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Kinetics of columnar abnormal grain growth in low-Si non-oriented electrical steel
Materials Science and Engineering A 385 (2004) 449–454
Kinetics of columnar abnormal grain growth in low-Si non-oriented electrical steel Mykola Dˇzubinsk´y∗ , Yuriy Sidor, Frantiˇsek Kov´acˇ Institute of Materials Research of Slovak Academy of Sciences, Watsonova 47, 04353 Koˇsice, Slovakia Received 26 February 2004; received in revised form 28 June 2004
Abstract Decarburising annealing in the two-phase region is a well-known method to provide abnormal columnar grain growth in non-oriented electrical steel. Previous investigations have revealed that columnar-grained material produced in this way has an increased intensity of the “random cube” {1 0 0}0 v w and reduced {1 1 1}u v w deformation texture components in comparison with the same material with a typical equiaxed microstructure. In the present paper, the kinetics of the microstructure and texture development during the columnar grain growth is investigated by means of electron backscatter diffraction with emphasis on the early stages of the process. It has been found that texture changes reflect the character of the columnar grain growth when with further propagation of the columnar grains from the surface to the midplane the global texture becomes similar to the one of the near surface region in primary recrystallised material. © 2004 Elsevier B.V. All rights reserved. Keywords: Columnar grain growth; Microstructure; Texture; Kinetics
1. Introduction Much work has been carried out on steels to tailor their properties. A difficulty in many cases is the heterogeneity of the microstructure and the texture of the steel as a result of the production process. This is of particular importance in electrical steels. For non-oriented electrical steel it is possible to apply a decarburising annealing in the intercritical region that leads to the columnar-grained microstructure with a specific type of the texture, thus avoiding the heterogeneity [1]. Although, by application of such an annealing, it is possible to enhance desirable so-called “random cube” {1 0 0}0 v w texture component in the final material, which cannot be obtained by applying ordinary rolling and annealing processes.
∗ Corresponding author. Present address: IMMPETUS-Institute for Microstructural and Mechanical Process Engineering, The University of Sheffield, Sir F. Mappin Building, Mappin Street, Sheffield S13JD, UK. Tel.: +421 55 6338115/44 114 2227858; fax: +421 55 6337108/44 114 2227890. E-mail addresses: [email protected], [email protected] (M. Dˇzubinsk´y).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.046
First time it has been found by Assumus that high temperature annealing of very thin Si–Fe sheets leads to an abnormal growth of the cube grains [2], when the secondary recrystallisation process is controlled by the gas–metal interfacial energy, which is lowest at the (1 0 0) orientation in a high purity inert atmosphere or in vacuum [3–5]. The cross rolling and subsequent high temperature annealing of the silicon steel sheets leading to increasing the intensity of the cube texture component was investigated in [6]. Tomida et al. suggested to apply annealing in vacuum to remove the manganese and subsequent annealing in the ␥ or ␣ + ␥ two phase region in a decarburising atmosphere [7,8], or high-temperature annealing in vacuum with application of an oxide separator between the sheets [9] with the aim to achieve high intensity of the cube orientation component in the material. Nevertheless, all above described processes cannot be applied under industrial conditions. Alternatively, the two-step decarburising annealing process, which leads to the columnar grain growth, without application of the preliminary vacuum annealing, has been proposed [1,10]. In the present paper, kinetics of the columnar grain growth assisted by decarburising annealing is investigated.
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2. Experiment Semi-processed electrical non-oriented steel, with the following chemical composition: C = 0.05 wt.%, Mn = 0.36 wt.%, Si = 0.24 wt.%, P = 0.068 wt.%, S = 0.008 wt.%, Al = 0.109 wt.%, N = 0.005 wt.%, was used as the experimental material. Samples with a nominal thickness of 0.45 mm were taken from the industrial production process after temper rolling. Afterwards, alternatively to the process described in [1,10], the samples were annealed in a simplified manner, i.e. isothermally at 880 ◦ C in the typical decarburising atmosphere of cracked ammonia. Different annealing times were applied: 30, 60, 90, 120 and 150 s. To obtain “initial” state after primary recrystallisation, one sample was annealed in Ar atmosphere at 750 ◦ C for 6 min. Prepared in such a way the samples were scanned utilising Phillips XL40/TSL EBSD facilities, and subsequently data were analysed by OIMTM software package. The step length during the scanning was increased with increasing coarseness of the microstructure, from 1.5 m (the samples after primary recrystallisation and 30 s of decarburising annealing) to 10 m (the samples after 120 and 150 s of decarburising annealing). From the collected data, grain boundaries maps were produced, stitched together, where it was possible, and subjected to an image analysis, when microstructural parameters of each particular grain were measured. In all samples, an average grain size d¯ and an effective average grain size d¯ eff [11] were evaluated as follows: n 2 √ d¯ = √ si n π
(1)
i=1
n n 1.5 ¯deff = √2 si / si π i=1
(2)
i=1
where si is an area of ith grain and n is the number of grains in the microstructure. Although there is no clear criteria yet how to conduct a comparison of the significantly different in terms of grain size and size homogeneity microstructures utilising EBSD, we tried to get the objective result including different number of scanned “unit areas” into texture analysis depending upon the microstructures coarseness and homogeneity. One “unit area” is the longitudinal whole-thickness cross-sections of the 450 m × 600 m dimensions. A single area was used for texture calculation in the sample after primary recrystallisation. In the fine-grained sample after 30 s of annealing 2 u.a. and in the mixed-grained sample with predominantly coarse grains after 60 s of annealing 16 u.a. were taken into consideration. As during holding after 90 s at the temperature negligible microstructure changes were observed, EBSD data from all columnar-grained samples (after 90, 120 and 150 s) were combined together, i.e. 3 × 22 = 66 u.a. were taken into account. Apart from this, “zone” microstructure and texture analyses were conducted in the samples after 30 and 60 s of annealing, where all microstructure and texture pa-
Fig. 1. Microstructures (EBSD obtained grain boundaries maps) and zones in those: (a) sample after primary recrystallisation; (b) sample after 30 s of annealing; (c) sample after 60 s of annealing; (d) typical for samples after 90, 120 and 150 s of annealing.
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rameters were additionally evaluated for these conventional zones, which are “surface”, “centre” and “intermediate” in the sample after 30 s annealing and “surface” and “centre” in the sample after 60 s annealing (see Fig. 1b and c, respectively). Results of the texture measurements are presented in the form of spatial diagrams of ϕ2 = 45◦ sections of the orientation distribution function (ODF) utilising Bunge Euler angles.
3. Results and discussion 3.1. Microstructure analysis In Fig. 1, four distinctive types of microstructure of the material under investigation are presented: after primary recrystallisation, after 30 s of annealing, after 60 s of annealing and typical microstructure of the samples after 90, 120 and 150 s of annealing. In Fig. 2, the dependences of the average ¯ the effective average grain size d¯ eff and the d¯ eff /d¯ grain size d, ratio upon time are presented (0 s corresponds to the state after the primary recrystallisation). It is obvious from Fig. 2 that during the first stage of the process (up to 90 s) comparatively fast abnormal grain growth occurs, and during second stage, after columnar grains have reached the midplane, a normal grain growth is observed, which results in minor microstructure changes during this stage. Analysing kinetics of the d¯ eff /d¯ ratio simultaneously with microstructures, presented in Fig. 1, it is possible to conclude that this parameter describes heterogeneity of the microstructure. The ratio in the homogeneous columnar microstructures as well as in the initial microstructure has approximately the same value of 1.35 for all four states: initial and after 90, 120 and 150 s of annealing. In highly heterogeneous microstructure after 60 s of annealing (Fig. 1c), consisted of the central band of the relatively fine primary recrystallised grains and the surface areas of the coarse columnar grains, d¯ eff /d¯ = 2.9. In the material with intermediate level of the homogeneity after 30 s of annealing, the value of d¯ eff /d¯ is 1.9.
¯ effective average grain size d¯ eff Fig. 2. Dependences of average grain size d, and d¯ eff /d¯ ratio upon annealing time.
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Table 1 Values of average grain size d¯ and effective average grain size d¯ eff for zones in samples after 30 and 60 s of annealing Zone
Sample after 30 s of annealing d¯ (m) d¯ eff (m)
Surface Intermediate Central Global microstructure
19.2 17.2 24.7 20.8
39.3 29.0 48.0 38.4
Sample after 60 s of annealing d¯ (m) d¯ eff (m) 101.8 – 29.5 41.1
140.2 – 38.4 119.8
In Table 1, both d¯ and d¯ eff for the homogenous conventional zones in microstructure of the samples after 30 and 60 s of annealing are presented. For the microstructure with a lower level of homogeneity, when abnormal columnar grains growth only commences, both d¯ and d¯ eff global values are close to those of the surface zone. In the case of a higher level of heterogeneity, when the microstructure consists of fine-grained central band (approx. one-fourth of the whole microstructure area) and rest of it has columnar character, effective “weighted” average grain size, as it is obvious from Table 1, describes better than “classical” average grain size partial contributions of the mentioned zones to the global microstructure. 3.2. Texture analysis In Fig. 3a–c, ϕ2 = 45◦ sections of ODF of the surface, the intermediate and the central zones of the sample after 30 s of annealing are presented. From these ODFs it is obvious that the central and the intermediate zones have the similar texture with a strong peak on ␣-fibre in between J {1 1 4}1 1 0 and I {1 1 2}1 1 0 texture components and weaker rotated Goss {1 1 0}2 2 5 component. At the same time, texture of the surface zone, apart from similar to the central and the intermediate regions strongly developed {2 2 5} || ND fibre and weak {1 1 0}2 2 5 peak, has a relatively sharp cube (0 0 1)(2,−1,1,0) peak of 6.2 times random intensity (further in text all orientation intensities without indicated units are times random). The global texture of this sample (Fig. 4b) reflects features of all three zones and has quite strong {2 2 5}1 1 0 peak of 6.6 intensity with developed adjacent end of {2 2 5} || ND fibre, weaker {1 1 0}2 2 5 texture component and quite weak near H {0 0 1}1 1 0 orientation component with intensity just above 1. Hence, a conclusion can be made from this sample texture analysis: although in the global texture the intensity of the cube component is minimal, in the surface region of abnormally growing grains a sharp cube peak is observed. In Fig. 3d and e, ϕ2 = 45◦ sections of ODFs of the surface and the central zones in the sample after 60 s of annealing are presented. As it is obvious from the ODFs, the texture of the central region (Fig. 3d) is similar to that of the sample after 30 s of annealing and has a strong {1 1 3}1 1 0 peak of 10.4 intensity and rotated Goss component of 2.5 intensity. Texture of the surface area (Fig. 3e) has several texture
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Fig. 3. ODFs section ϕ2 = 45◦ :(a) after 30 s of annealing, surface zone; (b) after 30 s of annealing, intermediate zone; (c) after 30 s of annealing, central zone; (d) after 60 s of annealing, surface zone; (e) after 60 s of annealing, central zone.
components of the moderate intensity: H {0 0 1}1 1 0 component, developed θ {0 0 1} || ND and α fibres, rotated G component and{1 1 2}1 1 1 one. A common feature of the surface zones of both samples after 30 and 60 s of annealing is the presence of the cube and the rotated G components, whereas in the central zone of the samples strong J/near J component and rotated G one of moderate intensity are registered. In Fig. 4a–c, ϕ2 = 45◦ sections of global ODFs of the samples after 30, 60 and 90 s of annealing are presented, and the ODF produced from the combined data of the samples after 90, 120 and 150 s of annealing is presented in Fig. 4d. In Fig. 4e, ODF of the primary recrystallised sample is presented [1]. Considering these ODFs, it is possible to conclude that texture kinetics during applied annealing process has a distinct pattern. Whereas at the start of annealing the global texture has a very strong J component, moderate {1 1 1}1 1 0 component and weak near H component (Fig. 4a), during the first stage of columnar growth the intensities of J and rotated G component remain approximately on the same level and the intensity of cubic component drops by a half (Fig. 4b). However, after further propagation of the columnar grains towards the midplane, the intensity of the initial strong J peak significantly decreases, intensity of the rotated G component
remains approximately on the same level and the intensity of the cube component increases in the sample after 60 s of annealing (see Fig. 4d). Initial decrease of the intensity of favourable cubic component is linked with the fact that the highest spatial density of the cube oriented grains in primary recrystallised matrix is not exactly at the surface, but at the depth approximately of one-eighth of sheet thickness, although density of cube oriented grains at the surface is still significantly higher than in central zone [1]. Hence, during the first stage of annealing, abnormally propagating grains from the surface, which is in comparison with mentioned subsurface layer less favourable for the cube orientation, “consumes” this layer with the highest density of the cube oriented grains. At the same time, the columnar grains layer at this stage is still insignificant in comparison with overall strip thickness, i.e. in the global texture of this state the main orientation components, inherited from the primary recrystallised matrix, are still dominating. After further propagation of the columnar grains towards the middle plane, they become a mayor component of the microstructure (see Fig. 1c), which reflects in decrease of the intensities of the primary recrystallised matrix texture components and steady intensity of cube texture component. When the whole microstructure is columnar already,
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Fig. 4. ODFs section ϕ2 = 45◦ : (a) after primary recrystallisation, global texture; (b) after 30 s of annealing; (c) after 60 s of annealing; (d) combined data of samples after 90, 120 and 150 s of annealing; (e) after primary recrystallisation, near surface layer [1].
the texture has developed θ fibre with strong cube H component, strong {1 1 1}1 4 4/near this component, and following orientation components of moderate intensities: rotated G {1 1 2}1 1 1/near this component and L {1 1 0}1 1 0 one (Fig. 4d). It is necessary to emphasise also that the texture of columnar grained microstructure is similar to the texture of near surface layer of the primary recrystallised material (Fig. 4e, [1]), which is possible to expect because of the employed mechanism of columnar grains growth. 3.3. Mechanism of columnar abnormal grain growth The mechanism of columnar abnormal grain growth, observed in current experiment, and especially early stages of it, are different from the “classical” case, when boundaries of certain grains in the primary recrystallised matrix have higher mobility. For example, a very well known case of such a behaviour is the abnormal grain growth of Goss grains, where minor fraction of {1 1 0}0 0 1 grains in the primary matrix grows extensively consuming grains with other orientation, which results in the final state, where all grains are Goss oriented [12]. Although if improper heating rate is applied, competitive normal grain growth can occur that results in deterioration of the texture of the final material [13].
In considered in the paper [1] case, the abnormal grain growth starts from the sheet surface and is driven by the decarburised interface movement towards the midplane of the sheet. When material is heated at correct rate, at temperaturesAc1 , phase transformation, which refines the primary recrystallised matrix, begins in the inside region. At the same time, surface layers are not subjected to the phase transformation because of the low local concentration of C, and ferritic grains continue to grow in these layers. When the temperature reaches T > Ac3 (twosteps process, described in [1]), the “inner” austenitic layer of the material has relatively fine-grained microstructure because of the phase transformation, whereas the decarburised front moves towards midplane and further abnormal growth of ferritic surface layers occurs. After dropping the temperature below Ac3 and, further still, below Ac1 another refining of inner material region occurs with simultaneous intensive decarburisation and continuation of the abnormal columnar grain growth of the ferritic “surface” grains until they reach the midplane. Generally, to obtain the columnar microstructure, it is not necessary to treat steel exactly in the above-described way
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when material is subjected twice to full phase transition ␣ → ␥ → ␣. In the present experiment the one-step annealing schedule was applied at a temperature within the intercritical region, and the abnormal columnar grain growth was successful. The reason of this fact was that abnormal growth of surface layer was faster than normal grain growth of inner grains at the earlier stages of process. At later stages of the process, the size advantage of surface ferritic grains in comparison with fine-grained inner material was high enough for continuation of the directional abnormal grain growth of surface layer towards midplane. It can be predicted, that even without phase transformation such a process is possible if decarburisation is intensive enough for driving abnormal columnar grain growth towards the midplane and, at the same time, during annealing the normal grain growth is inhibited, especially during the early stages of the process. Also the temperature gradient normal to the sheet thickness can act as an additional driving force for the described abnormal grain growth. Hence, two main conditions should be met for successful propagation of abnormally growing columnar grains from surface towards midplane: (1) proper rate of decarburisation process, which depends on the temperature/heating rate, decarburising potential of annealing atmosphere and chemical composition of steel; (2) during annealing the normal grain growth in inner region of the material should be inhibited, which again depends on the temperature schedule, chemical composition of steel, decarburising potential of atmosphere as well as on the level of stored energy during cold/temper rolling, the secondary particles system in the material and the average grain size of the primary recrystallised material.
4. Summary (i) Surface grains grow abnormally in the described process, creating columnar microstructure, as a sequence of: (1) movement of the interphase front from the sheet surface towards the midplane as a result of the decarburisation process; (2) size advantage of the surface grains in comparison with the primary recrystallised material. (ii) It is possible to obtain the columnar microstructure by the application of either one-step or two-step annealing schedule, although the columnar grain growth during
two-step process with proper parameters is supposed to be more stable. (iii) The main factors controlling columnar grain growth are: temperature of annealing, heating rate, decarburisation potential of the annealing atmosphere, chemical composition of steel, level of stored energy during cold/temper rolling preceding annealing process, system of the normal grain growth pinning sites (secondary particles mostly), grain size of primary recrystallised material. (iv) The main texture components of the final columnar state of the material under investigation are: developed θ fibre with strong cube H {0 0 1}1 1 0 component, strong {1 1 1}1 4 4 component, and following components of moderate intensities: rotated G {1 1 0}1 1 2, {1 1 2}1 1 1 and L {1 1 0}1 1 0. Acknowledgements This work was supported by the Slovak Grant Agency VEGA, project No. 2/4175/24. Authors are grateful to the Corus group for providing the opportunity to conduct texture measurements at Swinden Technology Centre, Corus Research, Development and Technology under Marie Curie Host Industrial Fellowship scheme. References [1] F. Kov´acˇ , M. Dˇzubinsk´y, Y. Sidor, J. Magn. Magn. Mater. 269 (2004) 333–340. [2] F. Assumus, K. Detert, G. Ibe, Z. Metallk. 8 (1957) 344–349. [3] K. Detert, Acta Metall. 7 (1959) 589–598. [4] J.L. Walter, Acta Metall. 7 (1959) 424–426. [5] J.L. Walter, C.G. Dunn, Trans. Metall. Soc. AIME 218 (1960) 914–920. [6] S. Taguchi, A. Sakakura, Kinzoku Butsuri 7 (1968) 221–226 (in Japanese). [7] T. Tomida, J. Appl. Phys. 79 (1996) 5443–5445. [8] T. Tomida, J. Mat. Eng. Perform. 5 (1996) 316–322. [9] T. Tomida, N. Sano, K. Ueda, K. Fujiwara, N. Takahashi, J. Magn. Magn. Mater. 254/255 (2003) 315–317. ˇ Niˇzn´ık, Kovov´e Materi´aly 34 (1996) 105–111 (in Slo[10] F. Kov´acˇ , S. vakian). [11] Y. Sidor, M. Dzubinsky, F. Kovac, Mater. Charact. 51 (2003) 109–116. [12] Y. Hayakawa, J.A. Szpunar, Acta Mater. 45 (1997) 1285–1295. [13] M. Dzubinsky, F. Kovac, Scripta Mater. 45 (2001) 1205–1211.
Kinetics of columnar abnormal grain growth in low-Si non-oriented electrical steel Mykola Dˇzubinsk´y∗ , Yuriy Sidor, Frantiˇsek Kov´acˇ Institute of Materials Research of Slovak Academy of Sciences, Watsonova 47, 04353 Koˇsice, Slovakia Received 26 February 2004; received in revised form 28 June 2004
Abstract Decarburising annealing in the two-phase region is a well-known method to provide abnormal columnar grain growth in non-oriented electrical steel. Previous investigations have revealed that columnar-grained material produced in this way has an increased intensity of the “random cube” {1 0 0}0 v w and reduced {1 1 1}u v w deformation texture components in comparison with the same material with a typical equiaxed microstructure. In the present paper, the kinetics of the microstructure and texture development during the columnar grain growth is investigated by means of electron backscatter diffraction with emphasis on the early stages of the process. It has been found that texture changes reflect the character of the columnar grain growth when with further propagation of the columnar grains from the surface to the midplane the global texture becomes similar to the one of the near surface region in primary recrystallised material. © 2004 Elsevier B.V. All rights reserved. Keywords: Columnar grain growth; Microstructure; Texture; Kinetics
1. Introduction Much work has been carried out on steels to tailor their properties. A difficulty in many cases is the heterogeneity of the microstructure and the texture of the steel as a result of the production process. This is of particular importance in electrical steels. For non-oriented electrical steel it is possible to apply a decarburising annealing in the intercritical region that leads to the columnar-grained microstructure with a specific type of the texture, thus avoiding the heterogeneity [1]. Although, by application of such an annealing, it is possible to enhance desirable so-called “random cube” {1 0 0}0 v w texture component in the final material, which cannot be obtained by applying ordinary rolling and annealing processes.
∗ Corresponding author. Present address: IMMPETUS-Institute for Microstructural and Mechanical Process Engineering, The University of Sheffield, Sir F. Mappin Building, Mappin Street, Sheffield S13JD, UK. Tel.: +421 55 6338115/44 114 2227858; fax: +421 55 6337108/44 114 2227890. E-mail addresses: [email protected], [email protected] (M. Dˇzubinsk´y).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.046
First time it has been found by Assumus that high temperature annealing of very thin Si–Fe sheets leads to an abnormal growth of the cube grains [2], when the secondary recrystallisation process is controlled by the gas–metal interfacial energy, which is lowest at the (1 0 0) orientation in a high purity inert atmosphere or in vacuum [3–5]. The cross rolling and subsequent high temperature annealing of the silicon steel sheets leading to increasing the intensity of the cube texture component was investigated in [6]. Tomida et al. suggested to apply annealing in vacuum to remove the manganese and subsequent annealing in the ␥ or ␣ + ␥ two phase region in a decarburising atmosphere [7,8], or high-temperature annealing in vacuum with application of an oxide separator between the sheets [9] with the aim to achieve high intensity of the cube orientation component in the material. Nevertheless, all above described processes cannot be applied under industrial conditions. Alternatively, the two-step decarburising annealing process, which leads to the columnar grain growth, without application of the preliminary vacuum annealing, has been proposed [1,10]. In the present paper, kinetics of the columnar grain growth assisted by decarburising annealing is investigated.
450
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2. Experiment Semi-processed electrical non-oriented steel, with the following chemical composition: C = 0.05 wt.%, Mn = 0.36 wt.%, Si = 0.24 wt.%, P = 0.068 wt.%, S = 0.008 wt.%, Al = 0.109 wt.%, N = 0.005 wt.%, was used as the experimental material. Samples with a nominal thickness of 0.45 mm were taken from the industrial production process after temper rolling. Afterwards, alternatively to the process described in [1,10], the samples were annealed in a simplified manner, i.e. isothermally at 880 ◦ C in the typical decarburising atmosphere of cracked ammonia. Different annealing times were applied: 30, 60, 90, 120 and 150 s. To obtain “initial” state after primary recrystallisation, one sample was annealed in Ar atmosphere at 750 ◦ C for 6 min. Prepared in such a way the samples were scanned utilising Phillips XL40/TSL EBSD facilities, and subsequently data were analysed by OIMTM software package. The step length during the scanning was increased with increasing coarseness of the microstructure, from 1.5 m (the samples after primary recrystallisation and 30 s of decarburising annealing) to 10 m (the samples after 120 and 150 s of decarburising annealing). From the collected data, grain boundaries maps were produced, stitched together, where it was possible, and subjected to an image analysis, when microstructural parameters of each particular grain were measured. In all samples, an average grain size d¯ and an effective average grain size d¯ eff [11] were evaluated as follows: n 2 √ d¯ = √ si n π
(1)
i=1
n n 1.5 ¯deff = √2 si / si π i=1
(2)
i=1
where si is an area of ith grain and n is the number of grains in the microstructure. Although there is no clear criteria yet how to conduct a comparison of the significantly different in terms of grain size and size homogeneity microstructures utilising EBSD, we tried to get the objective result including different number of scanned “unit areas” into texture analysis depending upon the microstructures coarseness and homogeneity. One “unit area” is the longitudinal whole-thickness cross-sections of the 450 m × 600 m dimensions. A single area was used for texture calculation in the sample after primary recrystallisation. In the fine-grained sample after 30 s of annealing 2 u.a. and in the mixed-grained sample with predominantly coarse grains after 60 s of annealing 16 u.a. were taken into consideration. As during holding after 90 s at the temperature negligible microstructure changes were observed, EBSD data from all columnar-grained samples (after 90, 120 and 150 s) were combined together, i.e. 3 × 22 = 66 u.a. were taken into account. Apart from this, “zone” microstructure and texture analyses were conducted in the samples after 30 and 60 s of annealing, where all microstructure and texture pa-
Fig. 1. Microstructures (EBSD obtained grain boundaries maps) and zones in those: (a) sample after primary recrystallisation; (b) sample after 30 s of annealing; (c) sample after 60 s of annealing; (d) typical for samples after 90, 120 and 150 s of annealing.
M. Dˇzubinsk´y et al. / Materials Science and Engineering A 385 (2004) 449–454
rameters were additionally evaluated for these conventional zones, which are “surface”, “centre” and “intermediate” in the sample after 30 s annealing and “surface” and “centre” in the sample after 60 s annealing (see Fig. 1b and c, respectively). Results of the texture measurements are presented in the form of spatial diagrams of ϕ2 = 45◦ sections of the orientation distribution function (ODF) utilising Bunge Euler angles.
3. Results and discussion 3.1. Microstructure analysis In Fig. 1, four distinctive types of microstructure of the material under investigation are presented: after primary recrystallisation, after 30 s of annealing, after 60 s of annealing and typical microstructure of the samples after 90, 120 and 150 s of annealing. In Fig. 2, the dependences of the average ¯ the effective average grain size d¯ eff and the d¯ eff /d¯ grain size d, ratio upon time are presented (0 s corresponds to the state after the primary recrystallisation). It is obvious from Fig. 2 that during the first stage of the process (up to 90 s) comparatively fast abnormal grain growth occurs, and during second stage, after columnar grains have reached the midplane, a normal grain growth is observed, which results in minor microstructure changes during this stage. Analysing kinetics of the d¯ eff /d¯ ratio simultaneously with microstructures, presented in Fig. 1, it is possible to conclude that this parameter describes heterogeneity of the microstructure. The ratio in the homogeneous columnar microstructures as well as in the initial microstructure has approximately the same value of 1.35 for all four states: initial and after 90, 120 and 150 s of annealing. In highly heterogeneous microstructure after 60 s of annealing (Fig. 1c), consisted of the central band of the relatively fine primary recrystallised grains and the surface areas of the coarse columnar grains, d¯ eff /d¯ = 2.9. In the material with intermediate level of the homogeneity after 30 s of annealing, the value of d¯ eff /d¯ is 1.9.
¯ effective average grain size d¯ eff Fig. 2. Dependences of average grain size d, and d¯ eff /d¯ ratio upon annealing time.
451
Table 1 Values of average grain size d¯ and effective average grain size d¯ eff for zones in samples after 30 and 60 s of annealing Zone
Sample after 30 s of annealing d¯ (m) d¯ eff (m)
Surface Intermediate Central Global microstructure
19.2 17.2 24.7 20.8
39.3 29.0 48.0 38.4
Sample after 60 s of annealing d¯ (m) d¯ eff (m) 101.8 – 29.5 41.1
140.2 – 38.4 119.8
In Table 1, both d¯ and d¯ eff for the homogenous conventional zones in microstructure of the samples after 30 and 60 s of annealing are presented. For the microstructure with a lower level of homogeneity, when abnormal columnar grains growth only commences, both d¯ and d¯ eff global values are close to those of the surface zone. In the case of a higher level of heterogeneity, when the microstructure consists of fine-grained central band (approx. one-fourth of the whole microstructure area) and rest of it has columnar character, effective “weighted” average grain size, as it is obvious from Table 1, describes better than “classical” average grain size partial contributions of the mentioned zones to the global microstructure. 3.2. Texture analysis In Fig. 3a–c, ϕ2 = 45◦ sections of ODF of the surface, the intermediate and the central zones of the sample after 30 s of annealing are presented. From these ODFs it is obvious that the central and the intermediate zones have the similar texture with a strong peak on ␣-fibre in between J {1 1 4}1 1 0 and I {1 1 2}1 1 0 texture components and weaker rotated Goss {1 1 0}2 2 5 component. At the same time, texture of the surface zone, apart from similar to the central and the intermediate regions strongly developed {2 2 5} || ND fibre and weak {1 1 0}2 2 5 peak, has a relatively sharp cube (0 0 1)(2,−1,1,0) peak of 6.2 times random intensity (further in text all orientation intensities without indicated units are times random). The global texture of this sample (Fig. 4b) reflects features of all three zones and has quite strong {2 2 5}1 1 0 peak of 6.6 intensity with developed adjacent end of {2 2 5} || ND fibre, weaker {1 1 0}2 2 5 texture component and quite weak near H {0 0 1}1 1 0 orientation component with intensity just above 1. Hence, a conclusion can be made from this sample texture analysis: although in the global texture the intensity of the cube component is minimal, in the surface region of abnormally growing grains a sharp cube peak is observed. In Fig. 3d and e, ϕ2 = 45◦ sections of ODFs of the surface and the central zones in the sample after 60 s of annealing are presented. As it is obvious from the ODFs, the texture of the central region (Fig. 3d) is similar to that of the sample after 30 s of annealing and has a strong {1 1 3}1 1 0 peak of 10.4 intensity and rotated Goss component of 2.5 intensity. Texture of the surface area (Fig. 3e) has several texture
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Fig. 3. ODFs section ϕ2 = 45◦ :(a) after 30 s of annealing, surface zone; (b) after 30 s of annealing, intermediate zone; (c) after 30 s of annealing, central zone; (d) after 60 s of annealing, surface zone; (e) after 60 s of annealing, central zone.
components of the moderate intensity: H {0 0 1}1 1 0 component, developed θ {0 0 1} || ND and α fibres, rotated G component and{1 1 2}1 1 1 one. A common feature of the surface zones of both samples after 30 and 60 s of annealing is the presence of the cube and the rotated G components, whereas in the central zone of the samples strong J/near J component and rotated G one of moderate intensity are registered. In Fig. 4a–c, ϕ2 = 45◦ sections of global ODFs of the samples after 30, 60 and 90 s of annealing are presented, and the ODF produced from the combined data of the samples after 90, 120 and 150 s of annealing is presented in Fig. 4d. In Fig. 4e, ODF of the primary recrystallised sample is presented [1]. Considering these ODFs, it is possible to conclude that texture kinetics during applied annealing process has a distinct pattern. Whereas at the start of annealing the global texture has a very strong J component, moderate {1 1 1}1 1 0 component and weak near H component (Fig. 4a), during the first stage of columnar growth the intensities of J and rotated G component remain approximately on the same level and the intensity of cubic component drops by a half (Fig. 4b). However, after further propagation of the columnar grains towards the midplane, the intensity of the initial strong J peak significantly decreases, intensity of the rotated G component
remains approximately on the same level and the intensity of the cube component increases in the sample after 60 s of annealing (see Fig. 4d). Initial decrease of the intensity of favourable cubic component is linked with the fact that the highest spatial density of the cube oriented grains in primary recrystallised matrix is not exactly at the surface, but at the depth approximately of one-eighth of sheet thickness, although density of cube oriented grains at the surface is still significantly higher than in central zone [1]. Hence, during the first stage of annealing, abnormally propagating grains from the surface, which is in comparison with mentioned subsurface layer less favourable for the cube orientation, “consumes” this layer with the highest density of the cube oriented grains. At the same time, the columnar grains layer at this stage is still insignificant in comparison with overall strip thickness, i.e. in the global texture of this state the main orientation components, inherited from the primary recrystallised matrix, are still dominating. After further propagation of the columnar grains towards the middle plane, they become a mayor component of the microstructure (see Fig. 1c), which reflects in decrease of the intensities of the primary recrystallised matrix texture components and steady intensity of cube texture component. When the whole microstructure is columnar already,
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Fig. 4. ODFs section ϕ2 = 45◦ : (a) after primary recrystallisation, global texture; (b) after 30 s of annealing; (c) after 60 s of annealing; (d) combined data of samples after 90, 120 and 150 s of annealing; (e) after primary recrystallisation, near surface layer [1].
the texture has developed θ fibre with strong cube H component, strong {1 1 1}1 4 4/near this component, and following orientation components of moderate intensities: rotated G {1 1 2}1 1 1/near this component and L {1 1 0}1 1 0 one (Fig. 4d). It is necessary to emphasise also that the texture of columnar grained microstructure is similar to the texture of near surface layer of the primary recrystallised material (Fig. 4e, [1]), which is possible to expect because of the employed mechanism of columnar grains growth. 3.3. Mechanism of columnar abnormal grain growth The mechanism of columnar abnormal grain growth, observed in current experiment, and especially early stages of it, are different from the “classical” case, when boundaries of certain grains in the primary recrystallised matrix have higher mobility. For example, a very well known case of such a behaviour is the abnormal grain growth of Goss grains, where minor fraction of {1 1 0}0 0 1 grains in the primary matrix grows extensively consuming grains with other orientation, which results in the final state, where all grains are Goss oriented [12]. Although if improper heating rate is applied, competitive normal grain growth can occur that results in deterioration of the texture of the final material [13].
In considered in the paper [1] case, the abnormal grain growth starts from the sheet surface and is driven by the decarburised interface movement towards the midplane of the sheet. When material is heated at correct rate, at temperatures
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when material is subjected twice to full phase transition ␣ → ␥ → ␣. In the present experiment the one-step annealing schedule was applied at a temperature within the intercritical region, and the abnormal columnar grain growth was successful. The reason of this fact was that abnormal growth of surface layer was faster than normal grain growth of inner grains at the earlier stages of process. At later stages of the process, the size advantage of surface ferritic grains in comparison with fine-grained inner material was high enough for continuation of the directional abnormal grain growth of surface layer towards midplane. It can be predicted, that even without phase transformation such a process is possible if decarburisation is intensive enough for driving abnormal columnar grain growth towards the midplane and, at the same time, during annealing the normal grain growth is inhibited, especially during the early stages of the process. Also the temperature gradient normal to the sheet thickness can act as an additional driving force for the described abnormal grain growth. Hence, two main conditions should be met for successful propagation of abnormally growing columnar grains from surface towards midplane: (1) proper rate of decarburisation process, which depends on the temperature/heating rate, decarburising potential of annealing atmosphere and chemical composition of steel; (2) during annealing the normal grain growth in inner region of the material should be inhibited, which again depends on the temperature schedule, chemical composition of steel, decarburising potential of atmosphere as well as on the level of stored energy during cold/temper rolling, the secondary particles system in the material and the average grain size of the primary recrystallised material.
4. Summary (i) Surface grains grow abnormally in the described process, creating columnar microstructure, as a sequence of: (1) movement of the interphase front from the sheet surface towards the midplane as a result of the decarburisation process; (2) size advantage of the surface grains in comparison with the primary recrystallised material. (ii) It is possible to obtain the columnar microstructure by the application of either one-step or two-step annealing schedule, although the columnar grain growth during
two-step process with proper parameters is supposed to be more stable. (iii) The main factors controlling columnar grain growth are: temperature of annealing, heating rate, decarburisation potential of the annealing atmosphere, chemical composition of steel, level of stored energy during cold/temper rolling preceding annealing process, system of the normal grain growth pinning sites (secondary particles mostly), grain size of primary recrystallised material. (iv) The main texture components of the final columnar state of the material under investigation are: developed θ fibre with strong cube H {0 0 1}1 1 0 component, strong {1 1 1}1 4 4 component, and following components of moderate intensities: rotated G {1 1 0}1 1 2, {1 1 2}1 1 1 and L {1 1 0}1 1 0. Acknowledgements This work was supported by the Slovak Grant Agency VEGA, project No. 2/4175/24. Authors are grateful to the Corus group for providing the opportunity to conduct texture measurements at Swinden Technology Centre, Corus Research, Development and Technology under Marie Curie Host Industrial Fellowship scheme. References [1] F. Kov´acˇ , M. Dˇzubinsk´y, Y. Sidor, J. Magn. Magn. Mater. 269 (2004) 333–340. [2] F. Assumus, K. Detert, G. Ibe, Z. Metallk. 8 (1957) 344–349. [3] K. Detert, Acta Metall. 7 (1959) 589–598. [4] J.L. Walter, Acta Metall. 7 (1959) 424–426. [5] J.L. Walter, C.G. Dunn, Trans. Metall. Soc. AIME 218 (1960) 914–920. [6] S. Taguchi, A. Sakakura, Kinzoku Butsuri 7 (1968) 221–226 (in Japanese). [7] T. Tomida, J. Appl. Phys. 79 (1996) 5443–5445. [8] T. Tomida, J. Mat. Eng. Perform. 5 (1996) 316–322. [9] T. Tomida, N. Sano, K. Ueda, K. Fujiwara, N. Takahashi, J. Magn. Magn. Mater. 254/255 (2003) 315–317. ˇ Niˇzn´ık, Kovov´e Materi´aly 34 (1996) 105–111 (in Slo[10] F. Kov´acˇ , S. vakian). [11] Y. Sidor, M. Dzubinsky, F. Kovac, Mater. Charact. 51 (2003) 109–116. [12] Y. Hayakawa, J.A. Szpunar, Acta Mater. 45 (1997) 1285–1295. [13] M. Dzubinsky, F. Kovac, Scripta Mater. 45 (2001) 1205–1211.