Effect of Coiling Temperature on the Evolution of Texture in Ferritic Rolled Ti-IF Steel

Transcription

1 J. Mater. Sci. Technol., Vol.23 No.3, Effect of Coiling Temperature on the Evolution of Texture in Ferritic Rolled Ti-IF Steel Zhaodong WANG, Yanhui GUO, Wenying XUE, Xianghua LIU and Guodong WANG The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang , China [Manuscript received May 2, 2006, in revised form July 31, 2006] The effect of coiling temperatures on the evolution of texture in Ti-IF steel during ferritic hot rolling, cold rolling and annealing was studied. It was found that texture evolution at high temperature coiling is absolutely different from that at low temperature one. The hot band texture includes a strong α-fiber as well as a weak γ-fiber after ferritic hot rolling and low temperature coiling. Both of them intensify after cold rolling and a γ-fiber with peak at {111}<112> is the main texture of annealed samples. However, the main component of the hot band texture after high temperature coiling is γ-fiber. After cold rolling, the intensity of γ texture reduces; α fiber (except {111}<110> component) intensifies and a strong and well-proportioned γ-fiber forms in the annealed samples. KEY WORDS: Ti-IF steel; Ferritic hot rolling; Coiling temperature; Texture 1. Introduction Interstitial free steels, namely IF steels, in which the remaining C and N in solution are scavenged as the precipitates by the addition of Ti and/or Nb, have been widely used, particularly in car body panels [1,2], due to their excellent formability. It has been recognized that the presence of favorable texture component in IF steels is responsible for their excellent deep drawablity, and strong {111} and weak {001} component parallel to the sheet plane produce good formability. Recently, much attention has been focused on the improvement of the final product through proper control of the hot rolling parameters [3 10]. In order to obtain high intensity of {111} recrystallization texture, final rolling in the ferrite region has been introduced in some hot strip mills [10]. Senuma and Yada [11] has reported that deep drawing steel sheets having an average plastic strain ratio, r value of higher than 1.5 can indeed be produced by ferritic hot rolling under lubricated rolling conditions and the recrystallization texture is dominated by the {111} texture. However, a low carbon steel sheet hot rolled in this possesses has poor deep drawability [12], because C in solution greatly influences the formation of a recrystallization texture [11]. The presence of C in solution in the ferritic rolling process hinders the formation of a recrystallization texture with strong <111>//ND orientations. Therefore, the IF steels are particularly suitable for this ferritic rolling process due to their ultra low C and N contents and high A r1 temperature (γ α transformation temperature). The products obtained by ferritic hot rolling can be divided into two kinds according to the coiling temperature [13]. One is a thin gauge soft and ductile hot rolled strip obtained by high temperature coiling for direct application which could be considered as a substitute for the conventional cold rolled and annealed sheet, and the other is a strained thin gauge hot strip gained by low temperature coiling for cold rolling and annealing, during which the Assoc. Prof., Ph.D., to whom correspondence should be addressed, [email protected]. recrystallization texture strengthens by the cumulating of hot rolling reduction and cold one. However, the texture evolution in the whole processing has not been discussed in detail in literature [3,8,14,15]. Therefore, the texture evolution during ferritic hot rolling, cold rolling and annealing under the condition of high temperature coiling and low temperature one was studied in this paper and the aim is to provide theory basis to obtain the best properties. 2. Experimental The experimental material was obtained from industrial trial and its chemical composition is shown in Table 1. Considering the precipitation of Ti 4 C 2 S 2 and preventing the coarsening of austenite grains, the slabs with a thickness of 230 mm were reheated at 1050 C, followed by rolling to a thickness of 46 mm during rough rolling to refine austenite grains. Finish rolling was performed in ferrite region with lubrication, and finishing temperature and the final thickness of slabs were 760 C and 5 mm, respectively. A high temperature 740 C and a low temperature 440 C were employed to investigate the effect of coiling temperature, respectively. The hot band was cold rolled with 75% reduction to a final thickness of 1.25 mm using a two-high cold reduction mill. The cold rolled samples were annealed in a special atmosphere furnace to simulate batch annealing. The annealing temperature employed in this work was 750 C and the annealing time was 1 h. The microstructure was analyzed by optical micrograph to observe the status of grains. The samples were prepared in a usual way and etched with 4% nital. For texture measurement, mid-thickness specimens were prepared by machining and paper grinding. Macroscopic textures were measured on an X Pert Pro X-ray diffractometer and three incomplete pole figures ({200}, {211} and {110}) were obtained. ODFs (orientation distribution functions) were then evaluated using Roe s method [16] with l max =16.

2 338 J. Mater. Sci. Technol., Vol.23 No.3, 2007 Table 1 Chemical composition of the test steel (mass fraction, %) Steel grades C Si Mn P S Ti Nb N Als IF3O Fig.1 Optical microstructures of samples in the condition of high temperature coiling: (a) hot rolled and high temperature coiled, (b) cold rolled, (c) annealed Fig.2 ϕ=45 ODF sections in the condition of high temperature coiling: (a) hot rolled, (b) cold rolled, (c) annealed Fig.3 Intensity distributions along ε fiber (a), α fiber (b) and γ fiber (c) in the condition of high temperature coiling

3 J. Mater. Sci. Technol., Vol.23 No.3, Fig.4 Optical microstructures of samples in the condition of low temperature coiling: (a) hot rolled and low temperature coiled, (b) cold rolled, (c) annealed Fig.5 ϕ=45 ODF sections in the condition of low temperature coiling: (a) hot rolled, (b) cold rolled, (c) annealed 3. Results and Discussion 3.1 Microstructure and texture evolution in the condition of high temperature coiling The optical micrographs of the test steels in hot rolled and high temperature coiled status, cold rolled as well as annealed one are shown in Fig.1. It can be seen from Fig.1(a) that after high temperature coiling, the deformed microstructure vanished and the recrystallization microstructure is characterized by uniform and equiaxed grains. Figure 1(b) shows that after cold rolling, the grains can not be discerned and the obvious characteristics of the cold rolled microstructure is the formation of the in-grain bands denoted by the arrow. The in-grain bands which are described as fish bone by Vanderschueren et al. [17] are vivid in the cold rolled microstructure of the steel in this work. As shown in Fig.1(c), deformed microstructure disappears and there are small and elongated grains after annealing. In order to obtain more equiaxed grains, the annealing temperature or the annealing time should be increased. Figure 2 shows the ϕ=45 ODF sections in hot rolled and high temperature coiled status, cold rolled as well as annealed one. The corresponding intensity changes of ε-fiber (<110>//TD), α-fiber (<110>//RD) and γ-fiber (<111>//ND) are also shown in Fig.3. It is clear that after ferritic rolling and high temperature coiling, the most prominent texture intensity is along the γ-fiber and the maximum is at {111}<112> with an intensity of about 12. All other texture intensities are quite low. This texture characteristic is in agreement with the recrystallized microstructure in high temperature coiled Ti- IF steel. After cold rolling, the intensity of γ texture (including {111}<110>) reduces, α fiber (except {111}<110> component) intensifies and the peak in the α-fiber is broad and extends towards {223}<110> and {111}<110> with an intensity of 8. This indicates that the grains with γ orientation rotate to α orientation, leading to high intensity of α fiber. After annealing, the intensities of γ-fiber and components between {223}<110> and {332}<110> in the α-fiber improve. The textures in γ-fiber stretch from {111}<110> to {111}<112> with a relatively strong intensity. Orientations all tend to rotate to γ-fiber, giving a sharp γ-fiber. In the TD fiber, the highest intensity is at {111}<112>, which are known to be the most stable orientation in this fiber. It can be seen from γ-fiber that the intensity of the strongest component {111}<112> reaches 15, only 1 higher than that of {111}<110>, resulting in a uniform γ-fiber. 3.2 Microstructure and texture evolution in the condition of low temperature coiling The optical micrographs of the test steels in hot rolled and low temperature coiled status, cold rolled as well as annealed one are shown in Fig.4. It is evident that a completely deformed microstructure is produced after hot rolling and low temperature coiling. Straighter grain boundaries and thinner deformation bands form after cold rolling. After annealing, the ferrite grains recrystallize completely, and small and uniform grains develop. Figure 5 shows the ϕ=45 ODF figures in hot rolled and low temperature coiled status, cold rolled as well as annealed one. The texture of hot band includes a strong α-fiber whose peak is at {001}<110>

4 340 J. Mater. Sci. Technol., Vol.23 No.3, 2007 Fig.6 Intensity distributions along ε fiber (a), α fiber (b) and γ fiber (c) in the condition of low temperature coiling as well as a weak γ-fiber whose main component is {111}<110>. The components in the α-fiber intensify and the intensity of {111}<112> in the γ-fiber changes little after cold rolling. A complete γ-fiber with the peak at {111}<112> develops and the components in α-fiber weaken evidently after annealing. The corresponding intensity changes of ε-fiber (<110>//TD), α-fiber (<110>//RD) and γ-fiber (<111>//ND) are also shown in Fig.6. It is clear that {001}<110> is the most prominent component with an intensity of 12. After cold rolling, the peak in the α-fiber moves to {114}<110> with an intensity of 18, {111}<110> intensifies, other components in the α-fiber weaken the intensities of {114}<221>- {111}<112> and {001}<110> components in the ε- fiber improve. After annealing, the intensities of all components in the α-fiber reduce with their majority being less than 2, while the intensities of components in the γ-fiber increase to higher than Discussion It is evident from above results that the evolution of texture in the high temperature coiling condition is absolutely different from that in the low temperature coiling one. Different coiling temperature results in different hot band texture, leading to different texture evolution during cold rolling and annealing. In fact, the difference in the hot band textures is attributed to whether the static recrystallization happens or not during coiling. After coiling at low temperature, static recrystallization does not happen, and grains are still in rolling status. As a result, the texture displays typical rolling texture, which consists of α fiber and γ fiber. Grains rotate continuously during cold rolling which is equivalent to increasing reduction, leading to higher intensity of α fiber. Furthermore, the most prominent component transfers from {001}<110> to {114}<110>, and the intensity of γ fiber improves a little, but is still much weaker than α fiber after cold rolling. However, the complete recrystallization microstructure is produced after high temperature coiling. The hot band texture is a recrystallization texture rather than a deformation texture resulted directly from rolling. Moreover, its characteristics are the same as that of the annealed texture in the condition of low temperature coiling, indicating that the hot band after high temperature coiling can be considered as a substitute for the conventional cold rolled and annealed sheet. After cold rolling, the intensities of γ fiber (including {111}<110>) reduce and those of α fiber (except {111}<110>) intensify. This indicates that grains with γ orientation rotate to α orientation, leading to high intensity of α fiber. The γ orientation shifts toward α orientation during cold rolling, which is in agreement with the results of Inagaki [18], who concludes that crystal rotates along two paths and one is {110}<001> {554}<225> {111}<112> {111}<110> {223}<110>. This shows that γ-fiber is not necessarily intensified during cold rolling and its change depends on the hot band texture. When the hot band texture consists of a strong α fiber and a weak γ fiber, the intensity of {111}<110> component improves and other components in the γ fiber change little after cold rolling. When the hot band texture includes a strong γ fiber, the intensities of all components in the γ fiber decrease after cold rolling. Texture change is also different during annealing. In the condition of low temperature coiling, α fiber disappears and the intensities of {111}<112> and {111}<123> increase dramatically after annealing. It can be seen from Fig.6 that the intensity of {111}<123> increases from 2 to 11, but the intensity of {111}<110> decreases, resulting in an inhomogeneous γ fiber, which gives strong anisotropy and goes against deep drawability. In the condition of high temperature coiling, an ideal texture to be beneficial to improving deep drawability is developed after annealing. Obviously, when the texture before annealing includes a strong α fiber and a weak γ fiber, an inhomogeneous γ fiber is obtained; and when the texture before annealing includes a strong γ fiber, an ideal γ

5 J. Mater. Sci. Technol., Vol.23 No.3, fiber is formed. This is in consistent with the results of Barnett and Jonas [19,20]. Their results show that, when rolling was finished at 70 C and 300 C, the hot band texture includes a partial α fiber and a complete γ fiber, and the annealed texture is an ideal γ fiber; when rolling was finished at 700 C, γ fiber does not show evident advantage, and the annealed texture is an γ fiber with {111}<112> in the ascent. Thus, in order to obtain a beneficial recrystallization texture, γ fiber should be the most prominent texture in the unannealed texture. 4. Conclusions (1) The hot band texture consists of a strong α fiber and a weak γ fiber after coiling at low temperature; both of them intensify after cold rolling and a complete γ fiber with {111}<112> dominating develops after annealing. (2) γ fiber is the only component in the hot band texture after high temperature coiling; the intensity of γ fiber reduces and that of α fiber increases after cold rolling and a uniform γ fiber is developed after annealing. (3) An ideal γ fiber to be beneficial to improving deep drawability can form after annealing only if γ fiber dominates in the texture before annealing. Acknowledgement The authors are grateful to the National Natural Science Foundation of China for financial support, under Grant No REFERENCES [1 ] R.Mendoza, M.Alanis, O.Alvarez-Fregoso and J.A.Juarez-Islas: Scripta Mater., 2000, 43(8), 771. [2 ] T.Senuma: ISIJ Int., 2001, 41(6), 520. [3 ] A.Tomitz and R.Kaspar: Steel Res., 2000, 71(12), 497. [4 ] V.J.Martinez, J.I.Verdeja and J.A.Pero-Sanz: Mater. Charact., 2001, 46(1), 45. [5 ] P.Juntunen, D.Raabe, P.Karjalainen, T.Kopio and G.Bolle: Metall. Mater. Trans. A, 2001, 32(8), [6 ] L.K.Tung, M.Z.Quadir and B.J.Duggan: Key Eng. Mater., 2002, (1), 437. [7 ] A.Tomitz and R.Kaspar: ISIJ Int., 2000, 40(9), 927. [8 ] Zhaodong WANG, Yanhui GUO, Zhong ZHAO and Daqing SUN: J. Northeastern Univ., 2005, 26(8), 747 (in Chinese) [9 ] H.Zhao, S.C.Rama, G.C.Barber, Z.Wang and X.Wang: J. Mater. Process. Technol., 2002, 128(1-3), 73. [10] P.J.Hurley and P.D.Hodgson: Mate. Sci. Eng. A, 2001, 302(2), 206. [11] T.Senuma and H.Yada: Proc. Int. Conf. on Phys. Metall. Thermomech. Proc. Steels Other Metals, Iron Steel Inst., Tokyo, Japan, 1988, 628. [12] T.Senuma, H.Yada, R.Shimizu and J.Harase: Acta Metall. Mater., 1990, 38(12), [13] Xinping MAO: Iron Steel, 2004, 39(5), 71. (in Chinese) [14] M.R.Toroghinejad, A.O.Humphreys, D.S.Liu, F.Ashrafizadeh, A.Najafizadeh and J.J.Jonas: Metall. Mater. Trans. A, 2003, 34(5), [15] Xiaojun GUAN, Yun LI and Zuocheng WANG: Iron Steel, 2004, 39(9), 58. (in Chinese) [16] R.J.Roe: J. Appl. Phys., 1965, 36(6), [17] D.Vanderschueren, N.Yoshinaga and K.Koyama: ISIJ Int., 1996, 36(8), [18] H.Inagaki: ISIJ Int., 1994, 34(4), 313. [19] M.R.Barnett and J.J.Jonas: ISIJ Int., 1997, 37(7), 697. [20] M.R.Barnett and J.J.Jonas: ISIJ Int., 1997, 37(7), 706.