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Štore, Slovenia

Kovacic M.,Store Steel | Jager R.,University of Nova Gorica
Materiali in Tehnologije | Year: 2015

Carbon steel C45 with an increased content of carbon is used for tempering in the automotive industry for highly stressed parts (axles, shafts), machine parts, screws, drills for wood, axes, knives, hammers and similar. In the present work an attempt of analyzing the influences of different steelmaking parameters is presented. On the basis of the monitored data about the casting-temperature changes, the total oxygen, the number of added aluminum rods, the chemical analyses before and after steelmaking, the added lime, the aluminum-cored wire, the calcium-silicon-cored wire, the sulphur-cored wire, the rolling dimensions, the casting speed, the opening of the ladle nozzle with oxygen and surface defects (scrap fraction) on rolled bars, a mathematical model was obtained with the help of the genetic programming method. The results show that the most influential parameters for the surface-defect occurrence on the C45 steel are the opening of the ladle nozzle with oxygen and aluminum. On the basis of the results, the steelmaking technology was changed. Instead of the aluminium-killed steelmaking technology the aluminium-calcium-free (ACF) steelmaking technology was used. The batches from an aluminium-calcium-free steelmaking period statistically have a significantly lower level of surface defects (scrap fraction). The scrap fraction was reduced from the average of 68.45 % to 1.92 % - by more than 35 times. Source

Sencic B.,Store Steel | Leskovsek V.,Slovenian Institute of Metals And Technology
Materiali in Tehnologije | Year: 2011

In the work the possibilities of the vacuum heat treatment of spring steel grade 51CrV4 are presented. Charpy-V notch (CVN) impact-test values are widely used in toughness specifications for spring steels, even though the fracturing energy is not directly related to the spring design. The plain-strain stress-intensity factor (K Ic) at the onset of unstable crack growth can be related to the spring design; however, K Ic values are not used in the toughness specifications. This is surprising since to the designer K Ic values are more useful than CVN values, because the design calculations for springs from high-strength steels should also take into account the strength and the toughness of materials to prevent rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture-toughness testing (ASTM E399-90), which is difficult to perform reliably for hard and low ductility materials, could be replaced with a non-standard testing method using circumferentially notched and fatigue-precracked tensile specimens. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell-C hardness -Fracture toughness K Ic - Tempering temperature) for the vacuum-heat-treated spring steel grade 51CrV4. Fractographic and metallographic analyses of the K Ic-test specimens used shows in steel the presence of positive and negative segregations. It was found that the width of the segregations bands and the distance between the positive and negative segregations influence significantly the fracture toughness due to the presence of bainite in the negative segregations. Source

Krajnc L.,Store Steel | Mrvar P.,University of Ljubljana | Medved J.,University of Ljubljana
Materiali in Tehnologije | Year: 2014

Non-metallic inclusion engineering is one of the most important aspects in steelmaking today. There are several thermodynamic and kinetic tools available that help to form non-metallic inclusions of the desired size, distribution and composition in steel. The goal of this work was to study the thermodynamic conditions responsible for the formation of multiphase inclusions in re-sulphurised steel grades that are produced for the automotive industry. Nozzle clogging on the continuous casting machine and surface defects on rolled rods are problems typically attributed to non-metallic inclusions. Samples of the 30MnVS6 steel grade were taken at five different stages of ladle treatment. The samples were prepared for metallography and images of inclusions were taken using a SEM. Thermodynamic calculations were made using the Thermo-calc software for each sample and for multiple inclusions in order to determine which phases are the most stable at a given stage of the ladle treatment. It was found that high-melting-point calcium sulphide (CaS) and spinel (MgO·Al2O3) inclusions and low-melting-point calcium aluminate (12CaO·7Al2O3) or calcium alumosilicate (2CaO·Al2O3·SiO2) inclusions are the most stable phases in current steelmaking practice. Source

Mramor K.,Cobik | Vertnik R.,Store Steel | Vertnik R.,University of Nova Gorica | Sarler B.,Cobik | And 2 more authors.
Computers, Materials and Continua | Year: 2013

This paper explores the application of Local Radial Basis Function Collocation Method (LRBFCM) [Šarler and Vertnik (2006)] for solution of Newtonian incompressible 2D fluid flow for a lid driven cavity problem [Ghia, Ghia, and Shin (1982)] in primitive variables. The involved velocity and pressure fields are represented on overlapping five-noded sub-domains through collocation by using Radial Basis Functions (RBF). The required first and second derivatives of the fields are calculated from the respective derivatives of the RBF's. The momentum equation is solved through explicit time stepping. The method is alternatively structured with multiquadrics and inverse multiquadrics RBF's. In addition, two different approaches are used for pressure velocity coupling (Fractional Step Method (FSM) [Chorin (1968)] and Artificial Compressibility Method (ACM) [Chorin (1967)] with Characteristic Based Split (CBS) [Zienkiewicz and Codina (1995); Zienkiewicz, Morgan, Sai, Codina and Vasquez (1995)]). The method is tested for several low and intermediate Reynolds numbers (100, 400, 1000 and 3200) and node arrangements (41x41, 81x81, 101x101, 129x129). The original contribution of the paper represents extension of the LRBFCM to Reynolds number beyond 400 and assessment of the method for two different types of RBFs and two different types of pressure-velocity couplings. The obtained numerical results, in terms of mid-plane velocities, are in a good agreement with the data calculated in several reference publications and by commercial code. Both RBF's used give approximately the same results. Both pressure-velocity coupling methods give approximately the same results, however the FSM turns out to be slightly more efficient. The advantages of the method are simplicity, accuracy and straightforward applicability in non-uniform node arrangements. Copyright © 2013 Tech Science Press. Source

Kovacic M.,Store Steel | Sencic S.,KOVA
Materiali in Tehnologije | Year: 2012

To implement sound air-quality policies, regulatory agencies require tools to evaluate the outcomes and costs associated with various emission-reduction strategies. However, the applicability of such tools can also remain uncertain. It is furthermore known that source-receptor models cannot be implemented through deterministic modeling. The article presents an attempt of PM10 emission modeling carried close to a steel production area with the genetic programming method. The daily PM10 concentrations, daily rolling mill and steel plant production, meteorological data (wind speed and direction - hourly average, air temperature - hourly average and rainfall - daily average), weekday and month number were used for modeling during a monitoring campaign of almost half a year (23. 6. 2010 to 12. 12. 2010). The genetic programming modeling results show good agreement with measured daily PM10 concentrations. In future we will carry out genetic programming based dispersion modeling according to the calculated wind field, air temperature, humidity and rainfall in a 3D Cartesian coordinate system. The prospects for arriving at a robust and faster alternative to the well-known Lagrangian and Gaussian dispersion models are optimistic. Source

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