What Is Structural Steel and How Is It Made?
Structural steel – steel intended for the manufacture of machine parts and various building structures. Therefore, spring, valve steels, steels for turbine blades, bearings, etc. should be attributed to the group of structural steels. However, a significant difference in the composition and properties of these steels makes it expedient to single them out into special groups; structural steels usually include only heat treatable and carburized steels. The main characteristic of structural steels is their mechanical properties under static and dynamic loads. Sometimes they are subject to additional requirements, such as resistance to corrosion, wear resistance, etc. In some cases, simple carbon steel has the necessary properties, and in other cases, structural steel must be alloyed with various elements. Structural steels are divided into building (mainly low carbon) and machine-building (medium and low carbon) by purpose. Low-carbon machine-building parts with a carbon content of up to 0.20–0.25% are used for the manufacture of parts subjected to carburization.
Therefore, the structural steel of this group is called carburized steel. Medium-carbon engineering machine steels acquire a good combination of mechanical properties after quenching and high tempering. Such heat treatment is called a mantempering, so these steels constitute a group of heat treatable steels. The low carbon content in building steels is due to the fact that these steels must be well welded.
Production of structural steels. Chromium-nickel and chromium-nickel-molybdenum steel. Production of structural chromium-nickel and chromium-nickel-molybdenum steels is carried out on a fresh charge and remelting. During smelting with full oxidation, the charge consists of chrome-molybdenum, nickel-chromium-molybdenum, manganese-molybdenum and carbon steel wastes. In order to prevent significant losses of chromium with oxidizing slag and the formation of thick chromic slag, the batching is based on a melt yield of not more than 0.40% Cr.
This limits the possibility of using a large amount of alloyed waste, so nickel, molybdenum and chromium have to be additionally introduced into the metal. Production of structural steel of a number of grades of this group can be carried out without a recovery period. In the production of structural steels with a reducing period, ferrochrome is seated in a bath immediately after flushing the oxidative slag before the introduction of slag-forming agents. After that, a reducing lime slag is induced which is intensively treated from the very beginning with carbon and silicon.
Chrome-tungsten steel. The production of structural chrome-tungsten steels is carried out with both full oxidation and remelting. Remelting wastes with partial oxidation with gaseous oxygen prevents the accumulation of nitrogen in the metal. Structural steel is predominantly smelted with full oxidation. Charging from waste carbon steel, iron and nickel carried out so that the melting of the carbon content in the metal was not less than 0.60%. For the early formation of slag in the course of melting, 1.0-1.5% lime is placed in a bath. After the charge is completely melted, the slag is partially flushed and updated and iron ore is placed in a well heated bath. At the end of the oxidation period, the temperature of the metal should be somewhat higher than the outlet temperature. The carbon content in the metal should be 0.09-0.11%, phosphorus – not more than 0.010%.