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Inclusion Shape Control with Calcium and Zirconium and Th eir Eff ects on Mechanical Properties of Cast Steel

Ersel Aydın, C. Fahir Arısoy, M. Kelami Şeşen

İstanbul Technical University - Türkiye

Abstract In this study, zirconium and calcium added to cast steel for evolving the inclusion morphology. Effect of the morphology changes found out by tensile test and charpy notch impact test. Significant improvements were determined on especially toughness value via inclusion modification. Microstructure characterization carried out by light microscope to identify the microstructure evolution and by scanning electron microscope and EDS to find out and characterize the inclusion morphology and possible composition. 1. Introduction In general, inclusion generation can be classified into two sources, indigenous and exogenous. Oxide particles are formed as a result of deoxidizer additions made to the steel ladle as a means to reduce the level of dissolved oxygen in the liquid steel. If these deoxidation products are not removed from the steel prior to casting, they will be present as oxide inclusions in the steel product. The inclusions generated via this process are inherent to the steelmaking process, and therefore are indigenous in nature. Exogenous source of inclusions may arise from uncontrolled oxidation of liquid steel (reoxidation) as well as excessive melt stirring resulting in slag entrainment and refractory erosion. Indigenous inclusions usually have modest influence on material properties due to smaller particle size. They can only be minimized through process control but cannot be eliminated completely. On the other hand, macro-inclusions with exogenous origin are detrimental to various material properties [1-6]. In general, oxide inclusions can be classified into:

• Single oxides; some common examples: FeO, Fe2O3, MnO, SiO2, Al2O3, Cr2O3, TiO2

• Complex oxides, often takes the general form of AO•B2O3, where metal A has +2 oxidation number and metal B has +3 oxidation number. Some common examples are FeO.Al2O3, MnO.Al2O3, MgO.Al2O3, FeO.Cr2O3

Complex oxide inclusions are sometimes known as spinel type (MgO.Al2O3) inclusions for their similarity in structures. Spinel type inclusions are characterized by faceted structure and high melting temperature, usually higher than steelmaking temperature of 1873 ºK. Spinel inclusions are especially harmful during steel processing as they do not deform during hot rolling and often cause poor surface finish [1-6]. The presence of non-metallic oxide inclusions is a major cause of incompatibility between the attainable and desirable level of cleanliness in many grades of steel. Generally, inclusions degrade the mechanical properties of the steel and thereby reduce the ductility and toughness of the cast metal and increase the risk for mechanical failure of the final product. Alumina inclusions occur as deoxidation products in the aluminum-based deoxidation of steel. Pure alumina has a melting point above 2000°C, these alumina inclusions are present in a solid state in liquid steel. The addition of calcium to steel which contains such inclusions changes the composition of these inclusions from pure alumina to CaO-containing calcium aluminates [3]. Sulphide inclusions are important to consider since it is common to have steel with oxygen content less than 0.02% while having sulphur content at around 0.03%. Liquid steel has a high solubility of sulphur where solid steel usually has significantly lower sulphur solubility. As liquid steel cools, sulphur segregates and forms FeS with melting point of 1460 ºK. FeS often causes embrittlement of steel. Therefore it has become a common practice to add sufficient amount of Mn, due to manganese’s stronger affinity for sulphur, to form MnS (Tm = 1870 ºK). Types of sulphide inclusions will also depend on manganese to sulphur ratio. Examples of common sulphide inclusions include MnS, FeS, (Mn,Fe)S and CaS [1-8]. In the as cast condition, MnS inclusions can be classified into three main morphologies [9-11] Type I: globular, when the oxygen solubility is high and the sulfur solubility is relatively low. Such inclusions are formed by a monotectic reaction in rimmed and semi-killed

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steels (when aluminum in the steel is less than 0.001 wt. %). Type II: formed in the interdendritic spaces of austenite with a fan-like morphology. In addition, most commonly formed at grain boundaries of steel. These are formed in aluminum killed steels with the first trace of aluminum above 0,005 wt %. Type III: angular inclusions are formed as isolated particles in the interdendritic spaces, when excess aluminum is used for deoxidation resulting in about 0.040 wt.% aluminum in the steel [14-15]. Type I sulfide inclusions contain usually an oxide core and are therefore harder than Type II sulfides. Type I oxy-sulfides are usually present in steel as individual particles while Type II MnS inclusions are formed by an eutectic reaction in the interdendritic spaces. In addition, Type II sulfides can deform to a larger extent than the inclusions of Type I during hot working of the steel. Therefore, they may be more harmful to the materials mechanical properties. Thus, MnS inclusions of Type II and III become elongated during rolling or other deformations of steel. These elongated inclusions introduce an anisotropy of the mechanical properties of steel which leads to an inferior strength, ductility and toughness in the short transverse direction [9-15]. The deformation of MnS inclusions in steels increases the interphase surface between inclusion and the steel matrix. This can lead to significantly decreased performance properties of steel e.g., plasticity and toughness. [16–18]. The harmful effect of MnS inclusions on the final mechanical properties can be reduced if the sulfur content can be decreased in cast steel (with a followed decrease of the steel machinability) or by a modification of MnS inclusions by an addition of Ca, REM (Rare-Earth-Metals) or Zr in the melt [9]. The main purposes of the modification process of MnS inclusions are usually to: - change the composition and properties (physical and chemical) of sulfides; - change the sulfide morphology (globalization); - decrease the size of the modified sulfides; - obtain a homogeneous distribution of precipitated sulfides in the solidified steel [9]. 2. Experimental Procedure Studies were carried out with sand casted test coupons (TC) that sized 80 mm X 120 mm X 250 mm. Zirconium (200 gram/ton) and calcium (500 gram/ton) additions were made in the ladle via box dipping method after deoxidation with aluminum. Chemical composition of steel specimens are given in Table 1-2.

Table 1. Chemical composition of steels for TC 1 and TC1-Zr (wt %)

C Si Mn P S 0.28 0.47 0.73 0.013 0.011 Cr Mo Ni Al

0.78 0.24 1.78 0.035

Table 2. Chemical composition of steels for TC 2 and TC2-Ca (wt %)

C Si Mn P S 0.23 0.59 1.02 0.014 0.010 Cr Mo Ni Al

0.75 0.58 1.80 0.038 2.1. Heat treatment processes Test coupons were heated to normalizing temperature at 100°C/h heating rate. When the temperature is stable samples were soaked for 3 hours to carry out normalizing, followed by cooled in still air to ambient temperature. After normalizing samples were again heated for quenching and soaked in the furnace for 3 hours. All specimens were finally tempered for 3 hours to achieve a high level of tensile strength and toughness. All parts were cooled in water medium after tempering process to avoid temper embrittlement. Heat treatment conditions are given in Table 3.

Table 3. Heat treatment processes of test coupons Specimen Normalizing Quenching Tempering

TC 1 870 °C – 3 h 840 °C – 3 h 600 °C – 3 h TC 1-Zr 870 °C – 3 h 840 °C – 3 h 600 °C – 3 h

TC 2 920 °C – 3 h 890 °C – 3 h 550 °C – 3 h TC 2-Ca 920 °C – 3 h 880 °C – 3 h 550 °C – 3 h

2.2. Test methods The mechanical properties including tensile strength, elongation and Charpy notch impact energy were measured using the average of three experimental results for each test coupons. Tensile tests were carried out according to ASTM E8 standards in order to determine yield strength and tensile strength of samples using Instron universal tensile testing machine. Charpy V-notch (CVN) impact testing was also carried out according to ASTM E23 standard. The fracture surfaces of tensile tested specimens were examined under field emission scanning electron microscope at an accelerating voltage of 15 kV for fractographic characterization. Analysis of chemical composition for the nonmetallic inclusions was carried out with energy-dispersive X-ray spectroscopy (EDS). 3. Results and Discussion The variation in yield strength, tensile strength, ductility (elongation) and toughness values of the Zr added and Ca added test coupons are presented in Table 4. According to

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the results of impact tests on specimens, Zr addition shows 68% and Ca addition 38% improvement in toughness compared to conventional casting method. Also yield and tensile strengths of the Zr added test coupon (TC 1-Zr) are slightly lower than conventional test coupon (TC 1). Figure 1 shows example of typical martensitic-bainitic micro structure of low alloyed cast steel.

Table 4. Mechanical results of specimens

Specimen Yield

strength (Mpa)

Tensile strength. (Mpa)

Elongation (%)

Toughness (Joule) at - 40° C

TC 1 797 941 10,71 48 TC 1-Zr 754 868 11,95 81

TC 2 1065 1171 10,16 36 TC 2-Ca 1053 1164 10,81 50

Figure 1. Martensitic-beynitic microstructure of TC1

specimen (500X – 20 μm scale length)

Because of high manganese and sulfur segregations to the interdendritic spaces during solidification, large amount of Type III-angular MnS inclusions can clearly be seen on the dendritic arms of TC1 specimen in Figure 2 and 3. It is evident that less harmfull type III MnS inclusions can be preferably formed instead of type II MnS inclusions with aluminum deoxidation.

Figure 2. MnS inclusions on dendrite arms of fracture

surface of tensile tested TC1 specimen

Type I globular oxy-sulfide inclusions can be seen on the fracture surface of TC1-Zr specimen in Figure 4 and 5. Zr addition evolved MnS inclusions from type III to type I that the most harmless inclusion type in steel. Due to the

inclusion modification, toughness value increased 68% in TC1-Zr specimen.

Figure 3. Type III MnS inclusions

Figure 4. Type I oxy-sulfides at TC1-Zr specimen

Figure 5. Type I oxy-sulfide inclusion

Figure 6: EDS analysis of oxy-sulfide inclusion

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Figure 7 shows the SEM image of calcium aluminate. Aluminum oxide inclusions were observed at the center of inclusion is surrounded by (Ca,Mn)S as shown in Figure 8. Due to the modification of alumina to calcium aluminates provided 38% improvement in toughness value of TC 2-Ca specimen without reducing the yield and tensile strength.

Figure 7. Calcium aluminate inclusion

Figure 8. EDS mapping image of calcium aluminate

4. Conclusion (1) Less harmfull type III inclusions can be formed preferably, instead of type II due to aluminum deoxidation. (2) MnS inclusions evolved from type III to type I via zirconium addition. (3) Zr addition in cast steel results in a significant improvement of 68% in impact toughness and slight decreasing in yield and tensile strength. (4) Ca addition in cast steel results in significant improvement of 38% in impact toughness without reducing the yield and tensile strength via forming of calcium aluminate inclusions. (5) Experimental study results shows that calcium and zirconium addition can be obtainable with box dipping method to the ladle after deoxidation without using any vacuum and wire feeder technology in foundry. References [1] Wu, C. P. P., Inclusion characterization in high strength low alloy steel. Master Thesis, University of Toronto, 2009.

[2] S. Millman, Clean steel – Basic features and operating practices, IISI Study on Clean Steel, International Iron and Steel Institute, Belgium, 2004. [3] C. S. S. Pires, A. Garcia, Modification of oxide inclusions present in aluminum-killed low carbon steel by addition of cacium, Metalurgia and Materials, 2004, vol. 57, pp. 183-189 [4] R. Kiessling and N. Lange, Non-Metallic Inclusions in Steel, The Institute of Materials (London), 1978, vol. 2, pp. 13-50 [5] R.E. Lismer and F.B. Pickering: JISI, 1952, vol. 170, pp. 48-50 [6] L. Zhang and B.G. Thomas, “State of the Art in Evaluation and Control of Steel Cleanliness – Review”, ISIJ International, 2003, vol. 43, no. 3, pp. 271–291 [7]A. Muan and E.F. Osborn, Phase Equilibria Among Oxides in Steelmaking, Addison-Wesley, Reading, Mass., USA, 1965, p. 4 [8] E.T. Turkdogan, Fundamentals of Steelmaking, The Institute of Materials (London), 1996, pp. 111-113 [9] Anmark, N., Karesev, A. and Jönsson, P. G., The Effect of Different Non-Metallic Inclusions on the Machinability of Steels, Materials 2015, 8, 751-783 [10] Sims, C.E.; Dahle, F.B. The effect of aluminum on the properties of medium carbon cast steel. Trans. Am. Foundrym. Soc. 1938, 46, 65–132. [11] Sims, C.E. The nonmetallic constituents of steel. Trans. Met. Soc. AIME 1959, 215, 367–393. [12] Steinmetz, E.; Lindenberg, H.-U. Einfluß von Kohlenstoff, Silicium und Aluminium auf die Morphologie der Sulfide in Eisenwerkstoffen. Arch. Eisenh ttenwes. 1976, 47, 713–718. [13] Itzkovich, G.M. Deoxidation of Steel and Modification of Nonmetallic Inclusions; Metallurgia: Moscow, Russia, 1981; p. 296. [14] Karasev, A., Inoue, R., Tilliander, A., Jönsson, P.G. Application of electrolytic extraction for three-dimensional investigation of inclusion characteristics in the steelmaking area. Technology of Process Metallurgy, Osaka, Japan, 15–16 April 2013; pp. 1–5 [15] Karasev, A.; Bi, Y.; Jönsson, P. Three-Dimensional Investigation of Large-Size Inclusions and Clusters in Steels by Using the Electrolytic Extraction Technique. In Iron and Steel Technology Conference, Pittsburgh, PA, USA, 6–9 April 2013; pp. 1–7. [16] Wranglen, G. Pitting and sulphide inclusions in steel. Corros. Sci. 1974, 14, 331–349. [17] Webb, E.G.; Suter, T.; Alkire, R.C. Micro electrochemical measurements of the dissolution of single MnS inclusions, and the prediction of the critical conditions for pit initiation on stainless steel. J. Electrochem. Soc. 2001, 148, B186–B195. [18] Williams, D.E.; Zhu, Y.Y. Explanation for initiation of pitting corrosion of stainless steels at sulfide inclusions. J. Electrochem. Soc. 2000, 147, 1763–1766.