Hot Deformation and Acicular Ferrite Microstructure in C Mn Steel Containing Ti 2 O 3 Inclusions

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1 , pp Hot Deformation and Acicular Ferrite Microstructure in C Mn Steel Containing Ti 2 O 3 Inclusions Jae-Hyeok SHIM, Jung-Soo BYUN, Young Whan CHO, 1) Young-Joo OH, 1) Jae-Dong SHIM 1) and Dong Nyung LEE School of Materials Science and Engineering, Seoul National University, Seoul Korea. jhshim@kist.re.kr. 1) Metals Processing Research Center, Korea Institute of Science and Technology, Seoul Korea. oze@kist.re.kr. (Received on February 21, 2000; accepted in final form on April 25, 2000) The influence of hot deformation on the formation of acicular ferrite in C Mn wrought steels containing Ti 2 O 3 inclusions has been investigated. A significant amount of acicular ferrite is formed even after hot deformation. From this, it seems that the ability of Ti 2 O 3 inclusions to induce the nucleation of intragranular ferrite remains valid within recrystallized austenite grains after deformation. However, the fractions of acicular ferrite are lower than those in the steels without hot deformation. A high fraction of acicular ferrite is obtained after hot deformation when Mn level is high, the formation of intergranular ferrite such as allotriomorphic ferrite and upper bainite being pronouncedly suppressed. Deformation at relatively low temperatures discourages the formation of acicular ferrite while encouraging intergranular ferrite by reducing prior austenite grain size and consequently by increasing the area of the austenite grain boundaries. KEY WORDS: acicular ferrite; hot deformation; C Mn steel; Ti 2 O 3 inclusion; prior austenite grain size. 1. Introduction For many decades acicular ferrite microstructures have been successfully adopted for the improvement of the toughness of weld metals in welding industry. 1 5) Acicular ferrite has known to provide an optimal combination of high strength and good toughness owing to its refined grain size and interwoven structure. 1 3,5) It has been basically regarded as upper bainite 5) or Widmänstatten ferrite 1) intragranularly nucleated at oxide particles dispersed in weld metals. It has been recognized that acicular ferrite is hardly observed in wrought steels because wrought steels has a much lower density of oxide particles than weld metals. 5) However, acicular ferrite microstructures have been often found in the heat affected zones (HAZs) of wrought steels containing particular oxide particles. 6 9) It has received much attention that there are particular oxide particles effective for producing acicular ferrite in the HAZs which inherit the compositions of wrought steels. In recent years, several attempts 10 14) have been made to produce acicular ferrite microstructures in wrought steels by utilizing nonmetallic inclusions including oxides as nucleation sites (inoculants) for acicular ferrite. Particularly, Ti 2 O 3 particles have been reported to be very effective for the formation of acicular ferrite in wrought steels ) It has also been reported that Ti 2 O 3 form a Mn-depleted zone (MDZ) around it by absorbing Mn from surrounding matrix and that the MDZ facilitates the nucleation of intragranular ferrite. 10,15) Although hot deformation processes such as hot rolling and hot forging are indispensable for the production of wrought steels unlike weld metals and HAZs, the effect of hot deformation on the formation of acicular ferrite has not yet been investigated. Hot deformation is expected to influence greatly the formation of acicular ferrite by reducing prior austenite grain size, since austenite grain boundaries serve as preferential nucleation sites for ferrite competing with non-metallic inclusions within austenite grains during the austenite ferrite transformation. The purpose of the present work is to study the effect of hot deformation on the formation of acicular ferrite in wrought steels. Particularly, much attention has been paid to whether a high fraction of acicular ferrite could still be maintained after hot deformation in C Mn steels. 2. Experimental Procedure Low carbon steels with different Mn contents were prepared using a laboratory vacuum induction melting and casting furnace. A small amount of Ti was added into some of the steel melts for deoxidation, which produces fine dispersive Ti 2 O 3 inclusions during the solidification of the melts. The chemical compositions of the steels are given in Table 1. Six out of the nine steels contain about 0.01 mass% Ti. The steels were hot rolled to plates and then machined to cylindrical specimens 8 mm in diameter and 12 mm in length. These specimens were austenitized at K for 20 min and then continuously cooled at 3 K/s to room temperature. Some of the specimens were 50% compressed axially in the temperature range between and K where the ISIJ

2 Table 1. Fig. 1. Chemical compositions of the prepared steels (mass%). Schematic illustration for the thermomechanical treatment. recrystallization of deformed austenite grains can take place followed by continuous cooling (Fig. 1). The thermomechanical treatment was performed using a thermomechanical simulator. All the specimens were polished and etched in 2% Nital for optical microscopy. The prior austenite grain size and the fraction of acicular ferrite were measured employing an image analysis computer program. The fraction of acicular ferrite was estimated by determining manually the regions of the individual microconstituents such as allotriomorphic ferrite, upper bainite and acicular ferrite in microphotographs taken by an optical microscope equipped with a high resolution digital camera and by measuring the areas of the regions using an image analysis program. 3. Results and Discussion Figure 2 shows optical micrographs of steels M1 to M5 containing a small amount of Ti cooled at 3 K/s following austenitization at K for 20 min. The microstructure of steel M1 having the lowest Mn content exhibits a mixture of coarse allotriomorphic ferrite formed along prior austenite grain boundaries and acicular ferrite formed within austenite grains. The authors confirmed in their previous paper 16) that fine Ti 2 O 3 inclusions, the sizes of which range from 0.1 mm to 5 mm, are uniformly dispersed in the same steels and that these particles are responsible for the formation of acicular ferrite. As the Mn content increases, the amount of intergranular ferrite such as allotriomorphic ferrite and upper bainite decreases and that of acicular ferrite increases. Whereas the fraction of acicular ferrite reaches more than 95% at the highest Mn content (M5), the formation of intergranular ferrite is very weak. An optical micrograph of steel M5 quenched at 849 K during continuous cooling in order to interrupt the transformation is shown in Fig. 3. It is obvious that ferrite laths emanate from a nonmetallic inclusion identified as Ti 2 O 3 within an austenite grain and that the formation of intergranular ferrite is suppressed. It has also been found that steels M1 to M5 contain very fine Ti(C,N) particles which are smaller than 30 nm in addition to Ti 2 O 3 particles. However, it has been reported 17) that non-metallic inclusions can serve as nucleation sites for intragranular ferrite when their sizes are larger than about 0.1 mm. Therefore, these Ti(C,N) particles uniformly dispersed in steels M1 to M5 are not likely to make a considerable contribution to inducing intragranular ferrite, nor to produce any change in the fraction of acicular ferrite. Figure 4 shows optical micrographs of steels M1 to M5 deformed at K and subsequently cooled at 3 K/s following austenitization at K for 20 min. Despite the hot deformation, acicular ferrite microstructures are well Fig. 2. Optical micrographs of steels (a) M1, (b) M2, (c) M3, (d) M4 and (e) M5 cooled at 3 K/s after austenitization at K for 20 min ISIJ 820

3 Fig. 3. Optical micrograph of steel M5 quenched at 849 K after austenitization at K for 20 min; the arrows indicate prior austenite grain boundary. formed. It is also found that the fraction of acicular ferrite increases with increasing Mn content and that the overall microstructure of steel M5 having the highest Mn content consists mainly of acicular ferrite, as is the case with the undeformed steels. Therefore, it is thought that Ti 2 O 3 inclusions remain effective nucleation sites for acicular ferrite within recrystallized austenite grains after hot deformation. Microstructures of steels M6 to M8 deformed at K are shown in Fig. 5. The microstructures of these steels exhibit not acicular ferrite but intergranular ferrite, i.e., allotriomorphic ferrite or upper bainite, which is in contrast to those of steels M1 to M5. This significant change in microstructure can be explained by the fact that steels M6 to M8 hardly contain Ti 2 O 3 particles which are effective nucleation sites for acicular ferrite. Figure 6 compares variations in the fraction of acicular ferrite with the Mn content in both deformed and undeformed steels. In both steels, the fractions tend to increase greatly with increasing Mn content. The authors have reported in their previous paper 16) that Mn is very effective for suppressing the formation of intergranular ferrite, hence relatively enhancing the nucleation of intragranular ferrite. This can explain well the variations in the fraction of acicular ferrite with the Mn content. The fraction in the deformed steels is found to be significantly lower than that in the undeformed steels over the Mn content range. In both steels, the fractions are, however, about 95% at the highest Mn content (M5), intergranular ferrite being pronouncedly suppressed. As shown in Fig. 7, the prior austenite grain sizes of the deformed steels are found to be about 80 mm, which are significantly lower than those of the undeformed steels (about 200 mm) because of the recrystallization of deformed austenite grains. Prior austenite grain boundaries compete with non-metallic inclusions such as Ti 2 O 3 particles within austenite grains as nucleation sites for ferrite during the austenite ferrite transformation. Therefore, the nucleation of intragranular ferrite becomes relatively unfavorable as prior austenite grain size decreases, since the area of the grain boundaries increases. This may be one of the main reasons for the differences in the fraction of acicu- Fig. 4. Optical micrographs of steels (a) M1, (b) M2, (c) M3, (d) M4 and (e) M5 deformed at K and subsequently cooled at 3 K/s after austenitization at K for 20 min. Fig. 5. Optical micrographs of steels (a) M6, (b) M7 and (c) M8 deformed at 1423 K and subsequently cooled at 3 K/s after austenitization at K for 20 min ISIJ

4 Fig. 6. Influence of the Mn content on the fraction of acicular ferrite in the deformed and undeformed steels. Fig. 7. Variation in the prior austenite grain size of the deformed and undeformed steels with the Mn content. Fig. 8. Optical micrographs of steel M9 deformed at (a) 1 423, (b) 1 323, (c) and (d) K and subsequently cooled at 3 K/s after austenitization at K for 20 min. lar ferrite induced by hot deformation. Optical micrographs of steel M9 deformed in the temperature range between and K and then cooled at 3 K/s following austenitization at K for 20 min are shown in Fig. 8. Whereas acicular ferrite covers overall microstructures in the steels deformed at and K, upper bainite grown from prior austenite grain boundaries is dominant at the expense of acicular ferrite in the steels deformed at and K. The polygonal shape of the austenite grains in all the steels confirms the complete recrystallization of the deformed austenite grains. Figure 9 shows variation in prior austenite grain size with deformation temperature. With decreasing deformation temperature the prior austenite grain size decreases significantly to less than 50 mm. It is thought that acicular ferrite microstructures are not well developed in the steels deformed at and K because the prior austenite grain sizes of these steels are not large enough to promote the intragranular nucleation. The result of the present work tells that hot deformation Fig. 9. Variation in the prior austenite grain size of steel M9 with the deformation temperature ISIJ 822

5 does not affect greatly the ability of Ti 2 O 3 particles to induce the ferrite nucleation within the recrystallized austenite grains and that acicular ferrite microstructures could be obtained through hot deformation. Although Barbaro et al. 18) proposed that prior austenite grain size must be larger than about 100 mm to obtain a fraction of acicular ferrite of more than 60% with a cooling rate of about 8 K/s, the authors could obtain about 95% with a prior austenite grain size of about 80 mm produced after hot deformation in relatively high Mn steels. Therefore, it is probable to produce acicular ferrite wrought steels through hot deformation processes such as hot rolling and hot forging. However, it should be mentioned that there are certain limits to hot deformation and one of them is the deformation temperature. Acicular ferrite microstructures were hardly observed in the steels deformed at relatively low temperatures of less than K in the present work, since the prior austenite grain sizes were too small to suppress the intergranular nucleation of ferrite and to encourage the intragranular nucleation of ferrite. So it might be difficult to expect the formation of acicular ferrite in the conventional production of plates and strips, since the finishing rolling temperatures are generally lower than K. 19) Although the production of acicular ferrite wrought steels still has several problems to be solved before it becomes a commercially viable process, the use of this unique microstructure will be promising in producing not only forging steels but also structural steels through developing continuous casting processes such as thin slab and strip casting processes in which the amount of deformation is relatively small. 13,14,20) 4. Conclusions The influence of hot deformation on the transformation of acicular ferrite in C Mn wrought steels containing Ti 2 O 3 inclusions has been investigated. Low carbon steels with different Mn contents were hot compressed and then continuously cooled following austenitization. Acicular ferrite microstructures are still maintained even after deformation at K. It seems that hot deformation does not affect greatly the ability of Ti 2 O 3 inclusions to induce the nucleation of intragranular ferrite within recrystallized austenite grains. It is found that the fraction of acicular ferrite in the deformed steels is lower than that in the undeformed steels. A high fraction of acicular ferrite of about 95% is obtained with a prior austenite grain size of about 80 mm after hot deformation in a relatively high Mn steel, the formation of intergranular ferrite being significantly suppressed. Acicular ferrite is hardly observed in the steels deformed at relatively low temperatures of less than K, since the prior austenite grain size of less than 50 mm are too small to suppress the nucleation of intergranular ferrite. REFERENCES 1) R. A. Ricks and P. R. Howell and G. S. Barritte: J. Mater. Sci., 17 (1982), ) Ø. Grong and D. K. Matlock: Int. Met. Rev., 31 (1986), 27. 3) R. A. Farrar and P. L. Harrison: J. Mater. Sci., 22 (1987), ) J. M. Gregg, H. K. D. H. Bhadeshia and L.-E. Svensson: Mater. Sci. Eng., A223 (1997), ) H. K. D. H. Bhadeshia: Bainite in Steels, The Institute of Materials, London, (1992), ) H. Homma, S. Ohkita, S. Matsuda and K. Yamamoto: Weld. J., 66 (1987), 301s. 7) J.-L. Lee and Y.-T. Pan: ISIJ Int., 35 (1995), ) K. Yamamoto, T. Hasegawa and J. Takamura: ISIJ Int., 36 (1996), 80. 9) H. Mabuchi, R. Uemori and M. Fujioka: ISIJ Int., 36 (1996), ) J.-H. Shim, Y. W. Cho, S. H. Chung, J.-D. Shim and D. N. Lee: Acta Mater., 47 (1999), ) J.-H. Shim, Y. W. Cho, S. H. Chung, J.-D. Shim and D. N. Lee: J. Kor. Inst. Met. Mater., 36 (1998), ) J.-H. Shim: Ph.D. Thesis, Seoul National University, Seoul, (2000). 13) I. Madariaga and I. Gutiérrez: Acta Mater., 47 (1999), ) F. Ishikawa and T. Takahashi: ISIJ Int., 35 (1995), ) J.-H. Shim, J.-S. Byun, Y. W. Cho, Y.-J. Oh, J.-D. Shim and D. N. Lee: Mn Absorption Characteristics of Ti 2 O 3 Inclusions in Law Carbon Steels, Scripta Mater., in press. 16) Y. W. Cho, J.-H. Shim, J.-S. Byun, J.-D. Shim and D. N. Lee: Influence of Mn on Microstructure of Ti-killed C-Mn Steel, submitted to Mater. Sci. Eng. A. 17) I. S. Bott and P. R. Rios: Scripta Mater., 38 (1998), ) F. J. Barbaro, P. Krauklis and K. E. Easterling: Mater. Sci. Technol., 5 (1989), ) T. Tanaka: Int. Met. Rev., 26 (1981), ) T. Maki: Mater. Jpn., 36 (1997), ISIJ