the optimal reduction is achieved at 1250 °C with a 48 min hold followed by air cooling while preserving the original “M”-shaped distribution characteristic after treatment. Increasing the heat treatment temperature, prolonging the holding time and reducing the cooling rate all contribute to reducing residual ferrite content.