Refining method for 400-series free-cutting stainless steel

The invention discloses a refining method for 400-series free-cutting stainless steel. The refining method comprises an AOD (Argon Oxygen Decarburization) refining process and an LF (Ladle Furnace) refining process; the technique of the method comprises the following steps: (1) controlling the final slag alkalinity of the AOD refining to be 2.1 to 2.3 and controlling the MgO content in final slag to be 8% to 10%; (2) during the LF refining process, observing viscous slag after heating in the furnace, adding steel which contains 0 to 4kg/ton of lime or steel which contains 0 to 2kg/ton of fluorite to regulate the viscosity of the furnace slag; adding steel which contains 0 to 0.6kg/ton of aluminum particles or ferrosilicon powder to perform deoxidized scorification on the furnace slag until white slag is produced; subsequently, regulating other alloy components excluding sulfur according to needs; then adding steel which contains 0 to 4kg/ton of quartz until the alkalinity of the furnace slag is 1.4 to 1.6; fi