The main key processes in the production of stainless steel include the following:
Steelmaking:
Raw material preparation: The main raw materials are ferrochromium, ferronickel, scrap steel, etc. These raw materials need to be proportioned in a certain ratio to ensure that the final product meets the required chemical composition and performance requirements. For example, when producing 304 stainless steel, the chromium content should be maintained at around 18% - 20%, and the nickel content at around 8% - 10.5%.
Smelting: The raw materials are added to an electric furnace or a converter for smelting. During the smelting process, by controlling parameters such as temperature and the composition of the slag, impurities are removed, and the chemical composition of the molten steel is adjusted. At the same time, gases such as oxygen and argon are blown into the molten steel for refining, which further reduces the content of harmful elements such as carbon, sulfur, and phosphorus in the steel, and improves the purity of the molten steel.
Continuous casting:
Mold vibration: The refined molten steel is poured into the mold of the continuous casting machine. The mold generates vibrations with a certain frequency and amplitude through the vibration device, so that the molten steel forms a uniform solidified shell during the solidification process, preventing cracks from occurring on the surface of the casting billet.
Cooling and solidification: The outside of the mold is equipped with a cooling system. The mold is cooled by circulating water, so that the molten steel quickly solidifies into a casting billet. As the casting billet moves downward, the secondary cooling zone continues to spray water on the casting billet for cooling, accelerating the solidification process and ensuring the quality and shape of the casting billet.
Hot rolling:
Heating: The casting billet obtained from continuous casting is heated to about 1100 - 1200°C to reach an appropriate rolling temperature. During the heating process, the heating rate and temperature uniformity need to be well controlled to avoid defects such as overheating and burning of the casting billet.
Rolling: The heated casting billet is sent to a hot rolling mill for rolling. The hot rolling mill usually consists of multiple stands of rolling mills. By gradually reducing the distance between the rolls, the casting billet is rolled multiple times, reducing its thickness gradually. At the same time, the internal structure of the steel is improved, and its mechanical properties are enhanced. After hot rolling, an oxide scale will form on the surface of the stainless steel strip, and subsequent treatments such as pickling are required.
Cold rolling:
Pickling: The stainless steel strip after hot rolling needs to be pickled first to remove the oxide scale on the surface. Pickling usually uses acid solutions such as sulfuric acid and hydrochloric acid. Through a chemical reaction, the oxide scale is dissolved, exposing the metallic luster of the stainless steel surface, preparing it for the subsequent cold rolling process.
Rolling: The pickled stainless steel strip is sent to a cold rolling mill for rolling. Cold rolling is a rolling process carried out at room temperature. Through cold rolling, the surface quality, dimensional accuracy, and mechanical properties of the stainless steel can be further improved. Cold rolling usually requires multiple passes of rolling, and the reduction amount of each pass is gradually reduced to reach the final product thickness requirement.
Annealing: During the cold rolling process, the stainless steel will undergo work hardening, making the material harder and more brittle. In order to restore the plasticity and toughness of the material, annealing treatment is required. Annealing is usually carried out in a protective atmosphere. The stainless steel strip is heated to a certain temperature (such as 800 - 1100°C), held for a certain period of time, and then slowly cooled, so that the structure of the material is restored and improved.
Surface treatment:
Polishing: The surface of the stainless steel is treated by methods such as mechanical polishing, chemical polishing, or electrolytic polishing to improve the surface finish, giving it a good appearance and corrosion resistance. For example, mechanical polishing uses a polishing wheel and polishing paste to polish the surface of the stainless steel, removing small defects and scratches on the surface to achieve a mirror effect.
Passivation: The stainless steel product is immersed in a solution containing passivation agents such as chromate and nitrate, so that a dense passivation film is formed on its surface. The passivation film can improve the corrosion resistance of the stainless steel and prevent the surface from rusting.
Coloring: Various colored coatings are formed on the surface of the stainless steel through methods such as electroless plating, electroplating, and spraying. This not only increases the aesthetic appeal of the stainless steel but also improves its corrosion resistance and wear resistance. For example, in electroless plating, a metal or alloy coating is deposited on the surface of the stainless steel through a chemical reaction. By controlling the composition of the plating solution and the process parameters, coatings of different colors can be obtained.