Vlačna ispitivanja nehrđajućih čelika u temperaturnom intervalu 800 - 1200°C

On different austenitic stainless steels tensile tests were carried out in the hot working temperature range for 800 to 1200ºC. The effects of δ ferrite on recrystallisation, of σ phase on deformability and the effect of temperature on recovery, recrystallisation and grain growth were established.Provedena su ispitivanja čvrstoće različitih austenitnih nehrđajućih čelika na povišenoj temperaturi 800-1200°C. Utvrđeni su utjecaji δ ferita na rekristalizaciju, σ faze na deformabilnost te temperature na oporavljanje, rekristalizaciju i porast zrna

Change of mould flux properties during continuous casting of ferritic stainless steel

During the continuous casting of stainless steel the performance of the mould powder is a key factor in this process. The content of Cr in the steel is up to 18 %, high corrosion resistance of the steel is improved with Al addition, up to 4 %. The aim of the paper was to study the change in physical properties of the casting powder, due to reactions between Al in the steel with SiO2 in the mould flux during continuous casting. In order to identify the change in the ability of lubrication casting slag with time, depending on the increase in the content of alumina in the slag. On the basis of these results, the most suitable casting powder for continuous casting of ferritic stainless steel with high aluminium content was determined

Reduction of chromium oxides with calcium carbide during thestainless steelmaking process

An efficient reduction of chromium from slag requires an appropriate reduction agent for the given steelmaking technology. The usual slag reduction praxis consists of carbon injections and additions of ferrosilicon and aluminum.Reduction of chromium containing slags with calcium carbide is an appealing alternative. Calcium carbide is a strong reduction agent that unlike ferrosilicon and aluminum also provides the possibility of foaming slag formation.Experimental work regarding chromium slag reduction with calcium carbide towards usual slag reduction praxis is described in this work. The results show that higher reduction rates in the stage of refining period of the melt and higher level of overall chromium reduction from slag can be reached with the blowing of CaC2

Microstructural characterisation of chromium slags

In this chromium slags that form during melting of chromium alloyed steels are examined. During melting and oxidation of these steel grades a considerable amount of chromium is lost, and gained back with slag reduction. Laboratory experiments were performed to study the mechanism of chromium oxide reduction by silicon. Slags chemistry and phase composition have a strong effect on the steelmaking process. Phase analysis revealed two types of chromium oxides, calcium chromites and chromite spinels dependent on chemistry and basicity of the slag

Submerged entry nozzle clogging during continuous casting of Al-killed steel

Nozzle clogging is a common problem in the production of continuously cast Al-killed steels. Clogging occurs when there are solid inclusions in molten steel at casting temperatures. SENs (Submerged entry nozzles) from continuous casting of Al-killed low alloy steel grades with increased content of sulfur (0,020 to 0,035 % S) were examined. The examinations revealed that the deposits are mainly alumina based, with spinel and sulfur inclusions and some entrapped steel melt. It was concluded that the process of clogging begins when the steel melt infiltrates the refractory and removes the protective zirconia surface, thus allowing the adhesion of fine solid aluminates, which form the deposits

Calculation of heat transfer coefficients

In forced-convection furnaces for reheating Al-alloys, convective heat transfer mechanism dominates. Al-body temperature prediction model uses measured furnace temperature as boundary condition. To calibrate such model, a convective heat transfer coefficient h is to be determined. Optimization technique is used here to determine h for every measured temperature sample so that measured temperatures match calculated, supposing radiative heat transfer coefficient constant and neglecting conductive heat transfer. Obtained h stably converges during normal reheating conditions. The obtained model is 4-fold cross-validated and obtained Root Mean Square Error of whole reheating profiles are [7,7; 20,4; 10,4; 12,5] °C

Oksidacija i redukcija kroma kod nehrđajućeg čelika u elektrolučnoj peći

The oxidation of chromium during the elaboration of stainless steels occurs with oxygen in solution blown inthe melt and with oxides in the slag. A higher content of silicon in the furnace charge decreases the extent of oxidation of chromium, however, the efficient reduction of chromium from the slag is of essential importance for a minimal loss of chromium. In this survey, the theory of the oxidation of chromium, its reduction from the slag and the conditions for the formation of foaming slag are discussedPrilikom izrade nehrđajućeg čelika u elektrolučnoj peći dođe do oksidacije kroma zbog upuhavanja kisika i zbog oksida u troscii. Viši sadržaj silicija u ulošku smanjuje oksidaciju kroma. Za smanjenje gubitaka kroma potrebno je smanjiti i količinu kroma u troski. U ovom radu se predstavlja teorija oksidacije kroma, njegova redukcija iz troske i uvjeti za formiranje pjenjene troske

Utjecaj elektromagnetskog miješanja na makro i mikrostrukturu kontinuirano lijevanih gredica

Die Anwendung des elektromagnetischen Rührens (EMR) beim Stranggiessen trägt zur besseren Qualität von Knüppeln bei. Einfluss von EMR auf das Erstarrungsgefüge ist beim Stranggiessen von Knüppeln 180 × 180 mm auf der Stranggiessanlage im Stahlwerk von Štore Steel untersucht worden. Durch die Anwendung von EMR wird eine Verbesserung der Oberflächengüte von Knüppeln und des Gefüges unter der Oberfläche und des inneren Gefüges erziehlt und zwar durch die Vergrösserung des mittleren globulitischen Bereiches der gegossenen Knüppel und Vorblöcke. Im Beitrag wird der Einfluss vom elektromagnetischen Rühren auf das Makro und Mikrogefüge von gegossenen Knüppeln vorgestellt.Uporaba elektromagnetskog miješanja (EMR) pri kontinuiranom lijevanju dovodi do poboljšanja kvaliteta lijevanih gredica. Istraživan je utjecaj EMR na strukturu skrućivanja kontinuirano lijevanih gredica 180 × 180 mm u čeličani Štore. Uporabom elektromagnetskog miješanja poboljšan je površinski i podpovršinski kvalitet gredica, unutarnja i vanjska makrostruktura kao i povećanje središnjih globularnih područja izlivenih gredica i poluproizvoda. U radu je prikazan utjecaj elektromagnetskog miješanja na makro i mikrostrukturu izljevenih gredica

Precipitation of metallic chromium during rapid cooling of Cr2O3 slags

The slag systems of CaO-SiO2- Cr2O3 and Al2O3-CaO-MgO-SiO2- Cr2O3 were analyzed. These slag systems occur in the production of stainless steel and are important from the process metallurgy point of view. Synthetic slag samples with different chromium oxide content were prepared and melted. The melted slag samples where then rapidly cooled on large steel plates, so that the high temperature microstructure was preserved. The samples were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The precipitation of different chromium oxide phases was studied, but most importantly the precipitation of metallic chromium was observed. These findings help us interpret industrial slag samples