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Stahlschlussel Key To Steel 2007

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Stahlschlussel Key To Steel 2007

The production of new steel by recycling old steel requires up to 10 times less energy than the primary production of steel from iron ore [5]. As foreseen in the steel industry, electric arc furnace (EAF) steelmaking, either scrap-based or based on hydrogen-direct-reduced iron, will contribute substantially to the reduction of CO2 emissions [4]. However, there will still be a need to introduce carbon into the EAF process, either to carburize the steel or to create foaming slag to improve the energy efficiency of the steelmaking process. To develop a fully green steel using EAF, it will be necessary to use alternative carbon sources that are either renewable or circular (e.g., biomass, plastic, rubber wastes, etc.) [4].

Production starts with scrap remelting in EAF followed by secondary metallurgy [5]. For a wide variety of steel grades, intensive scrap-steel recycling in an EAF is already common practice [6,7,8,9,10,11,12]. Scrap is usually well sorted already by suppliers and typically has a low impurity content. On the other hand, when end-of-life (EOL) steel scrap is recycled, the different steel grades are generally separated according to main alloying elements (Cr, Ni, Mo). Some nonferrous and nonmetallic contaminants (copper wires, aluminium, plastic from the shredding of cars, concrete residuals from demolition of buildings, etc.) are generally mixed in small quantities with the steel because of the imperfect separation of different materials prior to melting. Some impurities are removed as slag when the mixture is melted or during subsequent refining steps, and some elements evaporate, but some metallic elements cannot be simply removed (copper, lead). Consequently, the exploitation of steel scrap can lead to an overall increase in the impurity concentrations in steels that cannot be removed by metallurgical processes. Many problems are related to excessive levels of impurities that are prone to precipitation, segregation, and/or the formation of various complex nonmetallic inclusions [5,6,7,8,9,10,11,12]. Accordingly, studies that address open issues in scrap recycling are of great importance.

Initially, the role of the titanium added to steel as ferrotitanium was mainly to reduce the grain size and to act as a deoxidizer [25]. Titanium dissolved in steel is characterized by a high affinity for oxygen. Titanium lowers the activity of the oxygen in iron [22]. Since surface defects can occur on final steel products, control of the formation of oxide inclusions in Ti-bearing stainless steels is necessary. In addition, the deoxidation products can potentially form deposits within a submerged entry nozzle and thus clog the nozzle [26].

In steels, titanium is also highly reactive with carbon, nitrogen, and sulphur [27]. Nitrogen is an element that causes a great deal of concern. Since titanium nitride forms in preference to titanium carbide, based on thermodynamic considerations, it is essential (i) to add sufficient titanium to chemically bind the nitrogen first and then the carbon or (ii) to reduce the nitrogen as much as possible using other steelmaking techniques [27].

Titanium alloying can be made in the form of metal scrap, sponge, or as a ferrotitanium alloy. Ferrotitanium addition for binding interstitial elements is usually performed after the steel is refined in a ladle furnace (LF) [26]. For the cored wire, FeTi alloying is made as deep and late into the ladle as possible. Prior to the alloying, the liquid steel should be thoroughly deoxidized to reduce the oxidation of, and thus maximize the recovery of, the titanium [27]. In the case of titanium-stabilized stainless steels, the LF refining process involves deoxidation by Al followed by the addition of the Ti-alloy [26,28,29].

The aim of this study was to evaluate the influence of impurities on nonmetallic inclusion formation and the quality of hot-rolled plates manufactured from titanium-stabilized, austenitic stainless


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