Metallurgical features of the steel production for the automobile industry in Magnitogorski iron and steel works ("MMK")

The development of IF-steel at MMK started in 2002. The manufacturing of this product is organized according with the following production scheme: BOF shop - hot rolling mill 2000 (shop Nº10) - cold rolling mill 2500 (shop Nº5) -2-stand reverse mill (shop Nº5) - continuous hot galvanizing line (shop Nº6). Fine cleaning of the ferrite matrix from interstitial atoms of carbon and nitrogen in production of highly formable cold-rolled steels provides grain growth, formation of the required structure and good plastic properties. Minimization of the content of these interstitial atoms of carbon and nitrogen elements and their stabilization through binding into carbides and nitrides determine the technical and economical characteristics. The general trend of the production process of this technological development is obtaining minimal values for the carbon and nitrogen content. The best results are obtained with a carbon content below 0.002% and a nitrogen content below 0.003%. One of the most complex problems in production of 006/IF is the provision of a minimum carbon content in the final product. With this aim, the modernization of the circulation installation for steel vacuum degassing was performed. The modernization of the vacuum degassing installation made it possible to supply oxygen through a stationary installed tuyere mounted in the hood of the vacuum chamber. As a result of this modernization, a mean carbon content of 0.0035% (min.: 0.002%, max.: 0.006%) can be obtained in steel after vacuum degassing, nowadays. Deep decarburization of the metal can be only realized through vacuum degassing. This process is limited by diffusion of reagents to the reaction zone and liberation of products into gas phase. Therefore the organization of well-developed mass-transfer processes and gas liberation surfaces is a compulsory condition of this technology. With this in mind the prerequisite conditions of this process are the permanent support of the oxidation potential by means of oxygen lancing, agitation of metal by argon, decreasing of surface-active substances, in particular sulfur and nitrogen. High solubility of nitrogen in steel at temperatures of steelmaking, especially in reaction zones is characterized by higher temperature, caused by exothermic processes of nitrogen removal. It complicates the process of nitrogen removal. Obtaining low nitrogen content in steel requires complex measures of limiting its penetration into the metal during this technological process. Metallurgists use clean raw materials and apply oxygen blast and protect liquid metal from contact with air. Some changes were made in the technology of melting providing the variation of blowing intensity, slag foaming, metal jet and liquid pool surface protection during tapping, secondary treatment and continuous casting. The method of “low” temperature reheating of slabs was used during IF rolling on the hot rolling mill 2000. The optimum coiling temperature and corresponding schedules of hot rolling were also used. All these measures allowed for obtaining high quality IF-steel strip of the dimensions 2.2-3.2 x 1170-1541 mm on the hot-rolling mill 2000. (AU)