Developing a Technological Means of Obtaining Superior Properties and Quality for Quenched and Forged Structural Steels

Abstract

In order to establish the scientific principles of a technology for the production and ladle treatment of new steels that can be quenched at forming, steels which were made in factory and laboratory heats and had different carbon contents were analyzed to explore the key factors and processes that determine both their total content of nonmetallic inclusions (NIs) and their content of certain types of NIs – particularly the types that are harmful. It was found that the most important factors in producing steel which is clean based on its content of all types of inclusions (which has an NI rating no higher than 1.5 in the government standard GOST 1778) are the slag regime and the ratio of the weights of the lime and aluminum added to the steel-pouring ladle during tapping. The optimum ranges of values for these parameters are determined. The existing requirements on the slag-regime parameters are observed to ensure a low content of harmful corrosion-active nonmetallic inclusions (CANIs) in the steel. However, it is essential to maintain a low (at or below the lower limit of 0.02% specified for the grade of steel) aluminum content for the duration of the ladle treatment. Aluminum content can be increased during the final stages, if necessary. A study was made of the effect of the MgO content of the covering slag on the possibility and rate of formation of CANIs based on alumomagnesian spinel. it was found that the steel’s content of such inclusions rises sharply when the concentration of MgO in the covering slag is greater than 14.1%. An MgO content of up to 8% guarantees that the steel will be clean based on its content of all types of inclusions. The results obtained for the metal of two trial heats confirmed the adequacy of the key parameters that were identified and the ranges of values for those parameters which were determined to be optimum for obtaining a low CANI content and high resistance to local corrosion.