Horizontal Ingot Manufacturing and Modelling Results Yuriy Kostetsky 1,2, Yevhenii Volchenkov 2, Lev Medovar 2,3 1
Donetsk National Technical University, Krasnoarmiysk, Ukraine 2 E.O. Paton Electric Welding Institute, NASU, Kyiv, Ukraine 3 PC Elmet-Roll, Kyiv, Ukraine e-mail:
[email protected]
Abstract: Horizontal ingots (LH-ingots), having the ratio of a height to width less than one and solidifying in the conditions of mainly unidirectional heat removal, characterizes by high quality due to absence of segregation and shrinkage defects in the axial area. The solidification peculiarities of such ingots forming on the cast iron uncooled bottom plate were studied by numerical simulation. The results of numerical experiments were verified by experimental data on 18-ton horizontal ingot made from carbon steel. The possibility of horizontal ingots casting from die steel was investigated as well. It was found that uncooled bottom plate using allows producing high quality heavy ingots from high-alloyed steels for various applications. Key words: horizontal ingot, casting, numerical simulation, unidirectional solidification, heat removal, shrinkage porosity.
1. Introduction The interest to large cast unidirectionally solidified horizontal ingots arose at the 70–80-s of last century from demands to great size performs for various sheets and plates manufacturing for shipbuilding and oil and gas industry [1]. Horizontal ingots (LH (Low Head) ingots) have a ratio of height to width less than one. This, under certain conditions, gives prospects to realize successfully the advantages of unidirectional solidification and to receive heavy cast billet, which is free from segregation defects and physical non uniformity in the axial area. The mass of large steel ingots of this type reaches 90 t with a height up to one meter. The most actively the technology of LHingots manufacturing for thick sheets and plates production were developed in Japan and France [2–5]. In E. O. Paton Electric Welding Institute the complex of works on this technology development was performed in cooperation with Donetsk Polytechnic Institute (now the Donetsk National Technical University) and the first in the USSR large horizontal ingots with unidirectional solidification were produces in the same time [6, 7]. The created technology of horizontal ingots casting reduces or even fully removes harmful impurities under the top surface of the horizontal ingot [8–10] that allows providing the method of thick sheets production from hori-
zontal ingots without machining before rolling [11]. The permanent interest in casting technology of unidirectionally solidified LH-ingots remains due to high isotropic properties of metal both in cross section and along the length of the product that is essential for critical applications [12–14]. The desire to increase the competitiveness of production and increasing demands to metal products quality stimulates improvement and adaptation of the casting technology of unidirectionally solidified ingots. The numerical simulation of the casting and solidification processes can significantly improve the efficiency of the development of new technological solutions. Some interesting results of numerical simulation of the directional solidification of LH-ingots are given in the researches [15–17]. However, these results do not cover the full range of problems. This research studies the peculiarities of horizontal ingots casting using a cast iron bottom plate without water-cooling in order to clarify the possibility to produce high-quality ingots of die steel. 2. Description of the model and its parameters Bottom pouring is usually used for heavy LH-ingots casting. In order to organize the unidirectional heat removal, the liquid metal top
– 137 – BACK TO CONTENTS
Medovar Memorial Symposium • 2016 surface has to be covered by exothermic material. Side surfaces of the solidified metal have to be also insulated. As result, the main heat removal is directed to the bottom plate and the conditions for the predominantly unidirectional solidification can be achieved. Numerical simulation of pouring and solidification of LH-ingot was made for 18 tons (height 0.51 m, length 2.5 m, width 1.75 m) steel ingot that was casted onto iron bottom plate consisting of two parts of 0.25 m in thickness. The whole 3D model was created in the CAD software using actual dimensions of real equipment (Fig. 1). The geometrical model consists of the following structural elements: steel outer shell
а
b Fig. 1. CAD geometry view (a) of model and its structure (b): 1 — metal outer shell; 2 — refractory material; 3 — metal; 4 — sprue; 5 — gating system; 6 — upper part of iron baseplate; 7 — gating system refractory; 8 — lower part of iron baseplate. Exothermic layer on the top of ingot is conditionally not shown
(1); refractory material (2); ingot (3); sprue (4); gating system (5); upper part of iron baseplate (6); gating system refractory (7); lower part of iron baseplate (8). Exothermic layer on the top surface of ingot is conditionally not shown at the picture, had been taken into account during modelling. Simulation of the pouring and solidification processes of the ingot had been made using specialized software. The mesh size was chosen in the range of 10–40 mm in depending on the size and configuration of the elements of the model in order to achieve rational ratio between the calculation accuracy and computation performance. The materials and initial temperatures of components, which were used in the model, are given in the Tab. 1. The proper heat transfer coefficients have been set for each heat exchange boundaries. On the outer surfaces of the mould and on the top of the heat-insulating layer the conditions of socalled “free cooling on air” (including convective heat transfer and radiation heat losses) were set. The heat losses from the top surface of the liquid bath during mould filling as well as heat losses in the gating system were neglected. The thermodynamic and physical properties of the materials are listed in the Tab. 2. The calculations of LH-ingot formation process start from the beginning of mould filling and last up to the full solidification of cast metal. Thermal shrinkage of solidifying metal and air gap formation between the ingot surface and the mould was taken into account during simulation. 3. Simulation results of 18 tons LH-ingot casting The simulation results of ingot solidification dynamics are presented on the Fig. 2. It was shown that for the given conditions the solidification of metal occurs predominantly in one direction from the bottom plate. During the solidification the main part of the solidification front in the longitudinal and transverse axial sections of the ingot remains flat and moves from bottom to top. Fig. 3 shows dynamic of solid phase growth in the ingot cross section. The proportion of solid phase changes on square-root law. The
– 138 –
Medovar Memorial Symposium • 2016 Table 1. Model components and materials Component
Metal Iron bottom plates Refractory material Gating system refractory Exothermic layer
Material
Initial temperature, °C
Medium-carbon steel AISI 1026 Grey Iron EN-GJL-300 Chamotte Chamotte Exothermic material
1550 20 20 20 20
Table 2. Materials properties Parameter
Steel
Thermal conductivity [f(T)], W/m∙K Density [f(T)], kg/m3 Enthalpy [f(T)], kJ/kg Solidus temperature, °C Liquidus temperature, °C Ignition Temperature, °C Released energy of exothermic reaction, kJ/kg
Iron
Chamotte
48–36 21–36 0,85–1,50 7851–6817 7200–6300 1900 56–1424 153–1468 Cp 0,88–1,14 1516 1245 — 1430 1129 — — — — — — —
estimated duration of solidification completion of this ingot makes 350 minutes. The received shape of solidification front shows that the material chosen for the mould
Exothermic material
0,2–0,37 500 0,9 — — 500 1900
walls does not eliminate the heat losses through the side surfaces of the ingot. This leads to the formation of the solidification front, which moves from the walls of the mould to the ingot
τ=59,4 min (50% solid fraction)
τ=133,7 min (75% solid fraction)
τ=242,8 min (95% solid fraction) a b Fig. 2. The shape of the solidification front in a longitudinal (a) and transverse (b) axial cross-sections at various time steps during ingot solidification
– 139 –
Medovar Memorial Symposium • 2016
Fig. 3. Calculated change of the solid phase fraction while LH-ingot solidification
axis. Analysis of the temperature pattern in the cross section of the ingot at the end of solidification shows that the heat removal into the mould walls is caused by their heating (Fig. 4). The appearance of Λ-segregation can be predicted in the zone, where solidification fronts, which move in the horizontal and vertical directions, meet each other. Studies [18] have shown that the location of this type segregation in LH-ingots depends on the intensity of heat flow through the side surfaces. The obtained results show that the upper part of iron bottom plate accumulates the main part of heat from solidifying metal (Fig. 4). By the end of the solidification, the temperature
on the upper surface of the bottom plate, which contacts ingot directly, makes 700–800°C, and only near the mold walls the temperature decreases to about 630°C. The lower surface of this plate has temperatures 500–600°C. The lower bottom plate, where runners of the gating system are placed, also accumulates a certain amount of heat. In the central part, it heats on the half of thickness to a temperature above 320°C. Thus, the upper bottom plate withstands the main thermal load. However, both of bottom plates accumulate the ingot heat. The porosity formation in the ingot caused by shrinkage was calculated (Fig. 5) and estima ted by using the Niyama criterion (Fig. 5 a, b), which is widely used to predict the shrinkage porosity formation in castings [19]. The critical value of Niyama criterion for steel is close to one. The simulation results show that the critical value of this criterion occurs only in a thin surface layer under the top surface of the ingot. The simulation results of the porosity formation predict appearing the shrinkage porosity in this region also (Fig. 5 c, d). Verification of simulation results on the experimental data. Numerical simulation results were compared with those obtained at
a
b
Fig. 4. Temperature fields in the longitudinal (a) and transverse (b) cross sections of the ingot at the end of solidification
a
b
c
d
Fig. 5. Niyama criterion value (a, b) and porosity formation (c, d) in the longitudinal and transverse axial sections of the LH-ingot
– 140 –
Medovar Memorial Symposium • 2016 the real casting of horizontal steel ingot 18 tons in weight. Fig. 6 shows the results of monitoring the changes of the solidified layer thickness during LHingot solidification, which have been received by direct measurement of the liquid metal bath depth in the central area of the ingot using the steel rod as a dipstick. The calculated results of the dynamics of solidified layer thickness growth are given on the same picture. Computer simulation results agree well with the results of actual measurements, taking into account the fact that in the course of the measurements the metal rod does not penetrate to the entire depth of the twophase zone. From the axial part of LH-ingot the transverse template was cut, which macrostructure is shown in the Fig. 7. The analysis of macrostructure and porosity of the metal also can help to evaluate the adequacy of the computer simulation results. Fig. 8 compares the li quid metal bath profiles on the ingot macrostructure (a) and simulated picture (b). Fig. 9 compares the result of calculation of the presence and location of the shrinkage defects in the model (a) with the actual porosity observed in the transverse template (b) from experimental 18 t ingot. Slight shrinkage porosity located in the top nearsurface layer in axial area of
Fig. 6. Dynamic of vertical solidification front movement
Fig. 7. Macrostructure of ingot transverse axial templates
a
b
Fig. 8. Profile of liquid metal bath in the real ingot macrostructure (a) and calculated by model (b)
a
b
Fig. 9. Porosity in the head part of the experimental ingot (a) and predicted by model (b)
– 141 –
Medovar Memorial Symposium • 2016 the ingot as in the macrostructure of ingot, as in the results of simulation. Thus, the quite good matching of computer simulation results with experimental data of 18 t LH-ingot casting was achieved. 4. Simulation of the horizontal ingot steel casting from D2 die steel Using developed and verified model the numerical experiment to study the peculiarities of pouring and solidification of LH-ingot from tool die steel was made, using the same way (Fig. 1). The casting of ingot of thickness 0.15 m, width 1.5 m and length 2.0 m from AISI D2 steel was simulated. The structure of mould and uncooled bottom plates were the same, but the mould sizes have been adapted to the dimensions of the ingot. The boundary and initial conditions were the same. Fig. 10 shows the temperature distribution in the longitudinal and transverse sections in the ingot, mould and bottom plates at the end of the ingot solidification. Evidently, the smaller ingot weigh gives a smaller thermal load on both the mould and bottom plate. The wall of the mould was heated slightly for no more than ¼ of the thickness to temperature 600°C. Therefore, in practice, the thickness of the refractory layer can be reduced.
The main amount of heat was accumulated by upper bottom plate, while the lower plate was heated much less (up to 150°C). The estimated duration of the complete solidification of the ingot makes about 80 minutes. Analysis of the porosity distribution and calculations of Niyama criterion in the axial cross-sections of the ingot (Fig. 11, 12) shows that the unidirectional solidification of the ingot on the iron bottom plate with its thicknesses ratio equal to 0.6 allows forming a dense metal. The porosity can principally occur just in a thin nearsurface layer of the ingot under the top plane (marked by dark blue color in Fig. 11). This layer will be removed at further processing of the ingot. Calculated results did not also show shrinkage macroporosity formation in ingot (Fig. 12). The greatest predicted value of the porosity did not exceed 5% that characterizes the high-density of metal in ingot. Fig. 13 shows the results of calculation of the time of the mushy zone life. Note, that a strong cooling effect of the bottom plate results in the rapid solidification of the metal in a thin (1/5 of ingot thickness) layer that adjacent to the bottom plate surface. However, the main volume of the metal solidifies in the practically same conditions that allow predicting the uniform properties throughout the ingot cross-section. The cooling effect of the mould walls accele
a
b
Fig. 10. Temperature fields by the end of the ingot solidification: a — in the axial longitudinal section; b — in the axial cross-section a
a
b
b
Fig. 11. Niyama criterion distribution in ingot crosssections: а — in the longitudinal axial cross-section; b — in the axial transverse cross-section
Fig. 12. Results of porosity calculation: а — in the lon gitudinal axial cross-section; b — in the axial transverse cross-section
– 142 –
Medovar Memorial Symposium • 2016 a b Fig. 13. The pattern of solidification time for the ingot in its longitudinal axial section (a) and its transverse axial cross-section (b)
rates solidification of metal volume adjacent to the walls (Fig. 13). And joint cooling action of the mould walls and the bottom plate causes rapid solidification of the metal in the ingot corners that may cause thermal stress concentration in these areas. Indeed, the calculation results of the thermal stress distribution shows (Fig. 14) that the stress reach the largest value in the lower areas of the side faces and the areas of ingot connection with ingates of gating system. As known, the concentration of the thermal stresses may result in the formation of hot
cracks. Fig. 15 shows the calculated values of the hot tearing indicator. Analysis of these data shows that in the mentioned areas of stress concentration in the corners of the ingot the hot tearing indicator has elevated values. However, they are less than the critical one. Thus, the results of numerical experiments has shown that the using of considered casting tooling with uncooled iron bottom plate allows receiving of high quality sound LH-ingot of die steel free of coarse shrinkage defects and cracks. Conclusion Comparison of the results of numerical simulation of the 18-ton horizontal ingot casting with the experimental data has shown quite good level of their coincidence. It allows us to use this computer model for the development of casting technology of horizontal unidirectionally solidified ingots with different parameters.
a
b
Fig. 14. Distribution of thermal stresses on the surface of the ingot
a
b
Fig. 15. Distribution of values of hot tearing indicator on the ingot surface
– 143 –
Medovar Memorial Symposium • 2016 Computer modeling of the horizontal ingot casting of 0.15×1.5×2.0 m in size of die steel which solidified on the massive cast-iron uncooled bottom plate had shown a possibility to receive high-quality castings without rough shrinkage defects and surface cracks. References 1. Paton B.E.: Selecting a blank for rolling on the plate mill / B.E. Paton, B.I. Medovar, V.Y. Saenko et al. // Steel. — 1984. — N 3. — P. 13–15. 2. Setsuo O.: Development of production technique for high quality heavy steel plate by unidirection ally solidified ingot (LH ingot) / Okamoto Setsuo et al. // Proc. 68th Steelmaking Conf. Vol.68: Detroit Meet. Apr. 14–17, 1985. — Warrendele(Pa), 1985. — P. 197–204. 3. Development of unidirectionally solidified large scale ingots for heavy gauge plates / Kitagawa Toru et al. // Trans. Iron and Steel Inst. Jap. — 1985. — 25. — N 25. — P. 1227–1236. 4. Benhamou C.: Application of directional solidi fication ingot (LSD) in forging of PWR reactor vessel heads / C. Benhamou, I. Poitrault, J. Pisseloup, P. Bocquet // 10th International Forging Conference, Sheffild, 23–25 September, 1985. 5. Pisseloup J.: Adaptation du type et de la géométrie du lingot à la fabrication de grosses pièces for gées. Conséquences au niveau de la segregation / J. Pisseloup, I. Poitrault, J. Badeau, P. Bocquet // Mémoires et études scientifiques de la revue de métallurgie. — 1986. — 83(4). — P. 191–199. 6. Paton B.E.: Horizontal ingots for production thick sheets and plates / B.E. Paton, B.I. Medovar, A.G. Bogachenko, B.I. Shukstulsky, L.B. Medovar / Steel. — 1989. — N 12. — P. 20–22. 7. Patent № 1529144 USSR, B22D 7/06. Apparatus for bottom casting of horizontal ingots / B.I. Medovar, А.А. Troyansky, Y.V. Kostetsky et al. — Publ. 02.23.91. — Bull. N 7. 8. Medovar B.I.: Improving the quality of large hori zontal sheet ingots that require minimal prepara tion before rolling / B.I. Medovar, A.A. Trojansky, Y.V. Kostetsky et al.// Steel. — 1995. — N 3. — P. 14–17. 9. Patent № 1613245 USSR, B22D 7/00. Process for bottom casting of ingots / B.I. Medovar, V.Y. Saenko, B.I. Schukstulsky, Y.V. Kostetsky et al. — Publ. 20.11. 90. — Bull. N 46. 10. Troyansky A.A.: Development of the casting tech nology of large horizontal sheet ingots / A.A. Tro-
yansky, Y.V. Kostetsky // Metal and Casting of Ukraine. — 1996. — N 5. — P. 15–17. 11. Patetnt № 169015 USSR, B21B 1/02 1/40 Thick sheets manufacturing method / V.M. Klimenko, Y.I. Yurchenko, A.A. Troyansky, Y.V. Kostetsky — Publ. 07.12.91. — Bull. N 45. 12. Horikawa K.: Application of Extra-High Tensile Strength Steel for Hydropower Plants in Japan / K. Horikawa, N. Watanabe // Proceedings of the International Conference on High Strength Steels for Hydropower Plants, July 20–22, 2009, Takasaki, Japan. — 8 p. 13. Peng X.D.: Study on unidirectional solidification of slab ingot for special heavy plate / X. D. Peng, L. Hu, P. X. Wang, L. G. Zhao // Advanced Materials Research. — 2011. — Vol. 189–193. — P. 4033–4036. 14. Kostetsky Y.: Application of the technique of direc tional solidification in the casting of large ingots / Y. Kostetsky, A. Troyansky, M. Efimov, P. Yavtu shenko // Proceedings of 5th International Congress on the Science and Technology of Steelmaking 2012, Dresden, Oct. 1-3. — Dresden, 2012. — Paper ID 1263. 15. Ao G.W.: The studies on numerical simulation of unidirectional solidification process in 23t steel in got / G.W. Ao, M.G. Shen, Z.S. Zhang, L.L. Hong // Advanced Materials Research. — 2012. — Vol. 502. — P. 46–50. 16. Ao G.W.: Study on Influence of Water-Cooled Stool During the Process of Unidirectional Solidifi cation / Guangwu Ao, Minggang Shen, Zhenshan Zhang, Xiaodong Li // The Open Materials Science Journal. — 2015. — Vol. 9. — P. 158–161. 17. Poitrault I.: Characterization and prediction of solidification structures and macrosegregations in heavy steel ingots / I. Poitrault, D. Cardinaux, S. Corre, M. Zaloznik, C. David // Proceedings of the 6th International Conference on Mode ling and Simulation of Metallurgical Processes in Steelmaking — Steel Sim 2015, Bardolino, Lake Garda, Italy, 23–25 September, 2015. 18. Nakada M.: Phenomena of solidification and macrosegregation in the unidirectionally solidi fied ingots / M. Nakada, T. Kitagawa, K. Yano et al. // Tetsu-to-Hagané. — 1983. — Vol. 69, No 11. — P. 1433–1439. 19. Niyama E. , Uchida T., Morikawa M. , Saito S.: A Method of Shrinkage Prediction and its Appli cation to Steel Casting Practice. Am. Foundrymen’s Soc. Int. Cast Met. J., vol. 7 (3), p. 52–63 (1982).
– 144 – BACK TO CONTENTS
