Nonmetallic Inclusions and Promising Principles for Improving the Set of Properties and Quality Characteristics of Steel

Abstract

Results of a detailed study of continuously-cast low-alloy pipe steel metal demonstrate a significant difference in micro- and macrostructural states, including the presence and extent of equiaxed and columnar crystal areas in relation to the later stages of ladle treatment. A marked difference is also recorded in impact strength indices, hydrogen corrosion cracking resistance, and other service properties obtained for different rolled billets. It is established that as a result of adding large amounts of silicon- and manganese-containing ferroalloys in the later stages of ladle treatment or in the process of steel continuous casting there is formation of liquid silicate inclusions. Due to a high silica content during steel crystallization these inclusions are converted into a supercooled liquid or glassy condition and acquire irregular shape, filling the space between growing crystals. As a result of this they effectively reduce the intensity of mass- and heat-transfer during steel crystallization, block development of liquation processes, and facilitate an increase in the degree of metal structural and chemical inhomogeneity. As a consequence there is an increase by a factor of 2–2.5 in impact strength to extremely high values (KCV–40 more than 400–420 J/cm2), and rolled product hydrogen cracking resistance.