Effect of hot rolling condition on the development of textures in ultra low carbon steel

Journal of

Materials Processing Technology ELSEVIER

J. Mater. Process. Technol. 45 (1994) 471-476

E f f e c t of hot rolling condition on the development of t e x t u r e s in ultra low carbon steel Y. B. P a r k a, D. N. Lee b and G. Gottstein c aDept, of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, 540-742, K o r e a bDept, of Metallurgical Engineering, Seoul National University, Seoul, 151-742, K o r e a CInstitut ft~r Metallkunde und Metallphysik, R W T H Aachen, K o p e r n i k u s s t r . 14, 52056 Aachen, G e r m a n y

T h e d e v e l o p m e n t of hot and cold rolling textures in T i - b e a r i n g ultra low carbon steel h a s been i n v e s t i g a t e d laying emphasis on effects of various p r o c e s s i n g p a r a m e t e r s such as slab r e h e a t i n g temperature, hot rolling temperature, partition of hot rolling and cold rolling reductions. {225}<554>, {001}<110> and {225}<110> are s u g g e s t e d as a t t r a c t o r s of t e x t u r e components during deformation, which have 29.5 ° rotation relationship to a common <110> axis with r e s p e c t to each other. In chaotic rotation of g r a i n s during deformation, if the material lies under shear condition, {225}<554> appears as a main t e x t u r e component. Due to crystallographic instability, {225}<554> easily alters to {001}<110> and further to {225}<110> under plane strain condition a n d / o r with i n c r e a s i n g rolling reduction.

1. ~ T R O D U C T I O N

Plastic a n i s o t r o p y is one of the most important mechanical properties to obtain good formability of steel sheets. Due to the direct relevance of plastic anisotropy to the crystallographic texture, all p r o c e s s e s to produce steel sheets are aimed to control the t e x t u r e development. It h a s long been recognized that the stronger the development of { 111 } parallel to sheet plane, so called the 7 - f i b r e texture, the better the deep drawability. T h u s g r e a t efforts have been made for the development of the T-fibre t e x t u r e in deep d r a w i n g steels and interstitial free (IF) steels have been introduced as an alternative.

Recently, however, much higher formability is needed in the steel sheets for a special use, especially for automotive panel in order to meet the demand such as complex shape, o n e - b o d y forming, w e i g h t reduction and so on. F o r this reason attempts to obtain the optimum processing p a r a m e t e r s of I F steels have been made. B e c a u s e one p r o c e s s i n g p a r a m e t e r is related with the others, its small c h a n g e b r i n g s about the different development of t e x ~ r e s and consequently a g r e a t difference of deep drawability in final products [1]. M a n y studies on I F steels h a v e reported uncompatible results even with similar processing p a r a m e t e r s so that the development of their t e x t u r e s can not be

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472

exactly predicted u n d e r some processing conditions. T h i s mainly results from the fact that the basic mechanism of the texture evolution is not yet clarified in steels. Nevertheless, it is noticeable that they have commonly suggested a few important processing parameters affecting the development of textures [2-3]. As summarized, not only the annealing process which defines the recrystallization textures is important b u t also the hot rolling condition and the degree of s u b s e q u e n t cold rolling should be considered to develop the prerequisite textures for the former. In the p r e s e n t work to systematically investigate the texture evolution of IF steel, hot rolling textures have been made to be significantly different u n d e r various hot rolling conditions such as slab reheating temperature, rolling schedule and rolling temperarure. The formation and the effect of shear tex~res on the s u b s e q u e n t texture development have been examined with the help of ODF analysis. It is also one of the objectives to illucidate the occurrence of texture inhomogeneity through the sheet thickness during hot and cold rolling. T h u s the origin of shear texture and its effect on the development of subsequent textures have been examined and discussed in relation with processing parameters. 2. E X P E R I M E N T A L W O R K T i - b e a r i n g ultra low carbon steel (Ti = 0.04% and C < 0.002%) produced from Pohang Steel Company (POSCO) in Korea were used in the present work. T h e hot roiling conditions of specimen A, B, C and D were varied as follows. T h e slab reheating temperature of only specimen A was 1200 °C and the others were soaked at 1100 °C. For specimen D, the hot rolling reduction increases with pass and thus great deformation was made at the finishing rolling whereas for

others, the rolling reduction was the smallest at the finishing pass. Specimen A and B were hot rolled in the region of 7-austenite. On the other hand, the 4th and the 5th passes of specimen C and D were carried out in the region of a -ferrite. T h e coiling temperature was 620 °C in all specimens. T h e hot strips were subsequently cold rolled using a laboratory mill to reductions of 70%. 80°/6, 85% and 90%, respectively. T e x t u r e s of various thickness layers from center to surface of the specimens were measured with the help of X - r a y diffraction. ODFs were determined b y series expansion method (lmax--22) [4] from the four pole figures of {110}, {200), {112} and {103}. T h e orientation densities were displayed either in an orientation space formed b y the three Euler angles, ~1, O, ~2 or along the orientation tubes such as a-fibre, T-fibre and so on as shown in Figure 1

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Figure 1. Reduced Euler space with some important fibres and orientations for bcc materials.

473

expected to be satisfied, r e v e a l s the considerable development of rolling texture. In specimen A, w h e r e the slab reheating t e m p e r a t u r e is 1200 °C and thus hot rolling is performed at the higher temperature, {001}<110> appears as a texture component with the m a x i m u m orientation density. W h e n the slab reheating and hot rolling temperatures are lowered, the orientation density of {001}<110> i n c r e a s e s as well as {hkl}<011> t e x t u r e components are developed along the a - f i b r e as shown in specimen B and C. O D F s of specimen D is similar to that of cold rolling textures, which is attributed to the severe deformation during the finishing rolling in the ferrite region. It is, therefore, recognized that the lower the slab

3. Hot Roiling T e x t u r e O D F s at the surface layer (s = 1.0) of hot rolled specimen A, B, C and D reveal diffused t e x t ~ e s mainly due to dynamic recrystallization in austenite region and s u b s e q u e n t p h a s e transformation from austenite to ferrite during cooling. As shown in F i g u r e 2(a) to 2(d), the distinctive s h e a r texture, the m a x i m u m component of which is {225}<554>, is f o r m e d at the thickness layer close to surface (s = 0.8). A t the m i d - t h i c k n e s s l a y e r b e t w e e n the surface and the center (s

= 0.4),

textures

are

diffused

because

of the t e x t u r e transition that the shear t e x t u r e components d i s a p p e a r and w e a k {001}<110> s t a r t s to appear. A s shown in F i g u r e 2(e) to 2(h), the center layer (s = 0.0), w h e r e plane strain condition is

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(a) specimen A

(b) specimen B

(c) specimen C

(d) specimen D

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F i g u r e 2. O D F s of hot rolling t e x t u r e s at s = 0.8 ( a - d ) and s = 0.0 (e-h).

474

r e h e a t i n g and fi n i s h i n g temperatures, the more fiberous the hot rolling textures and c o n s e q u e n t l y the closer to cold rolling t ex t u r es . It follows from the examination of main t e x t u r e c o m p o n e n t s developed at each thickness l ay e r that there exist special rotation relationships. {225}<554> at s : 0.8 and {001}<110> at s = 0.0 h a v e a 29.5 ° rotation relationship to a c o m m o n <110> axis. {110}<001> at s = 0.8 and {554}<225> at s = 0.0 also h a v e the 29.5 ° <110> rotation relationship. Besides, the L-fibre distinguished at s = 0.8 and the "t-fibre developed at s = 0.0 show about 35 ° <110> rotation relationship. On the other hand, at the same center layer, {225}<110> dominating in specimen D also has the 29.5 ° <110> rotation relationship with {001}<110> appearing as the m a x i u m peak in the other specimens.

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SPECIMEN SPECIMEN SPECIMEN SPECIMEN

These orientation relationships illucidate the stability of t ex t u r e components during deformation as well as the profile of the t ex t u r e inhomogeneity through the sheet thickness. T h e sh ear t ex t u r e component {225}<554> mainly o b s e r v e d at s = 0.8 is crystailographically unstable and thus easily t r a n s f o r m e d to {001}<110> at s = 0.0, if shear condition be unsatisfied, as shown in F i g u r e 3 and 4. T h e t e x t u r e transition is likely to easily occur by m o v e m e n t of atoms on the {110} plane, the closest packed plane in bcc material. However, {001}<110> s e e m s not to be the most stable end orientation but, rather correctly, metastable. At the c e n t e r section of specimen D, in which the microstructure is similar to that of cold rolled steels, the d ev el o p m en t of {225}<110> becomes outstanding as a result of 29.5 ° <110> rotation of {001}<110>.

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F i g u r e 3. O D F s of {225}<554> at various t h i c k n e s s layers of the hot strips.

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F i g u r e 4. ODFs of {001}<110> at v ar i o u s thickness layers of the hot strips.

475

4. Cold Rolling Texture Cold rolling textures of steel sheets are characterized b y the partial a - f i b r e from {001}<110> to {111}<110> and the complete "t- fibre of {111} < u v w > as shown in Figure 5. The texture component with the m a x i m u m orientatin density is generally located between $ = 30 ° and 40 ° on the a - f i b r e in Euler space. However, in the present work, such a texture development of peak type as shown in Figure 6 appears abnormally depending upon the processing history, which will be discussed later. Figure 7 shows changes of ODFs along the a-fibre with the sheet thickness in cold rolled sheets with 80% reduction. T h e texture inhomogeneity through the sheet thickness is negligible in specimen A and B, hot rolled in austenite region. Only the surface layer (s = 1.0) shows occurrence of shear

,

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textures, not serious, at which the intensity of {001}<110> is a little higher than that at other thicknesses. T h e difference between them disappears with increasing reduction ratio and the stable end orientation converges to {112}<110>. On the other hand, in specimen C and D, hot rolled in the ferrite region, the texture inhomogeneity through the sheet thickness is significant. In spite of the difference of orientation densities the profile of the texture inhomogeneity through the sheet thickness shows similarity to an extent in both specimens. At the surface section (s = 0.8 to 1.0) of specimen C and D, {001}<110> appears as the maximum texture component and the orientation densities of the a-fibre, decrease with i n c r e a s i n g $. As hot rolled, specimen C and D reveal the strong development of the shear texture component {225}<554>, mainly at the thickness layer close to

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Figure 5. Orientation density distribution at t02 = 450 section of Euler space in specimen C, cold rolled with 80% reduction (s = 0.0).

Figure 6. Orientation density distribution at ~ = 45 ° section of Euler space in specimen C, cold rolled with 80% reduction (s = 0.8).

476

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F i g u r e 7. O D F s along a - f i b r e in the cold rolled sheets with 80% reduction. (o : s = 1.0, ~ : s : 0.8, Lx : s : 0,4 and x : s : 0.0) surface, which is crystallograpically u n s t a b l e and thus r o t a t e s b y 29.5 ° around <110> a x i s to {001}<110> during cold rolling. T h i s is the reson w h y the d e v e l o p m e n t of {001}<110> is distinctive at the s u r f a c e section of specimen C and D. However, {001}<110> is m e t a s t a b l e and would rotate to more stable orientation. Rotation b y 29.5 ° around <110> a x i s b r i n g s {001}<110> along the a - f i b r e to {225}<110>, the orientation d e n s i t y of which, therefore, i n c r e a s e s with rolling reduction.

the T-fibre, developed under p l a n e strain condition, and {001}<110> as the main texture component of hot rolling t e x t u r e s and {112}<110> dominating cold rolling t e x t u r e s in specimen A and B, hot rolled in austenite region. ACKNOWLEDGEMENT This work is supported by the international collaboration programme (446 K O R - 113/22/0) of the Korea Science and E n g i n e e r i n g Foundation and Deutsche F o r s c h u n g s g e m e i n s h a f t .

5. C O N C L U S I O N REFERENCES T h e d e v e l o p m e n t of hot and cold rolling t e x t u r e s in T i - b e a r i n g ultra low carbon steel h a s been s y s t e m a t i c a l l y i n v e s t i g a t e d with various processing p a r a m e t e r s . {225}<554>, {001}<110> and {225}<110> are s u g g e s t e d as a t t r a c t o r s of texture components during deformation, which h a v e 29.5 ° rotation relationship to a common <110> axis with r e s p e c t to each other. Besides, 35 ° <110> rotation relationship is recognized b e t w e e n the ~ - f i b r e of s h e a r t e x t u r e and

1. Y. B. Park, S. K. Chang, D. Raabe and K. Li~cke, Proc. of 10th ICOTOM, Germany, Clausthal (1993) in printing. 2. C. D. Dfirmann-Nowak, T e x t u r e s & Microstructures, 14-18 (1991) 813. 3. S. Hashimoto, T. K a s i m a and T. Inoue, T e x t u r e s & Microstructures, i 4 - 1 8 (1991) 841. 4. H. J. Bunge, M a t h e m a t i s c h e Methoden der Texturanalyse, Akademie-Verlag, Berlin (1969) 31.