A treatment solution is applied on a surface of a base iron; and the treatment solution is baked and dried. The treatment solution contains a first component: 100 parts by mass in solid content, and a second component composed of particles of one or more kinds selected from a group consisting of a polyolefin wax, an epoxy resin and an acrylic resin, the particles having an average particle size of 2.0 μm to 15.0 μm and a melting point of 60° C. to 140° C.: 5 parts by mass to 45 parts by mass in resin solid content. The first component contains a colloidal silica: 100 parts by mass, and an emulsion of one kind selected from a group consisting of an acrylic resin, an epoxy resin and a polyester resin which have an average particle size of 0.05 μm to 0.50 μm, or an emulsion of a mixture or copolymer of two or three kinds selected from the group: 40 parts by mass to 400 parts by mass in resin solid content.