Factors Affecting Texture Memory Appearing through!! Transformation in IF Steels

Transcription

1 Materials Transactions, Vol. 48, No. 8 (7) pp. 36 to 4 Special Issue on Crystallographic Orientation Distribution and Related Properties in Advanced Materials #7 The Japan Institute of Metals Factors Affecting Texture Memory Appearing through!! Transformation in IF Steels Naoki Yoshinaga, Hirofumi Inoue, Kouichi Kawasaki 3, Leo Kestens 4 and B. C. De Cooman 4; * Steel Research Labs., Technical Development Bureau, Nippon Steel Corporation, Futtsu 93-85, Japan Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai , Japan 3 Advanced Research and Technology Center, Niihama National College of Technology, Niihama , Japan 4 Department of Metallurgy and Materials Science, University of Ghent, Technologiepark 9, BE-5, Zwijnaarde (Gent), Belgium In case where IF steel is heat treated in region,! and! transformation during heating and cooling, i.e.!! transformation, take place during heating and cooling, respectively. The initial texture of is potentially weakened due to two times transformation in general. On the contrary, the!! transformed texture resembles the initial texture clearly under specific circumstances. Authors call this phenomenon texture memory. A very distinct texture memory effect was found in IF steels. Lowering the! transformation temperature pronounces texture memory significantly. The variant selection at grain boundary of seems to play an important role. Moreover, it is considered that the initial texture has to be sharp to in order to realize texture memory. [doi:.3/matertrans.ma74] (Received February 3, 7; Accepted April 3, 7; Published July 5, 7) Keywords: texture memory,!! transformation, ferrite, austenite, recrystallization, ultra low carbon steel, Interstitial Free steel, cold rolled sheet steel, synchrotron radiation, high strength steel, variant selection, Kurdjumov-Sachs relationship. Introduction It is well known that heat treatment in the single phase region randomizes the hi == ND texture through!! transformation in deep-drawable cold-rolled sheet steels.,) On the other hand, it has been observed that the initial texture hardly changes in microalloyed low-carbon steels with a considerable amount of Mn such as.9 mass%c-.6 mass%mn-.3 mass%nb, 3). mass%c-.35 mass%mn-.3 mass%nb 4) and.9 mass%c-.35 mass% Mn-.6 mass%nb 5) controlled-rolled sheet steels, in which martensitic transformation took place during cooling. In these studies, the mechanism was not discussed. One of the present author 6 8) has found that the initial recrystallization texture is almost completely recovered even after!! transformation in ultra low-carbon sheet steels without need for a martensite-like transformation. This phenomenon will be called texture memory, 7,8) hereafter. Some factors, which may affect texture memory, were investigated in order to understand its mechanism.. Experimental Procedure. Influence of Mn on!! transformation texture formation Vacuum melted ultra low carbon steels were used. The chemical compositions and processing conditions are listed in Table. Steel NT is a Nb and Ti added ultra low carbon mild steel. Steel MP contains Mn and P. The ingots were hotrolled, heat treated for coiling simulation and cold-rolled. After cold-rolling, the heat treatments, which are shown in Fig., were performed. In order to complete recrystallization *Present address: Materials Design Laboratory, Graduate Institute for Ferrous Technology, Pohang University of Science and Technology before transformation, the heat treatment in ferrite region at 88 C and 84 C for 6 seconds were performed in steels NT and MP, respectively. It was confirmed by dilatometry that 96 C was the temperature of the single region for both steels. 7) Conventional X-ray diffraction method was used to measure pole figures. In case where the!! transformation texture resembles the recrystallization texture, the texture memory effect is considered to appear. The samples used for the texture measurement were taken from the quarter-plane of the thickness. Orientation distribution functions (ODFs 3) ) were calculated from the {}, {}, {} and {3} complete pole figures.. Measurement of austenite texture at high temperature by synchrotron radiation The high temperature in-situ texture measurement in the single phase region by synchrotron radiation (SR) was carried out at Photon Factory in National Laboratory for High Energy Physics in Japan. Using the advantage of the high brightness of SR, a method for rapid projection of crystal orientation distribution was developed. 9 ) Figure gives a schematic drawing of the equipment used in this technique. The cold-rolled steels were mechanically and chemically polished to obtain a thickness of.5 mm. The quarter-plane of the thickness in the cold-rolled sheet was taken as a center of the specimen for SR measurement. The sample sheet was set on the sample stage of the diffractometer and a screen with an arc-shaped window was fixed behind the sample stage to detect a part of the {} diffraction cones. An Imaging Plate (IP) was positioned behind the screen to project the crystal orientation distribution. The samples were heated to 96 C at the rate of C/s and kept at 96 C for 6 seconds in a furnace with a carbon heater. Argon gas was fed into the furnace to protect the sample against oxidation. The X-ray beam, which was monochromatized to a wavelength of

2 Factors Affecting Texture Memory Appearing through!! Transformation in IF Steels 37 Table Chemical compositions and processing parameters. C Si Mn P S N Al Ti Nb SRT, C FT, C CT, C CR, % NT MP SRT: Slab Reheating Temperature, FT: Finishing Temperature, CT: Coiling Temperature, CR: Cold-Rolling Reduction Table Chemical compositions. (mass%) Samples C Si Mn P S N Al Ti Nb B TB NB NB Figure 4 T C, 6s 8 C/s C/s T C, 6s 96 C, 6s C/s 8 C/s Table 3 Hot-rolling and cold-rolling conditions. Sanples SRT, C FT, C CT, C CR, % TB NB NB Fig. Heat treatment after cold-rolling. T: 88 C for steel NT, 84 C for steel MP. heat cycle for recrystallization, heat cycle for!! transformation. T C-6s 85 C-6s C/s C/s C/s 95 C-t(min) 85 C-6s C/s C/s C/s T, C:, 5, t, min :, 3, 6, 8 Fig. 3 Heat-treatment after cold-rolling. Fig. Schematic illustration showing the equipment for texture measurement at high temperature using SR..6 nm, was focused on the sample. The IP was rotated with an angular velocity synchronized with the velocity of the sample rotation. The crystal grains were projected over an angle range of 5. The texture measurement started just after the heat treatment at 96 C for 6 seconds, mentioned above, and then took seconds at the same temperature..3 Influence of! transformation temperature The following attempt was made to confirm the influence of the! transformation temperature on texture memory. Vacuum-melted ingots of steels TB, NB and NB (Table ), were hot-rolled and cold-rolled under the conditions listed in Table 3. The specimens were heated up to the temperatures ranging from to followed by cooling at the various rates from to 8 C/s..4 Influence of heating condition in austenite region The industrially produced hot-rolled IF high strength steel with the chemical composition listed in Table 4, was used to investigate the influence of the heating temperature and time in the phase region. For this purpose, cold-rolled sheets were heat-treated at higher temperatures or for longer times, as indicated in Fig Influence of externally applied elastic stress The IF-6 material industrially hot-rolled and cold-rolled in a laboratory mill (Table 4) was used to investigate the influence of an external elastic stress on the!! transformation texture formation. Figure 4 represents the kinetics of the! transformation at 75 C. After holding for 6 seconds at 75 C, the transformation progressed more than 5%. Therefore, the nucleation of the! transformation is thought to be almost completed, thus the transformation variant should already be selected after 6 s. Furthermore, it is important not to introduce significant amount of plastic deformation to the transformed grains since the texture of can be changed as a result. For these Table 4 Chemical compositions of steels used (mass%) and processing parameters. C Si Mn P S N Al Ti Nb B SRT, C FT, C CT, C CR, % IF

3 38 N. Yoshinaga, H. Inoue, K. Kawasaki, L. Kestens and B. C. D. Cooman Fraction transformed C-6s 85 C-6s C/s C/s 75 C-8s C/s 6. 6 Time at 75 C, t/s Fig. 4 Isothermal! transformation kinetics at 75 C. PHI= 45 PHI= C-6s 95 C-6s Load stress at 75 C σ: 5~MPa Strain rate : 3-3 /s Fig. 6 Recrystallization texture and!! transformation texture ( ¼ 45 section) in steels NT. C/s 75 C-6s Unload after 6s C/s C/s 4 4 Fig. 5 Experimental conditions to examine the effect of external stress applied during! isothermal transformation on!! transformation texture formation in IF-6. reasons, the experimental conditions were determined as described in Fig. 5. The stress level was changed from the elastic to the plastic region, i.e. in the range from 5 to MPa, the temperature was 75 C and the time for which the stress was applied was 6 s. 3. Experimental Results 3. Influence of Mn on recrystallization and!! transformation textures The textures obtained after recrystallization and!! transformation in steels NT and MP are shown in Figs. 6 and 7, respectively. It is clear that the hi == ND fiber is sharply developed in both steels. In steel NT, however, the texture is weakened remarkably after the!! transformation. On the other hand, the!! transformation texture is very similar to the recrystallization texture in steel MP, although the intensity of the fghi component increases slightly. In other words, the recrystallization texture is memorized even after the!! PHI= 45 PHI= 45 Fig. 7 Recrystallization texture and!! transformation texture ( ¼ 45 section) in steels MP. transformation in steel MP, which is a Mn-added steel. It is worth mentioning that the microstructure of steel MP after the!! transformation is significantly different from the recrystallization microstructure (Figs. 8 and 9). 3. Texture of at high temperature measured by SR In order to clarify the mechanism of texture memory mentioned above, the high temperature in-situ texture measurement in the single region was carried out by means of SR X-ray diffraction. After the background was subtracted, the intensity was normalized under the assumption that the average intensity of the whole area is.. An absorption correction was also made. Comparison between the texture of steel NT and that of steel MP reveals no essential differences, as seen in Fig.. Fig. 8 Microstructures after recrystallization and!! transformation texture ( ¼ 45 section) in steels NT.

4 Factors Affecting Texture Memory Appearing through!! Transformation in IF Steels 39 Fig. 9 Microstructures after recrystallization texture and!! transformation texture ( ¼ 45 section) in steels MP. RD RD 36 Fig. MP. TD {} pole figures of measured by SR, steel NT and steel TD Fig. Numbering of the meshes in the inverse pole figure applied to eq. (). R Steel TB Steel NB Steel NB Steel MP Steel NT Ar 3 temperature, C Fig. Relationship between Ar 3 temperature and R (Recovery ratio, see eq. ()). 3.3 Influence of! transformation temperature on texture memory The! transformation temperature of the cold-rolled sheet, whose chemical compositions are listed in Tables and, were measured by dilatometry. The factor R in equation (), which represents the appearance of the texture memory, was calculated and plotted in Fig.. The parameter R is defined by equation (). It means that texture memory appears more significantly, when the R is closer to. It is clear from Fig. the texture memory appears when Ar 3 transformation temperature is below around 8 C. R ¼ I =I ðþ I : Integrated intensity of orientations 8, 35 and 36 (Fig. ) in the recrystallization texture. I : Integrated intensity of orientations 8, 35 and 36 (Fig. ) in the!! transformation texture. In order to assess the effect of the! transformation temperature more critically, the following test was conducted. Steel NT was water-quenched after holding at 93 C where it is in the single phase range. The texture memory occurs even in steel NT if a sufficiently rapid cooling rate, which results in the lowering of the! transformation temperature, is used. This effect can be clearly seen in Fig. 3. It should be noted that the microstructure is still polygonal even though water quenching is carried out and the grain size is smaller compared to the specimen cooled at 8 C/s. 3.4 Influence of heating temperature and time in the austenite region Figure 4 represents the influence of holding temperature and time in the range. It is obvious from Fig. 4 that the recrystallization texture formed at 85 C is weakened as the heating temperature and the holding time increase. This result is different from the results obtained by Hutchinson et al. ) There are a couple of differences between their experiment and the present one. Although they used a similar chemical composition, their Ti content was much higher, i.e. Fe-.3 mass%c-. mass%n-. mass%mn-.7 mass% Ti. Moreover, their austenization temperature was 95 C whereas it was 95 C or higher in the present study. The

5 4 N. Yoshinaga, H. Inoue, K. Kawasaki, L. Kestens and B. C. D. Cooman (c) Fig. 3 Influence of cooling rate after soaking in the single region (93 C 6 s) on!! transformation texture of steel NT (Table ). {} pole figures. recrystallization texture, cooled at 8 C/s from 93 C, (c) water quenched from 93 C. (c) (d) PHI= 45 PHI= 45 PHI= 45 PHI= 45 Fig. 4 Changes in!! transformation texture in IF-6 (Table 4) with changes in holding temperature and time in region. a: 85 C 6 s, b: 95 C 6 s, c: 95 C 8 min., d: C 6 s. (c) (d) PHI= 45 PHI= 45 PHI= 45 PHI= 45 Fig. 5 Effect of the external tensile stress applied during the! isothermal transformation at 75 C on the!! transformation texture in IF-6. 5 MPa, 5 MPa, (c) 75 MPa and (d) MPa. Yield strength of at 75 C is around 75 MPa in this material. Contour levels: chemical composition and the temperature could directly correlate to the grain growth behavior or stress and strain condition in region. It is speculated, therefore, that these difference could bring about the different observations. 3.5 Influence of external elastic stress The!! transformation textures when the external stresses are applied during the isothermal transformation at 75 C are shown in Fig. 5. The yield strength of at 75 C in IF-6 is approximately 75 MPa. Therefore, the textures shown in Fig. 5, and (c) are formed under the influence of elastic stress. The results of Fig. 5 therefore prove that the external stress does not have significant effect on the formation of a!! transformation texture. 4. Discussion 4. Characteristics of the texture A prediction of the phase texture was made using the ODF 3) to understand the measured texture shown in Fig.. In this calculation the measured recrystallization textures shown in Figs. 6 and 7 were used as initial textures. The K-S relationship 4,5) without variant selection was used to convert the recrystallization textures to textures. It is obvious from the calculated results in Fig. 6 that there is a very good agreement between the measured textures shown in Fig. and the simulated ones. Therefore, it can be concluded that textures of both steels are formed by! transformation based on K-S relationship without any.3.3 Fig. 6 Simulated textures in steels NT and MP based on K-S relationship without specific variant selection (cf. Fig. ).

6 Factors Affecting Texture Memory Appearing through!! Transformation in IF Steels 4 specific variant selections. In the textures, the hi == ND fiber whose main component is fgh3i is observed. This hi == ND fiber is considered to originate from the hi == ND fiber in the recrystallization texture. Rex- α A B 4. Possible mechanisms for texture memory The following experimental facts were obtained: a) Mn addition enhances the texture memory effect, b) if a sufficiently rapid cooling rate is applied during! transformation, the texture memory takes place even in a steel without Mn, c) the texture of in Mn alloyed steel is not different from that in the steel without Mn, d) the texture of is quite weak, which can be predicted by converting the initial recrystallization texture according to the K-S relationship without variant selection, e) the! transformation temperature has a dominant effect on the texture memory, i.e. when the! transformation temperature is lower than approximately 8 C, the texture memory is always observed regardless of chemical composition, f) holding at a higher temperature and a longer period in region suppresses the texture memory and g) an external elastic tensile stress does not influence on the texture memory significantly. From the results c) and d), the texture memory is considered to arise from a specific variant selection mechanism during! transformation. The results a) and b) can be considered equivalent to the observation e). Therefore, the model for the texture memory has to be developed for the variant selection during! transformation. Moreover, it has to be consistent with the observations e), f) and g). Numerous models for the formation of! transformation texture have been proposed so far.,5 3) Since the texture in the present study is very weak as shown in Fig., the model is required to deal with the behavior of!! transformations of individual grains or colony of grains in order to predict the distinct texture after transformation. From this aspect the conventional models which required either a macroscopic deformation such as rolling and tensile strain 5,6,8,9) or a clear texture prior to the transformation,) cannot be applicable. The model, which is only useful for the martensitic transformation related phenomenon 7) should be also ruled out because the texture memory effect is observed in the material in which the martensitic transformation is not involved. Surface effect related model ) cannot be applied either as the texture memory occurs in the bulk. As a consequence, the conventional models applicable to the texture memory effect could be the followings: ) the grain boundary model,,4 6) ) the internal stress based model. 6 8) The likelihood of ) above mentioned has been already discussed in the previous papers. 6 8) However, if the stress might play a potential role in texture memory the external stress also has to influence on it. Nevertheless, the experimental result g) above mentioned clearly shows this is not the case. Therefore, hereafter, the model ), i.e. the grain boundary model, will be discussed. The grain boundaries could play a role since the grain α γ γ α a A a Fig. 7 Schematic drawing of microstructure change through!! transformation. The grains a and b have ones of the K-S variants of grains A and B, respectively. A indicates the nucleus of transformation. boundaries are commonly recognized as one of the most potential nucleation site. Ameyama et al.,4) and Hakata 5) have investigated the variant selection according to the grain boundary characteristics precisely. The essence of their variant selection model is the following. Considering the grain nucleates at the grain boundary between two ferrite grains, i.e. A and B, there are in principle 48 variants which the grain may choose from since for each grain 4 variants can occur. The first principle which grain must obey is that the {} plane of the new grain satisfies the K-S relationship with one of the {} planes of A orb which has smallest angle to the grain boundary plane. The new grain selects one of twelve {} planes of AorBso that only 4 variants out of the 48 variants are possible. The second principle which is to be taken into account is the orientation relationship between and A in the case that a {} plane of obeys the first condition with respect to the B grain. Among 4 possible variants, the one which results in the smallest angle between the {} plane of the grain and the {} plane of A is selected. If there is the variant of which can satisfy K-S relationship with both A and B grains it is natural to consider this grain has to precipitate most likely. In fact, Ameyama and Maki 7) have clearly observed in duplex stainless steel. Considering the!! transformation in general, the crystal orientation of grain A is not equal to that of grain B in Fig. 7. Therefore, there is no specific orientation relationship between grains a and b of, generally. In this case there is no need that the orientation of A, nucleus of ferrite, coincides with that of grain A or B. On the contrary, when the orientation of grain A is very similar to that of grain B, i.e. a rather small angle grain boundary exists between the grains A and B, the grains a and b have ones of the 4 K-S variants of orientation of grain A (or B). This situation can be realized when the recrystallization texture is significantly sharp. In this specific case, the grain A has common K-S variant for the grain a and the grain b. Namely this is the b b

7 4 N. Yoshinaga, H. Inoue, K. Kawasaki, L. Kestens and B. C. D. Cooman orientation of grain A (or B). This way the texture memory can appear. It can be therefore concluded that the distinct texture memory after!! transformation can appear only when initial has a significantly sharp texture. Next, the consistency of this mechanism with respect to the experimental results e), f) and g) mentioned above will be checked. When the! transformation temperature decreases the interfacial energy due to generation of nuclei must increase, since atomistic accommodation by diffusion becomes difficult to occur at the interface. K-S relationship must be obeyed more critically in case where the! transformation temperature becomes lower in order to minimize the interfacial energy. Therefore, it is considered that the texture memory can be more pronounced, when the transformation temperature decreases. The increase in holding temperature and time in region can lead to grain growth. Therefore the orientation relationship between the adjacent grains can change. At such newly formed grain boundaries, the variant selection rule at the grain boundary mentioned above cannot hold in most cases. In this manner, the texture memory tend not to appear clearly in case that the holding temperature and time in region. Since the orientation relationship between the grains does not seem to change due to the externally applied elastic stress the variant selection rule at grain boundary can still hold. Therefore, the texture memory can appear even when the macroscopic stress is applied during! transformation. Thus, the mechanism for the texture memory proposed above is considered to be consistent to the experimental results e), f) and g). 5. Conclusion The texture memory effect, whereby the recrystallization texture is completely recovered after an!! transformation, was found to occur in ultra low carbon cold rolled sheet steels. Systematic investigations were carried out in order to reveal the mechanism of the texture memory. The following experimental facts were obtained. () Mn addition enhances the texture memory effect. () If a sufficiently rapid cooling rate is applied during! transformation, the texture memory takes place even in the steel without Mn. (3) The textures at high temperature were measured using synchrotron radiation. The texture of in Mn alloyed steel is not different from that in steel without Mn. (4) The texture of is quite weak, which can be predicted by converting the initial recrystallization texture according to K-S relationship without variant selection. (5) The! transformation temperature has a significant effect on the texture memory, i.e. when the! transformation temperature is lower than approximately 8 C, the texture memory is always observed regardless of chemical composition. (6) Holding at a higher temperature and a longer time in the region suppress the texture memory. (7) The external elastic tensile stress does not influence on the texture memory significantly. The texture memory arises clearly from a specific variant selection mechanism occurring during the! transformation. The variant selection at grain boundaries is considered to play an essential role in texture memory. When the initial texture is very sharp the probability, which the K-S variant of common to both grains at grain boundary exists, is considered to increase. This variant should coincide with the main orientation of the sharply developed initial texture. The influence of the initial texture should be investigated in detail in the future. Acknowledgments We are grateful to Prof. T. Maki in Kyoto University for his useful comments on the present work. Thanks are also extended to Dr. M. Takahashi for his helpful advice. REFERENCES ) R. M. S. B. Horta, D. V. Willson and W. T. Roberts: J. Iron and Steel Inst. (97) 4. ) O. Hashimoto, S. Satoh and T. 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