HIGH-TEMPERATURE THERMAL ANALYSIS OF SPECIFIC STEEL GRADES

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1 HIGH-TEMPERATURE THERMAL ANALYSIS OF SPECIFIC STEEL GRADES Karel GRYC a, Bedřich SMETANA b, Monika ŽALUDOVÁ b, Petr KLUS a, Karel MICHALEK a, Markéta TKADLEČKOVÁ a, Jana DOBROVSKÁ b, Ladislav SOCHA a, Bohuslav CHMIEL c a VŠB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Department of Metallurgy and Foundry, and Regional Materials Science and Technology Centre, Czech Republic, karel.gryc@vsb.cz, b VŠB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Department of Physical Chemistry and Theory of Technological Processes, and Regional Materials Science and Technology Centre, Czech Republic, bedrich.smetana@vsb.cz c TŘINECKÉ ŽELEZÁRNY, a.s., Průmyslová 1000, Třinec-Staré Město. Abstract This paper deals with determining the temperatures of phase transformations in real steel grades. The study of phase transformations in the high-temperature area was realized by dynamic methods of thermal analysis in the newly formed Laboratory for Modelling of Processes in the Liquid and Solid Phases. The scientific team consists of following members: Department of Metallurgy and Foundry and Department of Physical Chemistry and Theory of Technological Processes. Experiments were carried out using direct methods of thermal analysis (large samples approx. 23 g) and differential thermal analysis - DTA (small sample approx. 200 mg) for different rates of heating and/or cooling. The paper mainly discusses the possibility of determining the solidus and liquidus temperatures under precisely defined conditions of heating and/or cooling. A series of experimental measurements were carried out to obtain temperature "equilibrium" (near the equilibrium temperature) as well as experiments with settings that are close to real operating conditions. The discussion is focused on comparison of characteristic temperatures of phase transformations obtained using different methods of thermal analysis with clearly defined and controlled experiments (experimental conditions). The basic mathematical and statistical analysis of the data was also carried out. This paper was created in the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. Keywords: steel, thermal analysis, phase transformation, solidus, liquidus, heating, cooling 1. INTRODUCTION Modern steel production technology needs to keep better control of the entire production cycle from selection of quality raw materials, through proper control of primary and secondary metallurgy processes, and finally, the optimum settings of casting and solidification conditions. In the refining processes, optimizing the slag regimes [1, 2], thermal and chemical homogenization of the melt [3] or filtration of steel [4, 5] is very important to solve. In the casting and solidification of steel studies, works toward optimizing the process of solidification of heavy forging ingots [6] are currently being implemented. The methods of study of metallurgical processes are also based on knowledge of thermodynamic properties of materials occurring in a given technology nodes. Knowledge of solidus and liquidus temperatures of the studied steels is one of the most important factors - especially in dealing with the processes involved in the casting and solidification. These temperatures are critical parameters for proper adjustment of models

2 20 mm (physical or numerical) or in the final stage of applied research of the real process. It is significantly affecting the final quality of the as-cast steel (billets or ingots). Therefore, this paper is devoted to discussion of findings obtained during the utilization of dynamic thermal analysis methods to identify the solidus and liquidus temperatures. 2. EXPERIMENTAL EQUIPMENT, USED METHODS As already mentioned in previous work [7], new Laboratory for Modelling of Processes in the Liquid and Solid Phases within the project RMSTC was formed at the Faculty of Metallurgy and Materials Engineering. This laboratory has also acquired new equipment for high-temperature thermal analysis - JUPITER (Fig. 1). The conditions for initiation of intensive research activities in the field of dynamic thermal analysis methods for steel are based on years of experience of team members with the issue of laboratory studies of metallurgical processes and the ability to use other equipment of this type - SETSYS 18 TM (Fig. 2) create. Fig. 1 Netzsch STA 449 F3 Jupiter Fig. 2 Setaram SETSYS 18 TM In less than a year after the installation of new equipment JUPITER, dozens of measurements to define and subsequently to use the practical potential of this device - especially given by the opportunity to study the thermodynamic properties of the material on the "big" steel samples (over 20 g) - have already been done. This paper discusses methods and results of thermal analysis of samples (Table 1) taken from two grades of tool steels (A and B), with an approximate carbon content of 0.6 / 0.5 wt. %, [Cr] = 5 or 8 wt. %, and up to 2 wt. % of other alloying elements (Ni, V). Table 1 Steel samples with dimensions specified for each analysis Sample for method: NETZSCH STA 449 F3 Jupiter SETARAM Setsys 18 TM Ø14 mm Dimensions:

3 Two methods for dynamic thermal analysis were used to measure the solidus (T S ) and liquidus (T L ) temperatures: Differential Thermal Analysis (DTA) - SETSYS 18 TM, Direct Thermal Analysis - JUPITER. The principles of both methods are described for example in [8]. Method of Differential Thermal Analysis (DTA) S type measuring rod for TG / DTA (thermocouple: Pt / PtRh 10%) was used to acquire temperatures of phase transformations at the SETSYS 18 TM equipment. Samples were analysed in alumina (Al 2 O 3 ) crucibles with a capacity of ml. Weight of analysed steel samples was approximately 200 mg. Measures based on comparison of steel sample with an empty crucible were done. Constant dynamic atmosphere - inert argon with purity ( %) - was maintained during measuring. Such high purity gas is accomplished by using a gas filtering device (SAES Getters system). Differential thermal analysis was used to determine the temperatures of solidus and liquidus temperatures close to equilibrium in the frame of studied steel grades. This method and equipment was proved for determination of equilibrium temperatures in the past. Small sample size limits the negative effects including processes related to the complex structure of the studied steels. The heating rate 10 and 15 C.min -1 in the critical temperature range was used to obtain the relevant temperature. Liquidus temperatures were then corrected according to generally accepted methods [9]. Example of the DTA curve obtained and analysed for the steel grade A is shown in Figure 3. Fig. 3 DTA curve for measured steel A ([C] = 0.6 wt. %, [Cr] = 5 wt.%), heating rate 10 C.min -1 Direct Thermal Analysis S type measuring rod for TG (thermocouple: Pt / PtRh 10%) was used for to obtain temperatures of phase transformations at the STA 449 F3 Jupiter equipment. Samples were analysed in the corundum (Al 2 O 3 ) crucible with a capacity of about 4 ml. Weight of analysed samples was about 23 g. Constant dynamic atmosphere - inert argon with purity ( %) - was maintained during measuring.

4 Direct thermal analysis was used to determine the solidus and liquidus temperatures for the conditions corresponding to the solidification process of studied real steel grades. This method was chosen intentionally, it allows studying the behaviour of complex structures contained in "large" samples of steel. Measurements were carried out for the cooling rate 1, 2 and 6.5 C.min -1. Example of measured and subsequently analysed curve obtained from direct thermal analysis measurement for steel B is shown in Figure 4. Fig. 4 Curve from direct thermal analysis for steel B ([C] = 0.5 wt. %, [Cr] = 8 wt.%), cooling rate 6.5 C.min DISCUSION OF THERMAL ANALYSIS RESULTS Figure 5 summarizes the obtained solidus and liquidus temperatures for both analysed steel grades. a) steel A b) steel B Fig. 5 Liquidus and solidus temperatures of large samples in dependence on cooling rate (lines are equilibrium temperatures corrected values obtained from heating of small samples)

5 Another phase transformation was identified for both types of steels and both methods in the temperature interval between solidus and liquidus temperatures (Fig. 3, 4). This phase transformation is not discussed in this paper. Figure 5 shows that the solidus and liquidus temperatures (horizontal lines) determined using the DTA (close to equilibrium values) define the temperature range for temperatures obtained for different cooling rate on large samples. Processes occurring at cooling rates 1, 2 and 6.5 C.min -1 in large samples significantly narrow two-phase region between the T L (liquidus temperature) and T S (solidus temperature). The temperature interval between the "equilibrium" T L ( C) and T S ( C) is C for steel A. Among its most significant narrowing occurs at cooling rate 2 C.min -1 (59.4 C). Non-equilibrium solidus and liquidus temperatures (due to the complex mechanism of steel solidification depending on cooling rate) obtained using direct method of thermal analysis were then determined for each cooling rate as follows: For cooling rate 1 C.min -1 : T S = C, T L = C; For cooling rate 2 C.min -1 : T S = C, T L = C; For cooling rate 6.5 C.min -1 : T S = C, T L = C. The temperature interval between the equilibrium T L ( C) and T S (1338 C) is C for steel B. Among its most significant narrowing occurs at cooling rate 1 C.min -1 (60.8 C). Non-equilibrium solidus and liquidus temperatures obtained using direct method of thermal analysis were then determined for each cooling rate as follows: For cooling rate 1 C.min -1 : T S = C, T L = C; For cooling rate 2 C.min -1 : T S = C, T L = C; For cooling rate 6.5 C.min -1 : T S = C, T L = C. 4. CONCLUSION A number of methodological experiments and measurements under various operating conditions in the frame of applied research have already been done in the Laboratory for Modelling of Processes in the Liquid and Solid Phases (RMSTC) for a period of one year after installing of new equipment for high-temperature thermal analysis (JUPITER). This paper was aimed to determine "equilibrium" solidus and liquidus temperatures and such temperatures under conditions of large samples cooling. Two different dynamic thermal analysis methods were used (direct and differential). Lower temperatures T L and higher temperatures T S measured at cooling rates 1, 2 and 6.5 C.min -1 in large samples (compared to results for "equilibrium" conditions - small samples) are probably caused by complex factors affecting the of the process of solidification of multi-componential system - steel. The shift to lower temperature values (T L ) could also be related to the degree of super-cooling of the melt, with the formation of first critical germs of solid phase, and also with the kinetics of phase transformations. The influence is most likely attributable to the additional other phenomena taking place during cooling, such as formation of microstructure forming outside or inside the metal matrix. The aim of the work is to find temperatures that are relevant to real steel systems and may differ from the one value that is characteristic for equilibrium states. It is evident that solidification process is affected not only by chemical composition but also by structure and metallographic cleanliness of steel. Deeper understanding of such effects will require more detailed study involving also other sophisticated analytical methods.

6 AKNOWLEDGEMENT This paper was created in the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. REFERENCES [1] MICHALEK, K., ČAMEK, L., GRYC, K., TKADLEČKOVÁ, M., HUCZALA, T., TROSZOK V. Desulphurization of the High-Alloyed and Middle-Alloy Steel under the Conditions of an EAF by Means of Synthetic Slag Based on CaO-Al2O3. MATERIALI IN TEHNOLOGIJE, 2012, vol. 46 no.3, Slovenia. pp ISSN [2] SOCHA, L.; BAŽAN, J.; GRYC, K.; STYRNAL, P., PILKA, V.; PIEGZA, Z. Assessment of Briquetting Fluxing Agent Influence on Refining Effects of Slag during Steel Processing at the Secondary Metallurgy Unit. In 20th Anniversary International Conference on Metallurgy and Materials: METAL 2011, p ISBN [3] MICHALEK, K., GRYC, K., MORAVKA, J. Physical Modelling of bath Homogenization in Argon Stirred Ladle. METALURGIJA. Vol.48, issue:4, p: , Oct-DEC 2009.ISSN: [4] JANISZEWSKI, K. Influence of Slenderness Ratios of a Multi-Hole Ceramic Filters at the Effectiveness of Process of Filtration of Non-Metallic Inclusions from Liquid Steel, Archives of Metallurgy and Materials. Vol.57, issue 1/2012. DOI: /v p [5] JANISZEWSKI, K. Liquid Steel Filtration in the Process of Steel Casting in the CC Machine, In 20th Anniversary International Conference on Metallurgy and Materials: METAL METAL 2011, p ISBN [6] TKADLEČKOVÁ, M., MACHOVČÁK, P., GRYC, K., KLUS, P., MICHALEK, K., SOCHA, L., KOVÁČ, M. Setting of Numerical Simulation of Filling and Solidification of Heavy Steel Ingot Based on Real Casting Conditions. MATERIALI IN TEHNOLOGIJE, 2012, vol. 46 no.3, Slovenia. pp.7-10 ISSN [7] GRYC, K., SMETANA, B., MICHALEK, K., SIKORA, V., TKADLEČKOVÁ, M., ZLÁ, S., ŽALUDOVÁ, M., DOBROVSKÁ, J. Connection of Basic and Applied Research in the Field of Thermo-Physical Study of the Properties of Steels, Slag and Ferro-Alloys In 20th Anniversary International Conference on Metallurgy and Materials METAL 2011, p ISBN [8] GALLAGHER, P.K. Handbook of Thermal Analysis and Calorimetry: Principles and Practice. 2nd ed. Elsevier, p. [9] SMETANA, B., ZLÁ, S., DOBROVSKÁ, J., KOZELSKÝ, P. Phase Transformation Temperatures of Pure Iron and Low Alloyed Steels in the Low Temperature Region Using DTA. International Journal of Materials Research, 2010, vol. 101, no. 3, p ISSN