PHYSICAL METALLURGY CHARACTERISTICS OF INCLUSIONS AND MICROSTRUCTURAL RESPONSE IN LOW CARBON STEELS

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1 PHYSICAL METALLURGY CHARACTERISTICS OF INCLUSIONS AND MICROSTRUCTURAL RESPONSE IN LOW CARBON STEELS FYZIKÁLNĚ METALURGICKÉ CHARAKTERISTIKY INKLUZÍ A MIKROSTRUKTURNÍ ODEZVA V NÍZKOUHLÍKOVÝCH OCELÍCH Eva Mazancová a Zdeněk Jonšta a Petr Wyslych a Karel Mazanec a a VŠB-TU Ostrava, Tř. 17. listopadu 15, Ostrava-Poruba, ČR, eva.mazancova@vsb.cz Abstract The grain refinement represents an effective way how to improve strength and ductility of steels without deteriorating the achieved toughness. The beneficial effect on grain refinement consisting in facilitating the nucleation of intragranular acicular ferrite (AF) is influenced by evenly dispersed fine inclusions in steel. In particular fine inclusions (oxides, sulphides, nitrides) are especially effective in AF nucleation. The considered effect in austenite decomposition in AF has been developed in connection with the concept of oxide metallurgy. The preferential of AF nucleation is realized at intragranular sites (inclusions) in austenitic matrix. Although the mechanism of AF formation is not elaborated yet, it is clear the nucleation chemistry plays an important role. In applied oxide metallurgy concept it is necessary to control the oxide particles during steelmaking process. In present work, an attempt has been performed to contribute to the elucidation of physical metallurgy mechanisms controlling the Mn-depletion zone formation based on the analysis of conditions leading to MnS initiation on MnOSiO 2 inclusions (Si/Mn steel deoxidization). The unfavourable effect of Al 2 O 3 on AF nucleation is elucidated (the evaluation and cluster formation inclusive attraction between pair of inclusions). The Mg-addition is also taken into consideration and compared with its influence on Al 2 O 3 and/or MnOSiO INTRODUCTION The beneficial effect of inclusions on austenite decomposition into ferrite has been recognized and termed oxide metallurgy, recently [1]. Inclusions can act as intragranular nucleation sites for acicular ferrite (AF) resulting in reduced grain size and in improvement physical properties of steel, consequently. The realized mechanism of considered process is not elucidated in details yet. It is clear inclusion chemistry plays an important role in the AF nucleation. At the same time utilisation of non-metallic particles as heterogeneous nucleation sites is an alternative way of grain refinement [2]. Although the application of this method has been realized for decades by grain refinement of the weld materials and HAZ of weld joins, it has been recently attracting as an alternative technique of grain refinement obtained in wrought steels [3], especially for thin slabs to which heavy deformation rolling can not be applied. The resulting AF microstructure is characterized by inter-weaved of ferritic plates nucleated from finely dispersed catalyst particles present in the interior part of austenite grains. The AF morphology presents a very promising combination of high strength and high toughness by deflecting propagation of internal cleavage cracks developed in service [4]. Fine oxides inclusions sulphides and nitrides are formed during solidification as potential AF nucleants. Among the most effective non-metallic nucleants can be classified Ti 2 O 3, TiN, CuS and MnS. A numbers of different mechanisms are suggested to explain how the inclusions facilitate the AF nucleation. The most persuasive suggestion is the existence of a 1

2 zone around the inclusions where Mn is depleted (zone acting as nucleation site for the AF). Zone being Mn-depleted is formed around the nucleating (MnOSiO 2 ) inclusion by MnS nucleation on it and by subsequent Mn-diffusion in the steel matrix into the MnS nucleated particles. A recent study has demonstrated that the Mn-depleted zone can be formed around Ti 2 O 3 inclusions leading to the enhancement of AF volume fraction. In this connection it is interesting to remember the additional way concerning the Mn-depleted zone formation around oxides containing an excess of cation vacancies. This process is e.g. realized in Ti 2 O 3 and it is due to direct Mn diffusion from matrix into above mentioned oxide inclusion. The present work is devoted to the finding of physical/chemical sources causing the above considered behaviour of inclusions and contributing to the AF nucleation. The analysis will be made for steel containing MnOSiO 2 oxides (Mn/Si deoxidization) at aiming to elucidate the physical metallurgy causes of detected AF nucleation activity. 2. MORPHOLOGY ANALYSIS OF SECONDARY NON-METALLIC INCLUSIONS The non-metallic inclusions formed after Si/Mn deoxidization corresponds to the manganese-silicate (MnOSiO 2 ) and on these inclusions MnS particles precipitate. In summary, three categories can be obtain, namely alone MnOSiO 2, alone MnS and MnOSiO 2 with MnS. The majority of MnS was usually found to accompany oxide inclusions. In accordance with this finding, the incidence of isolated MnS was very low even in the case of applied high cooling rate. From the morphological features of the detected inclusions and on the basis of the EDS analysis, it can easily be inferred that the inclusions are MnOSiO 2 oxides containing a very small sulphur amount and that MnS has formed on the oxide inclusions by precipitation and growth. This process can be observed after prolonged steel heating (e.g. at 1200 ºC/1.2 ks) when more than 90% of the MnOSiO 2 oxides can coexist with MnS [5]. Fig. 1. Equilibrium of precipitation of MnS low carbon steel The inclusions morphology is dependent on the cooling rate level. In case the cooling rate is increased, the size of inclusions and the volume fraction of MnS in the oxide inclusions decrease. In the relation to the above mentioned cooling rate effect, the formation probability of Mn-depleted zone around the inclusions is affected by the thermal history of steel with respect to the realization of corresponding diffusion process. The achieved morphology results make possible to present some additional remarks to the mechanism of MnS precipitation on MnOSiO 2 inclusions and to the formation of Mn-depleted zone. The 2

3 formation process of considered inclusions is described in Fig.1. This figure shows the equilibrium precipitation of MnS in dependence on temperature. It is seen in Fig.1, if the composition change of the inclusions follows the thermodynamic equilibrium path, the inclusions become saturated with sulphur at the solidus. Further cooling results in precipitation of MnS from the inclusions due to a decrease in sulphur solubility (it corresponds to the level of sulphide capacity of the inclusions) [5]. 3. MECHANISM OF INCLUSION FORMATION Figure 2 demonstrates the compositional change of the MnOSiO 2 inclusions containing a small sulphur amount with isothermal holding at 1200 ºC (cooling rate = 50 ºC/min.). In case the application of a higher cooling rate (500 ºC/min.), the obtained dependence MnO/ MnOSiO 2 vs solubility of MnS is practically the same as in the dependence given in Fig.2. This dependence shows, the manganese and sulphur contents in inclusions are far removed from the saturation level than this has been possible in the literature [6]. Based on this finding, we can imply the inclusions are not in equilibrium with steel matrix. That is to say, the steel matrix is supersaturated with both Mn and S in regard to the inclusion. These results lead to the conclusion the above mentioned elements have a tendency to move from the steel matrix into the inclusion. The hypothetical paths of MnS precipitation during cooling are presented in complex form in Fig.3. This figure shows the change of sulphur content in an inclusion with temperature. The ABC path represents the sulphur content in the inclusion being in equilibrium with the steel matrix as it follows from Fig.1. This path represents the maximum sulphur content that can be detected in inclusions and above which MnS should precipitate out. In this case the amount of sulphur that has been rejected in the form of MnS from inclusion until cooled to 1200 ºC is depicted in Fig.3 as DC distance. This result leads to the conclusion the ABC path presented in Fig.2 is not real. The modified dependence is lying below the above given path. The path indicated AEF corresponds to a schematic representation of a route that the sulphur might follow in a red nonequilibrium case. The distance CF can be held for the compositional driving force contributing to the transfer of sulphur from the steel matrix to the inclusion. It is possible to deduce the sulphur content in inclusion will increase toward the point C, if the sample is isothermally held at 1200 ºC. The manganese transfer should be the same as that of sulphur since the steel matrix is also supersaturated with these elements [5]. [11] Fig. 2. The composition change of the MnOSiO 2 incliusions (lower S content, cooling rate of 50 C/min) 3

4 Fig. 3. Theoretical paths of MnS precipitation during cooling In addition to the above realized analysis the homogeneity of inclusion has been evaluated. The achieved results have demonstrated that a concentration gradient of Mn and S across inclusions exists. Manganese content becomes higher toward the inclusion surface. This implies the manganese and sulphur transfer from the steel matrix to the inclusion is occurring. This evaluation has been performed in the sample prepared by cooling to 1200 C at a rate of 500 C/min. and after subsequent isothermal heating for 60 minutes. Due to continual manganese and sulphur transfer from the steel matrix the outer part of investigated inclusions become richer in both manganese and silicium. Therefore, the transfer of these elements to the inclusions will result in the MnS formation at the margin of inclusions. Concerning the concentration gradient in inclusions, it is important to remark, the transfer rate of manganese and sulphur at the interface of metal and inclusion is higher while the mass transfer within the inclusions is sluggish. It leads to the creation of concentration gradient across the inclusion. Schematic illustration of described mechanism realised by the MnS formation and creation of Mn-depleted zone during isothermal holding at 1200 C after solidification is given in Fig.4 [5]. Fig. 4. Transfer of Mn and S at the metal/inclusion interface A) concentration gradient across the inclusion B) precipitation and growth of MnS particles at the outer part on the inclusions 4

5 Therefore, it is necessary to assure both the formation of Mn-depleted zone and also its durability around inclusions together with MnS precipitation on the inclusions at the temperature of austenite decomposition in ferrite. The filling of this transformation procedure requires a careful control of chemical metallurgy parameters set. The above presented analysis leads to the conclusion: the inclusions having a high sulphide capacity are favourable to form the Mn-depleted zone and to fulfil the required nucleation conditions for MnS precipitation on inclusions. The obtained results demonstrate on a basis of performed materials engineering analysis how is possible to directly control the phase transformation process by application of the chemical metallurgy technique. 4. INFLUENCE OF COMPLEX INCLUSIONS ON THE AUSTENITE TRANSFORMATION PROCESS INTO AF It is well known, alumina inclusions are not attractive from point of view of their influence on austenite decomposition in ferrite. Alumina inclusions show a week capability for MnS to precipitate on alumina inclusions. In view of such behaviour alumina inclusions are not useful by the control of AF formation by austenite decomposition [7]. Alumina inclusions appearing in the normal Al-killed steel have a strong attractive force between these inclusions what results in their coagulation and/or in the formation of clusters. In relation to this behaviour of alumina inclusions, it was realized an investigation how to modify the remembered alumina behaviour after addition of the second element. The source of the mentioned unfavourable alumina influence is an existence of strong attractive force between individual alumina particles. As it follows from the previous chapter in the concept of oxide metallurgy, the oxide particles are controlled from the beginning of the steelmaking process for the subsequent nucleation ones. A successful realization of this concept is based on the application of new processing consisting in the heterogeneous nucleation combining the steelmaking process with the following heat treatment and rolling process. In order to make oxide inclusions useful for this purpose the composition size and distribution must be controlled carefully. Alumina inclusions are known (found in the Al-killed steel) with strong attractive force between them what results in a high coagulation tendency. On the contrary, it leads to the existence of a very weak capability for the MnS formation on these inclusions [7]. Yin et al [8] observed that a strong long-range attraction exists among alumina inclusions in low-carbon Al-killed steel. On the contrary, the complex inclusions of the aluminamanganese type behave in different manner. The small modification in chemical composition of inclusions leads to the fundamental change in the behaviour of considered complex inclusion consisting in the investigated conditions of 7%MgO (with 93% of Al 2 O 3 ). The additional chemical analysis (Auger spectra) of considered complex aluminamagnesia inclusion revealed a different chemical composition in the centre and in the periphery of inclusion (Mg presence can be only found in the inclusion core) [7]. The attractive force has an important influence in this connection. The derived attractive forces between different types of inclusions are shown in Fig.5. For a very interesting could be held the detected attractive inclusions behaviour of 93% Al 2 O 3-7% MgO or MgO type. The attractive force between the inclusions containing MgO is 0.1 of the force between the alumina inclusions, approximately. The maximum acting length for Al 2 O 3 MgO complex inclusions and MgO inclusions are 21 and 22µm, respectively. These values correspond to 2/5 of attractive distance considered for Al 2 O 3 (see 22 vs 55µm) [7]. Alumina-magnesia complex inclusions and magnesia inclusions only coagulate when the distance between these inclusions is smaller than the distance of Al 2 O 3 inclusions. The surprising is the finding concerning the inclusions behaviour based on 93% Al 2 O 3-7% MgO and pure MgO (same level of attractive force) when the Al 2 O 3 MgO inclusion complex only consists in the periphery of Al 2 O 3. Under this condition the contact angle must be the 5

6 same at the periphery as that for pure alumina. If so, the attraction force between complex inclusions in the pair must be the same as that between the alumina in the pair. The probable cause of this surprising result consists in the different oxygen content in the molten steel and in the corresponding influencing of a contact angle between the inclusion and molten steel (dependent on the oxygen concentration in the molten steel). The content angle in the molten steel containing a higher oxygen concentration is around 100º. This value is lower than that of pure alumina (contact angle is 135º, approximately). For this reason, the attraction force between alumina magnesia inclusions becomes lower. The attractive force between MgO inclusions is lower because the contact angle of those particles in the molten steel is lower than the one found in Al-killed steel. Al 2 O 3 -SiO 2 Al 2 O 3 Fig. 5. Attractive forces between pair of different inclusions 5. CONCLUSIONS The nucleating potency of individual comprising complex inclusions represents a decisive inoculation parameter in austenite/ferrite transformation. It has been established the higher density number of evenly dispersed fine particles (0.5 2µm) are very effective in the refinement of ferrite by nucleation of AF. The type of non-metallic particles is mainly determined by deoxygenation process while spatial distribution of inclusions is influenced by their wettability with steel melt for their even also dispersion in it. The occurrence evenly dispersed fine oxide inclusions facilitate the heterogeneous AF nucleation what leads to the attainment of refined microstructure. REFERENCES [1] TAKAMURA, J., MIZOGUCHI, S. In Proceed. 6 th International Confer. Iron and Steel, ISIJ, Tokyo, vol.1, 1990, p [2] JONES, S.J. BHADESHIA, H.K.D.H.: Acta Mater., 45, (1995), p [3] ISHIGAWA, E., TAKAHASHI, T. ISIJ Inter., 35, 1995, p [4] KIM, H.S., CHANG, C.H., LEE, H.G. Scr. Mater., 53, 2005, p [5] KIM, H.S., LEE, H.G., OH, K.S. Met. Mater. Trans.A, 32A, 2001, p

7 [6] HASAGAWA, A., MORITA, K., SANO, N. Tetsu to Hagané, 81, 1995, p [7] KIMURA, S., NAKAJIMA, K, MIZOGUCHI, S. Met. Mater. Trans.B, 32B, 2001, p [8] Yin, H., SHIBATA, H., SUZUKI, M. ISIJ Inter., 37, 1997, p