FERRITIC ROLLING TO PRODUCE DEEP-DRAWABLE HOT STRIPS OF STEEL

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1 FERRITIC ROLLING TO PRODUCE DEEP-DRAWABLE HOT STRIPS OF STEEL Radko Kaspar Andreas Tomitz a) MAX-PLANCK-INSTITUT FÜR EISENFORSCHUNG GmbH, Düsseldorf, Germany b) HOESCH HOHENLIMBURG GmbH, Hohenlimburg, Germany Abstract The aspired good deep-drawability (high r- and n-values, Δr. ) is basically achieved by a definite anisotropic flow mechanism. Such necessary anisotropy can be ensured in a deepdrawable steel sheet by a preferential {111} ND-texture. In a conventional production, a hot rolling in austenite and a cold rolling at room temperature together with a subsequent recrystallization annealing are applied for such texture development in the final cold strip. As a cost saving replacement for this, a thin-gauge hot strip with a required deep-drawability can be employed. As a promising realization of cost saving thin-gauge deep-drawable hot strips of steel, a ferritic rolling can be implemented. In this new practice the finishing is shifted down into the temperature region of ferrite. To optimize the process parameters, extensive laboratory tests on IF steel were carried out by using the hot deformation simulator WUMSI. By the measurements of the texture development as well as by the computing of r-values, the texture formation could be optimized achieving a deep-drawability in hot strips comparable to that of a cold strip after a conventional austenitic rolling. 1. DEEP-DRAWABILITY OF STEEL A good cold workability (low proof strength, high uniform elongation) additionally to a sufficient strength (after cold forming) and, particularly, a good deep-drawability are the properties that are of a large importance for many flat products of steel. The aspired good deepdrawability can be realized by a favorable anisotropic material flow during the deep-drawing process. For the anisotropic flow behavior of a polycrystalline material the distribution of the orientations of individual grains plays a decisive role and is determining for the r-values 1,) 3.. parallel to sheet plane Outgoing from a statistic disorderly distributed grain orientations, r-value increases with increasing fraction of. {111}-oriented grains and decreasing {111} {1} amount of those with {1} orientation parallel to sheet plane, Fig. 1 3). r -value [-] m 1.. DEEP-DRAWABLE STRIPS PRODUCED BY CONVENTIONAL ROLLING To realize the requirements for deepdrawable steels with a pronounced {111}- texture by a conventional route, hot rolling is traditionally carried out in the austenite range followed by cold rolling with a. orientation: rm-value theoretical: {111}.9 {55} Volume ratio of {111} and {1} grains (I{111} / I {1}). {11}.3 {11} 1. {1} Figure 1. Effect of texture on the mean r-value r m. I{111} and I{1} are the texture intensities of the corresponding orientations.1

2 sufficient deformation (7 - %) and a subsequent batch or continuous annealing for recrystallization. A cold strip is a final product in such case. In order to reduce production costs there is a tendency to achieve a sufficient deep-drawability without cold rolling, that means already with a hot strip as a final product. Fast developments of the rolling technique make it possible to produce so-called thin-gauge hot strips with minimum thicknesses that are nowadays within the range of those of cold strips, Fig. ). Such production requires unavoidably lowering finishing temperatures because of large heat losses of thin hot strips. But, the austenitic rolling with low finishing temperatures is not easy to perform because of high transformation temperatures of extra low carbon steels (ELC), ultra low carbon steels (ULC) and low carbon interstitial free (IF) steels with manganese content less than. %. The reheating temperatures above 15 C would be necessary for thicknesses of - 5 mm. Hot strips thinner than 1. mm are not producible at all by a conventional austenitic hot rolling. 3. DEEP-DRAWABLE THIN-GAUGE HOT STRIPS PRODUCED BY FERRITIC ROLLING The difference in the processing routes of a conventional austenitic rolling and a novel ferritic rolling is apparent from Fig. 3. Consider-able cost reductions may be achieved in different way. The most evident is a reduced reheating temperature in the ferritic rolling practice, which gives also the potential for an increased throughput of the furnace. Lower reheating temperatures for ferritic rolling (between 95 and 15 C) result to a reduced AlN dissolution (enhancing ferrite recrystallization kinetics) and a smaller initial austenite grain size. This low rolling temperature practice leads also to an im-proved hot rolled product Flow stress, MPa Minimum hot strip thickness, mm % of cold strip production future Figure. Minimum thicknesses of hot strip as a potential substitution of cold strip a) b) roughing finishing ELC IF - transformation ELC - transformation IF Deformation temperature, C Figure. Flow stress for =. of ELC and IF steel over the range of deformation temperatures (strain -1 rate 1 s ) quality with less surface defects, improved flatness of the hot strips due to reduced internal stresses owing to the fact that steel strips are already transformed prior to cooling on the run-out table. Temperature C Mn Figure 3. Comparison of conventional austenitic (a) and ferritic (b) rolling roughing finishing

3 Fortunately, moderate rolling loads in the finishing mill enable the application of ferritic rolling even on existing mills. As shown in Fig. 5), the flow stresses - and so the rolling loads - of the IF steel are lower in the temperature range between 7 and 7 C than those at 95 C in the conventional austenitic temperature region. At lower temperatures higher flow stresses of ELC steel are measured presumably due to dynamic strain aging in these steel grades. Two different groups of ferritic rolled deepdrawable thin-gauge hot strips can be produced, Fig. 5: Soft hot strip: In this product group the condition must guarantee a complete recrystallization in the coil (becoming soft), Fig. 5a. For this, the finishing and temperatures must be appropriate high. `HardA hot strip annealed: By further lowering finishing temperatures in this processing, compared to the production of soft hot strip, thinner hot strips can be produced (< 1 mm). Such hot strip does not recrystallize in coil (becoming hard) and must additionally be recrystallized by annealing, Fig. 5b. Temperature a) b) finishing annealing Figure 5. Production of soft hot strip (a) and hard hot strip additionally annealed (b) -fibre 1 {111}<11> {111}<11> { 11}<11> {1}<11> {11}<11> {111}<11> -fibre Figure. Eulerian space showing the position of - and -fibres (a). TEXTURE DESCRIPTION For the description of texture development the method of grain orientation distribution (ODF) is the most applied way. ) In a so-called Eulerian space each point corresponds to one orientation, defined by three Eulerian angles Φ, ν 1 and ν, Fig.. For practical purposes the key textures in steel sheets can be followed by the focusing only some special lines in this space, so-called fibres, in the Eulerian space. For deep-drawable steel sheets two fibres are important: α-fibre (<11> RD) and γ-fibre ({111} ND). In this way the pole densities can be expressed only along these two fibres in the form of two D-diagrams. The texture development in simulated warm (ferritic) rolled strips was the main objective of this work. The study was focused on the potential final products mentioned above: a soft hot strip and a hard hot strip additionally annealed. Finishing temperature, C 7 7 partially recrystallized completely recrystallized hardly recrystallized not realizable Coiling temperature, C Figure 7. The region of a complete ferrite recrystallization of the IF steel presented in the coordinate system of finishing temperature and temperature

4 5. MATERIAL AND EXPERIMENTAL TECHNIQUE The investigations were done on an IF-steel as a typical representative of deep-drawable steels. The chemical composition was as follow (in mass %): C:.%, Si:.7%, Mn:.97%, P:.1%, S:.%, N:.3%, Al:.%, Ti:.3%, Nb:.7%. The laboratory tests were done on the hot deformation simulator WUMSI 7) by using the plane strain hot compression test as a simulation of rolling. The texture was measured on a Siemens D5 texture goniometer. The r-values were computed from the texture measurements by an ANIS-MPI program of the University Birmingham. ) Texture intensity 1 Texture intensity 1 - fibre : <11> RD fibre: {111} ND {1} {11} {111} {11} <11> <11> 71 C 71 C 1 C 7 C austenite 9 C [ ] 1 [ ] Figure. Deformation texture for different finishing temperatures. RESULTS AND DISCUSSION For the design of the rolling schedules in ferrite region the determination of the range of the γ- α-transformation temperatures as well as the knowledge of the recrystall-ization behavior of ferrite is indispensable. Fig. 7 shows the range of temperatures in which ferrite can recrystallize completely in coil in the production of soft hot strips. Coiling bellow this temperatures leads to a hard hot strip that has to be additionally annealed to achieve a designed texture..1. Deep-drawable "soft" hot strip In the development of texture during the production of deep-drawable hot strip, the deformation texture just after finishing is decisive for the quality of the recrystallized texture after. Generally, the deformation texture should involve a sufficient intensity of γ-fibre including typically some component of {1} as well. After finishing with ε = x.3 the deformation - fibre: {111} ND texture was measured for various finishing temperatures in ferrite and compared with that after finishing in austenite, Fig.. The high finishing temperature in ferrite (1 C) leads to an unfavorable rolling texture with a poor coverage of {111} and the maximum amount of grains with α-fibre near to{1} component. By reducing finishing temperature (7 C or lower) a more distinctive γ-fibre components with a strongest coverage of α-fibre in the range of {11} can be observed. In contrast to ferritic rolled specimens, there is nearly irregular texture with random oriented grains after the γ-α-transformation of austenitic rolled steel, as also reported in. 9) Fig. 9 displays the texture development of a favorable deformation texture (a low - fibre : <11> RD {1} {11} {111} {11} after finishing at 71 C after at 7 C <11> <11> 75 9 [ ] [ ] 1 Figure 9. Texture development in the simulated soft hot strip showing the change of the deformation texture just after finishing into a recrystallized texture after (see arrows)

5 finishing temperature of 71 C) due to the recrystallization in coil. The formation of a typical annealing texture with a strong γ-fibre orientation and a reduced coverage of {1} component of α-fibre can be observed. There is a striking increase in the <11> component of γ-fibre. As observed, the lowering finishing temperature improves the deformation texture with an increasing amount of {111} oriented grains which supports the formation of a sharper {111} recrystallized texture after. This is reflected by a significant increase in r-values (as computed from the texture measurements) with decreasing finishing temperature, Fig Deep-drawable hard hot strip By a further lowering of finishing temperature thinner hot strips with a more favorable rolling texture can be produced. Texture intensity 1 r-value [-] ferrite deformation: = 1. finishing at 71 C 7 C C (r) 5 (r 5) 9 (r 9) Angle to rolling direction [ ] Figure 1. Effect of finishing temperature on the r-values of simulated soft hot strips Nevertheless, the temperature becomes too low for a complete recrystall-ization of the warm deformed material in the coil and, therefore, an additional recrystallization annealing is necessary by using batch or continuous processing. The texture development during a batch annealing of specimens finished at C with two different temperatures is given in Fig. 11. Whereas the higher temperature of 55 C leads to a rather low γ-fibre coverage, the lower temperature of C brings about a significant improvement in texture with a high level of γ-fibre showing a maximum at the {111}<11> component. This is supposed to reflect the recovery processes during the at higher temperatures that reduce the stored energy surplus of {111}-oriented grains and so diminish their amount after recrystallization. The r-value distributions after the simulation both of the possible annealing processes (batch and continuous), are displayed in Fig 1. The distribution of r-values as a function of the angle to RD shows considerably higher values, especially in RD (r ), in comparison to those of soft hot strips. So the form of the curves is more similar to that of cold strips. The mean r- values > 1.5, Δr <. as well as other mechanical properties (elongation A > 5 %,.%-proof strength R p. <1 MPa, strengthening exponent n..3) achieved in hot strip are quite sufficient for a lot of applications without necessity of manufacturing cold strips. 7. CONCLUSIONS G The beneficial texture development, similar as by the production of the conventional cold strip, can be achieved already in a thin-gauge hot strip by finishing in a temperature region of ferrite by so-called ferritic rolling. - fibre : <11> RD [ ] - fibre: {111} ND {1} {11} {111} {11} <11> <11> deformation texture just after finishing ( = 1. at C) 1 at C at 55 C [ ] Figure 11. Effect of temperature on the development of the texture of the hard hot strip showing the change of the deformation texture just after finishing into a recrystallized texture after additional simulated batch annealing

6 G G G By reducing finishing temperature of ferritic rolling a more favorable initial rolling texture can be generated as a pre-condition for a beneficial final recrystallized texture. A minimum temperature (7 C for the IFsteel tested) must be met if producing deepdrawable soft hot strips directly after. By a further lowering of finishing and temperature (to achieve even lower hot strip thicknesses) hard (not recrystallized) hot strips must additionally be annealed after to guarantee a complete recrystall-ization. Lower temperatures are more desirable for these products. r-value [-] batch annealing at 7 C continuous annealing at 7 C (r) 5 (r 5) 9 (r 9) Angle to rolling direction [ ] Figure 1. r-values after different simulated annealing procedures applied on the hard hot strip (finishing at C, at C) REFERENCES [1] SCHLIPPENBACH, U. v.; LÜCKE, K.: Deformation and recrystallization tex-tures in a low-csteel, ICOTOM 7, Holland, 19, pp [] PLUTKA, B.: Untersuchungen zu Vorgängen bei der Bildung von Rekristallisationstexturen in Tiefzieh-stählen, Dissertation RWTH Aachen, Germany, [3] RAY, R. K.; JONAS, J. J.; HOOK, R, R. E.: Cold rolling and annealing textur-es in low carbon and extra low carbon steels, Int. Mat. Rev. 39 (199), pp [] BALD, W.; KNEPPE, G.; ROSENTHAL, D.; SUDAU, P.: Innovative Techno-logie zur Banderzeugung, stahl und eisen 119 (1999) 9, pp [5] TOMITZ, A.; KASPAR, R.: Laborsimulation zur Optimierung der Prozeßparameter und der Produktqualität von Tiefziehstählen beim Warmwalzen im Ferritgebiet, Proc. of the 13. Aachener Stahlkolloquium Umformtechnik Stahl- und NE-Werkstoffe, Ed. by R. Kopp, Verlag Mainz, Wissenschaftsverlag, Aachen, Germany, 199, pp [] BUNGE, H.-J.: Mathematische Methoden der Texturanalyse; Akademie-Verlag, Berlin, 199. [7] KASPAR, R.; STREIßELBERGER, A.; PEICHL, L.; PAWELSKI, O.: Fortgeschrittene Technik der Warmumformsimulation. Z. Werkstofftech. 1 (193), pp [] BATE, P. S.: Manual for ANSI-MPI, University of Birmingham, 1997: [9] RAY, R.K.; JONAS, J.J.: Transformation textures in Steels. Int. Mat. Rev. 35 (199) 1, pp