STUDY ABOUT DOWN GRADING VARIABLES BY INCLUSIONARY CLEANLINESS IN THE LADDLE FURNACE AT TERNIUM SIDERAR Alejandro Martín (Instituto Argentino de Siderurgia, Argentina) Elena Brandaleze (Instituto Argentino de Siderurgia, Argentina) Jorge Madías (Instituto Argentino de Siderurgia, Argentina) Roberto Donayo (Ternium Siderar, Argentina) Adolfo Gómez (Ternium Siderar, Argentina) Jorgelina Pérez (Ternium Siderar, Argentina) ABSTRACT In order to reassess the criteria about downgrading by microinclusionary cleanliness, already established in the plant operation practice, some microinclusion counting was carried out in steel samples taken at the final stage of the trimming station process, before being sent to the continuous casting machine. The assessment of metallic and non-metallic inclusions was carried out on a scanning electronic microscope by means of a program that lets us analyze the microinclusions and make the automatic counting at different stages. Also, it was possible to study the stirring time effect on inclusion floating, the addition of aluminium to steel and slag, and phosphorus reversion over internal cleanliness of the steel. It was determined how the microinclusion quantity evolves according to each variable and the results were compared with the limits considered at that moment. The resulting information permitted to understand the incidence of the analyzed variables over the internal cleanliness of the steel and it made possible to modify and optimize the limits of downgrading in the operation practice. 1. INTRODUCTION Ternium Siderar produces 2,8 Mt of steel annually, the route of the process is blast furnace-oxigen steelmaking and continuous casting of slabs; it is the main steel making producer in Argentina and the only one making flat products. The Argentine Iron and Steel Institute- IAS- is a non-profit entity that comprises the main siderurgical companies in Argentina, and it aims to carry out technological developments, as well as to provide technical assistance, analysis and testing to those companies.
Many different joint works have been performed which were directed at improving the steel cleanliness, in macro and microinclusions as well [1, 2]. After implementing the laddle furnace, Siderar adopted the calcium injected practice for every steel grades all heats, in order to improve the sequencing and conditions of steel flow in the mould, that influence on the superficial quality of the slabs [3]. There were new criteria adopted regarding downgrading for lack of microinclusionary cleanliness, in accordance with routine measures taken through the process in the laddle furnace. For this purpose, the steel is divided in five groups (table 1). Group Quality - Grade 1 API 2 High demand 3 Commercial and structural 4 Automobile 5 Automobile and critical parts Table 1. Identification of groups and steel grades defining the variables of material acceptance or rejection. In each group, several variables affecting the internal cleanliness of the steel are considered. Three of the variables involved are soft stirring time, the addition of aluminium in the laddle furnace and phosphorus reversion in the laddle. There exist certain application limits that define whether each casting complies with the required conditions or must be downgraded. The limits were fixed basing on data provided by other plants, bibliographic recommendations and own experience. The aim of the present work was to assess the existing relation between the limits considered so far and steel cleanliness in order to determine whether these were accurate or if they are overdemanding; and also, in accordance with the results to analyze the possibility of modifying process times or additions in the practices that were carried out. 2. METHODOLOGY There is a following-up stage through which samples are taken during the process in 500 heats of low carbon steel, including the 5 steel groups mentioned. Figure 1 shows the routine sampling sequence in the plant; in the present research the samples analyzed were the T11, taken at the end of the treatment in the trimming station, which were obtained through samplers without deoxidizer.
Converter LF Trimming CC P1 S0 L11 L12 T11 F11 F12 Process layout Sample Figure 1. Sequence of steel sampling during the process and nomenclature of samples. 2.1 Technique of inclusion counting. The inclusion evaluation is carried out on a scanning electronic microscope (SEM) by means of a program that permits the automatic counting of different phrases and their analysis EDS. Out of the 500 heats in which samples were taken, there was an inclusion counting in 0 of them; considering the number of samples and the need to obtain a quick assessment, it was decided to select a 5 mm 2 area using the technique described as follows. To start with, there is a visual observation of the samples and selection of the dirtiest area (with a bigger number of inclusions) and a picture of this using the backscattered electron detection system, this is followed by an automatic testing of inclusions by examining the selected area. The result is a text file which has information about the studied fields. Apart from the inclusions quantity, the area is measured as well as the perimeter, and diameter in each detected inclusion. In this case only inclusions bigger than 2 um were considered. 3. DEVELOPMENT There is further separate discussion about the general characteristics of the microinclusions that were found, and the effect of each of the studied variables: final soft stirring time, addition of aluminium and the phosphorus reversion. 3.1 Characteristics of microinclusions. Even though the main aim of the present work was not to study characteristics, the analysis resulting from this, allows us to infer, according to figure 2, that 90% of the microinclusions found were less than 6um and were mainly globular calcium aluminates, some of which presenting spinels crystals precipitated and calcium sulphides on the surface (Figure 3).
40 35 35 % 30 25 25 15 5 0 5,0 4,0 1,0 2-3 1 3-4 2 4-5 3 5-6 4 6-7 5 7-8 6 8-30 7 µm Micrones Figure 2. Typical distribution of microinclusion sizes. Mg Spinels SCa Ca Aluminate 5 µm Al Ca Mg S Figure 3. Typical microinclusion of calcium aluminate with spinels crystals and calcium sulphide on the surface.
3.2 Effects of the final soft stirring time. During the refining process, there generally exist two types of stirring through argon bubbling. A strong stirring to favour desulphurization, macroinclusions floating, as well as to achieve thermic homogeneity; and also there is a soft stirring. In the final stage of the soft stirring occur an agglomeration and inclusion floating, and a morphologic modification of those which have not floated through calcium injection [4]. In the conditions operating in Siderar at the moment of this sampling, one part of the final soft stirring was made in the laddle furnace and another part, including calcium addition, was made in the trimming station, before being sent to continuous casting. The effect of the minutes of final soft stirring that were added, were analyzed on the quantity of inclusions present, the rest of the studied variables were settled in narrow ranks. There was a selection of heats which showed a phosphorus reversion variation between 0 and 30 ppm and an aluminum addition between 40 and 70 kg (figure 4). It is clearly observed that the first minutes of the soft stirring process have a high incidence on the steel cleanliness. For longer periods (according to the oval) it can be inferred that the quantity of microinclusions per mm 2 remains stable, lower than. This means that a longer stirring time does not probably imply improving the microinclusionary cleanliness. 70 60 specifications groups 1, 2, 3 y 4 specification group 5 Microinclusions/mm2 50 40 30 0 0 5 15 25 30 35 40 Final soft stirring time (min) Figure 4. Evolution of the quantity of inclusions for mm 2 considering the final soft stirring minutes in the laddle furnace and trimming station.
3.3 Effect of aluminium addition. At Ternium Siderar they generally perform two types of aluminum addition in the laddle furnace: shot into the slag to complete deoxidation and wire into the liquid steel, in case it is necessary. Owing to the fact that microinclusions are mainly produced by deoxidation, it is interesting to assess the influence of those additions [5]. In the same way, in order to establish the rest of variables, there was a selection of heats presenting a phosphorus reversion from 0 to 30 ppm, stirring time from 15 to 25 minutes and the aluminium shot addition less than 70 kg. Figure 5 shows the counting results carried out in samples with different aluminium additions through wire into liquid steel. For the analyzed heats, it is observed that for additions up to 1 kg, the quantity of microinclusions per mm 2 remains lower than. For greater additions, the content of microinclusions increases, reaching a maximum of 30 per mm 2 for 140 kg additions. It is important to note that there were no more heats analyzed with high additions, because injections over 140 kg are rare. 50 45 specifications for groups 1, 2, 4 y 5 40 microinclusios/mm2 35 30 25 15 5 0 0 40 60 80 0 1 140 160 180 Al wire (kg) Figure 5. Quantity of inclusions versus aluminium wire additions into the liquid steel. In figure 6 it is possible to observe the additions of shots to deoxidize the slag, with the same stirring conditions and P reversion than the previous case, but with aluminium wire additions less than 60 kg.
Microinclusions/mm2 60 55 50 45 40 35 30 25 15 5 0 specifications groups 1, 2, 4 y 5 specification group 3 0 40 80 1 160 0 240 280 Al shot (kg) Figure 6. Evolution of cleanliness compared with Al shot addition to the slag. It can be noted that the addition of Al in shot to slag does not seem to influence on the steel cleanliness under 0 kg. From that point the quantity of microinclusions per mm 2 in metal begins to increase. 3.4 Effect of phosphorus reversion. The converter slag has high contents of FeO and MnO; it is well-known that the high levels of those oxides produce a harmful effect in the ladle slag, in relation to steel cleanliness because increase the total oxygen of the steel [6, 8]. A simple way of assessing this effect is calculating the phosphorus reversion, which occurs when the slag with high percentages of FeO and MnO, is deoxidated during the laddle furnace process. In this way it is possible to avoid dependence on the slag analysis in each heat, as routine chemical analysis of liquid steel are enough to determine the P reversion. In this case, in order to establish the rest of variables, there was a selection of heats made with a final soft stirring time ranging from 15 to 25 minutes and total aluminum additions between 40 to 70 kg. It can be noted in Figure 7 that with 40-50 ppm reversions, the cleanliness level keeps down 15 microinclusions per mm 2. From that point, there is a slow increase until it reaches 65-70 ppm; this is when the quantity of inclusions found go up dramatically. It is important to see that it is really difficult to find heats with a P reversion superior to 60 ppm that also comply with the filtering conditions of the rest of the variables.
50 45 specifications for groups 1, 2, 3, 4 y 5 40 Microinclusions/mm2 35 30 25 15 5 0 0 40 60 80 0 Phosphorus reversion (ppm) Figure 7. Phosphorus reversion versus quantity of inclusions per mm 2. 4. ANALYSIS OF RESULTS For API, Commercial-structural and high demand grades, the steel must be down graded if inclusions floating time is less than 12 minutes of final soft stirring. Particularly, if the steel belongs to the automobile and critical parts grades, the minimum time is minutes. The results of the following-up show that the minimum stirring time must be minutes to achieve the expected cleanliness levels. For higher stirring times, there is no considerable improvement in the steel cleanliness; the microinclusions density keeps steady possibly due to the small size of the remaining microinclusions and that they do not have enough surface to be dragged to the slag. On the other hand, if the final soft stirring time is excessive, there could be some reoxidation risk, because of the steel being exposed to the atmosphere for the opening of the slag layer that protects it. Other authors got similar results, assessing the total oxygen content versus stirring time [9, ]. In previous works [7], at Ternium Siderar, some measurements of oxygen level on the bath were carried out into the roof of trimming station during the stirring period. The results obtained vary between 1% and 15% of oxygen in different conditions, giving way to the proper conditions for steel reoxidation in case there is an opening in the slag layer.
Hence, considering the difficulties in the accurate control of the stirring flow rate and the cleanliness of laddle lip that influence on the correct support of the roof letting in air or not, it is essential to optimize stirring time. If we compare the limits of the present downgrading (figure 3) for different steel grades, it could be noted that the minimum value of 12 minutes considered for API, commercial and structural and high demand grades, it seems to be optimum as the lower the times, the greater the change is in the quantity of microinclusions present. For automobile and critical parts grade, it can be seen that the limit could move to lower stirring times (-12 minutes) as the steel cleanliness would not be harmed. If the downgrading is assessed in relation to aluminum wire addition in steel, the present operations consider that the steel is downgraded when the addition is higher than 1 kg. It can be noted how the microinclusions density increases along with the quantity of added aluminum, obviously for alumina generation resulting from deoxidation. Despite this tendency, the values keep down microinclusions per mm 2 up to 1 kg addition (figure 5). For greater additions there is a strong increase in the quantity of particles found. For this reason the limit of the rule (1 kg) seems to be a suitable value according to the results obtained in the present work. In accordance with the aluminium shot addition, the current operation considers that steel is downgraded if this addition in slag is higher than 80 kg for commercial-structural grades and 70 kg for the other grades. When analyzing the quantity of microinclusions obtained (figure 6), it can be observed that the shot addition seemed not to alter the steel cleanliness up to values near 0 kg, and from that point the quantity of microinclusions increases. The limits considered are to be found in the area where the microinclusion density is less than per mm 2 this would lead us to infer that those addition limits could extend to 90 or 0 kg. As regards phosphorus reversion, the downgrading occurs if the P reversion is higher than 40 ppm. The tendency in the curve obtained (figure 7) shows that the cleanliness level keeps steady up to 40-50 ppm. It goes up between 50 and 65 ppm and for higher values, the microinclusions density increases dramatically resulting from the interaction of the bath with a slag with high oxygen values because of high contents of FeO and MnO. When analyzing the current limit of the standard and basing on the studied notes, it could be considered the possibility of moving it up to 60 ppm of P reversion. 5. CONCLUSIONS The present operations were revised led to the conclusion that the stirring times could be shortened without diminishing the steel cleanliness. Particularly, the minimum times range from minutes in commercial and structural steel grades, to 14 minutes in high demanding quality and 15 minutes in critical parts grades.
Secondly, it can be seen that it is possible to increase the limit value of aluminium shot addition to slag for all steel grades to 0 kg and the phosphorus reversion limit to 60 ppm for commercial and structural quality. Finally the applications of these new limits lead to a second stage in order to assess their impact on the downgrading levels in the five steel groups studied. REFERENCES [1] L. FERRO, J. PETRONI, D. DALMASO, J. MADIAS, C. CICUTTI, Steel cleanliness in continuous casting slabs, ISS 55 th Steelmaking conference proceedings, (1996), pp. 45-48. [2] R. PANELLI, J. MADÍAS, R. ARES, J. AZCUAGA, L. FERRO, J. PETRONI, Improvements in ladle metallurgy at Siderar, Clean Steel 5, Balatonfured, Hungary, June 1997, Vol. 1, pp. 175-185. [3] C. CICUTTI, M. VALDEZ, T. PÉREZ, Optimization of calcium treatment to improve the castability, 01 Steelmaking conference proceedings, pp. 871 882. [4] LARS HELLE, Secondary metallurgy for steelmaking, 1989. [5] S. KR. RAY, Steel cleanliness: concept, influence on product quality and improvement measures, Vol. 28, Nº 2, October 05. [6] E. WONG; S. RITZA, Strategy to achieve reliable, cost-effective slag conditioning and calcium treatments at Algoma Steel Inc, July 1999, I&SM, pp. 23 27. [7] W. CHIAPPAROLI, G. TRAGLIA, R. LOPEZ, G. TORGA, Caracterización de la atmósfera sobre el baño metálico en los LF de Siderar y Siderca, 14 th Steelmaking conference 03, San Nicolás Argentina, pp 684-692. [8] Inclusión modelling in the metallurgical industry Work shop of inclusion modeling organized by the casting and simulation group of Mefos, Junio 1999. [9] Y. SUSUKI, T. KUWABARA, Ironmaking and steelmaking 1978, pp 80-88. [] T. OHNISHI, Secondary steelmaking for product improvement, Conference proceedings, London, 1985.