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Characterization of Impurities in Different Ferroalloys and Their Effects on the Inclusion Characteristics of Steels

Time: Thu 2021-06-03 10.00

Location: Publikt via Zoom, KTH, (English)

Subject area: Materials Science and Engineering

Doctoral student: Yong Wang , Processer, School of Industrial Engineering and Management (ITM)

Opponent: Professor Bart Blanpain, Dept. of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven

Supervisor: Professor Pär Jönsson, Materialvetenskap; Universitets lektor Andrey Karasev, Materialvetenskap

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     Ferroalloys have become increasingly important due to their indispensable role in steelmaking. As the performance requirements of steel materials increase, it is necessary to have a better understanding of the impact of impurities in ferroalloys on the steel cleanliness. The quality of the ferroalloy will directly affect the quality of the steel. This is especially important when ferroalloys are added during the late stage of the ladle metallurgy process. The goal of the present work is to gain knowledge about various ferroalloy impurities added in the steel production process and to study the influence of ferroalloy impurities on inclusions in the steel. The research work is divided into four main parts.

In the first part, previous works on impurities present in different ferroalloys as well as how these impurities can influence the steel cleanliness have been reviewed. The applications of different ferroalloys and their production trends were discussed. The possible harmful inclusions in different ferroalloys were identified. The results showed that: 1) MnO, MnSand MnO-SiO2-MnS inclusions from FeMn and SiMn alloys have a temporary influence on the steel quality; 2) The effect of trace elements, such as Al, Ca contents, should be considered before the addition of FeSi alloys to steel. Also, Al2O3 inclusions and relatively high Al contents are commonly found in FeTi, FeNb and FeV alloys due to their production process. This information should be paid more attention to when these ferroalloys are added to steel; and 3) specific alloys containing REM oxides, Cr(C,N), Cr-Mn-O, Al2O3, Al-Ti-O, and Ti(C,N) have not been studied enough to enable a judgement on their influence on the steel cleanliness. Moreover, the effect of large size SiO2 inclusions in FeSi and FeMoalloys on the steel cleanliness is not fully understood.

In the second part, the impurity assessment of 10 different ferroalloys (FeSi, FeCr, FeMo,FeV, FeTi, FeNb, FeW, FeB, MnN, FeCrN) was carried out by using various characterization techniques. The inclusions obtained in these ferroalloys were mostly silica or alumina, or the oxides of the base elements. Also, the main elemental impurities and inclusions were closely related to their manufacturing route. The advantages and disadvantages of different methods were compared, and the detection technology of ferroalloy inclusions was optimized. The results showed that the traditional two-dimensional method on a polished surface cannot always be applied for the investigation of inclusions in some specific ferroalloys. Moreover, the investigations of inclusions on metal surface after electrolytic extraction showed a big potential to use to detect larger sized inclusions. The results on both the film filter and metal surface should be grouped together to obtain more comprehensive information on the inclusion characteristics. Among these ferroalloys, FeCr and FeNb were found to be relatively less studied ferroalloys. Thus, they were selected for further studies.

In the third part, the early melting behaviours of FeNb, HCFeCr and LCFeCr alloys during additions in liquid iron was studied. The experiments were carried out by using the"liquid metal suction" technique. Here, the ferroalloy was contacted with liquid iron for a predetermined time and then quenched. The obtained samples were further studied to determine the microstructure and the formation of inclusions. It was found that the mutual diffusion between solid ferroalloy and liquid iron formed a reaction zone. Also, the initial dissolution mechanism of FeNbs alloy in liquid iron was proposed, and the mechanism was controlled by diffusion. The TiOx inclusions in FeNb alloy will partially or completely be reduced due to a reaction with Nb in the reaction zone. The original stable inclusions, such as Al2O3 in FeNb alloys and MnCr2O4 inclusions in LCFeCr alloys can move in this zone and keep their original forms without experiencing any changes. Under the same conditions, the melting speed of LCFeCr alloy is faster than that of HCFeCr alloy. The addition of FeNband FeCr alloys in steel certainly introduces inclusions to steel.

In the fourth part, the influence of the addition of LCFeCr alloys on the inclusions in Ti containing ferritic stainless steel was studied on a laboratory scale. It was found that the theMnCr2O4 inclusions in the LCFeCr alloy would react with TiN and dissolved Ti in the Ti containing steel to form TiOx-Cr2O3 system inclusions. In addition, the removal effect of slag on such inclusions was also studied. The results found that the slag addition can modify their-rich inclusions, but that the Ti content in the steel was significantly reduced. Therefore, a proper amount of TiO2 content should be added into the slag to get a low Ti loss in the steel melt, which should be studied further. Therefore, the composition of the steel directly affects the behaviour of the inclusions from ferroalloys in steel.