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 110004, 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 strongα-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 isγ-fiber. After cold rolling, the intensity of γtexture reduces;αfiber (except{111}<110>component) intensifies and a strong and well-proportionedγ-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 recog-nized that the presence of favorable texture compo-nent in IF steels is responsible for their excellent deep drawablity, and strong{111}and weak{001} compo-nent parallel to the sheet plane produce good forma-bility. 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 tex-ture, final rolling in the ferrite region has been intro-duced in some hot strip mills[10]. Senuma and Yada[11]
has reported that deep drawing steel sheets having an average plastic strain ratio,rvalue 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
fer-ritic rolling process hinders the formation of a recrys-tallization texture with strong <111>//ND orienta-tions. Therefore, the IF steels are particularly suit-able for this ferritic rolling process due to their ultra low C and N contents and highAr1temperature (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
duc-tile hot rolled strip obtained by high temperature coiling for direct application which could be consid-ered 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 coil-ing for cold rollcoil-ing and annealcoil-ing, durcoil-ing which the † Assoc. Prof., Ph.D., to whom correspondence should be
addressed, E-mail: [email protected].
recrystallization texture strengthens by the cumulat-ing of hot rollcumulat-ing 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 in-dustrial trial and its chemical composition is shown in Table 1.
Considering the precipitation of Ti4C2S2 and
pre-venting 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
temper-ature 740◦C and a low temperature 440◦C were
em-ployed 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 an-nealing. The annealing temperature employed in this work was 750◦C and the annealing time was 1 h.
The microstructure was analyzed by optical mi-crograph to observe the status of grains. The samples were prepared in a usual way and etched with 4% nital. For texture measurement, mid-thickness speci-mens were prepared by machining and paper grinding. Macroscopic textures were measured on an X0Pert Pro
X-ray diffractometer and three incomplete pole figures ({200},{211}and{110}) were obtained. ODFs (ori-entation distribution functions) were then evaluated using Roe0s method[16]withl
Table 1 Chemical composition of the test steel (mass fraction, %)
Steel grades C Si Mn P S Ti Nb N Als
IF3O 0.0037 0.015 0.12 0.007 0.007 0.068 0.005 0.0028 0.034
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) an-nealed
Fig.3 Intensity distributions alongεfiber (a), αfiber (b) andγ fiber (c) in the condition of high temperature coiling
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 con-dition 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 recrys-tallization 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 microstruc-ture 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 maxi-mum 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 recrys-tallized microstructure in high temperature coiled
Ti-IF steel. After cold rolling, the intensity of γ tex-ture (including{111}<110>) reduces,αfiber (except {111}<110>component) intensifies and the peak in theα-fiber is broad and extends towards{223}<110> and {111}<110> with an intensity of 8. This indi-cates that the grains with γ orientation rotate to α orientation, leading to high intensity of αfiber. Af-ter annealing, the intensities of γ-fiber and compo-nents between {223}<110> and {332}<110> in the α-fiber improve. The textures inγ-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 com-ponent {111}<112> reaches 15, only 1 higher than that of {111}<110>, resulting in a uniformγ-fiber.
3.2 Microstructure and texture evolution in the con-dition 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 ev-ident that a completely deformed microstructure is produced after hot rolling and low temperature coil-ing. Straighter grain boundaries and thinner deforma-tion 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 in-cludes a strongα-fiber whose peak is at{001}<110>
Fig.6 Intensity distributions alongε fiber (a), αfiber (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 inten-sify 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 com-ponents inα-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 inten-sity 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 theα-fiber reduce with their majority being less than 2, while the intensities of components in theγ-fiber increase to higher than 10.
3.3 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 tempera-ture coiling one. Different coiling temperatempera-ture results in different hot band texture, leading to different tex-ture evolution during cold rolling and annealing. In fact, the difference in the hot band textures is at-tributed to whether the static recrystallization hap-pens or not during coiling. After coiling at low tem-perature, static recrystallization does not happen, and grains are still in rolling status. As a result, the tex-ture displays typical rolling textex-ture, which consists of α fiber and γ fiber. Grains rotate continuously during cold rolling which is equivalent to increasing reduction, leading to higher intensity ofαfiber. Fur-thermore, 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 com-plete recrystallization microstructure is produced
af-ter high temperature coiling. The hot band texture is a recrystallization texture rather than a deforma-tion texture resulted directly from rolling. Moreover, its characteristics are the same as that of the an-nealed texture in the condition of low temperature coiling, indicating that the hot band after high tem-perature coiling can be considered as a substitute for the conventional cold rolled and annealed sheet. Af-ter 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 γorientation rotate toαorientation, leading to high intensity ofαfiber. Theγ orientation shifts to-ward α orientation during cold rolling, which is in agreement with the results of Inagaki[18], who
con-cludes that crystal rotates along two paths and one is {110}<001> → {554}<225> → {111}<112> → {111}<110>→ {223}<110>. This shows thatγ-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 strongαfiber and a weak γ fiber, the intensity of {111}<110> compo-nent improves and other compocompo-nents in the γ fiber change little after cold rolling. When the hot band texture includes a strongγfiber, the intensities of all components in the γfiber decrease after cold rolling. Texture change is also different during anneal-ing. In the condition of low temperature coiling, α fiber disappears and the intensities of {111}<112> and {111}<123>increase dramatically after anneal-ing. 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 inhomoge-neousγfiber, 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 an-nealing. Obviously, when the texture before annealing includes a strongαfiber and a weakγfiber, an inho-mogeneousγ fiber is obtained; and when the texture before annealing includes a strong γ fiber, an idealγ
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 partialαfiber 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 anγ 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 temper-ature; both of them intensify after cold rolling and a completeγfiber with{111}<112>dominating devel-ops after annealing.
(2)γfiber 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 idealγ 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 Sci-ence Foundation of China for financial support, under Grant No. 50104004.
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