Metalurgie vysoce čistých ocelí

Abstract

The dissertation thesis describes new possibilities in the steelmaking and forgings production of meeting present requirements imposed on the mechanical properties of the dynamically stressed parts of the power equipment. The main field, which is dealt with herein, includes steel making technology using the basic primary as well as secondary metallurgy equipment, and the process of casting of steel ingots with respect to the resultant usable properties and with a view to the link-up between the production technology and the achieved quality level defined by chemical heterogeneity and steel purity. The verification of the possibilities to achieve low concentrations of undesirable elements in the steel has pointed out problems concerning the impurification of raw materials for the preparation of a metallic charge in the production of steel in the electric arc furnace. The utilization of primary iron resources in the preparation of a metallic charge provides good prerequisites for achievement of the required chemical purity of the basic melt. Depending on the proportion of selected raw materials in a charge it is possible to predict the chemical composition of liquid metal and slag. It is possible, with high operating certainty, to ensure the required chemical composition of liquid metal for subsequent processing in the secondary metallurgy equipment, provided that the chemical composition of liquid metal in the EAF is respected and the slag regime in the phase of oxidation is controlled. In light of the improvement of metallographical purity or the reduction of a proportion of non-metallic inclusions in the steel, the main contribution is the final phase of finishing a heat after vacuum steel treatment. The time and method of inert gas blowing influence, in an obvious and provable manner, the concentration of oxygen as well as the amount of non-metallic inclusions in the steel. The casting of ingots is the process during which, under certain conditions, a considerable impairment of steel purity occurs due to an increased proportion of exogenous non-metallic inclusions. So the protection of the casting strand against secondary oxidation by oxygen contained in the atmosphere is a significant technological element in the production of steels of high purity. Numerical simulations have proved that the inside diameter of casting runners is the significant factor with influence on the intensity of the flow of liquid metal in a mould. Checking the flow rate of liquid metal in the mould enables to predict the risk of the impurification of cast steel due to casting powders.