MECHANISM OF GOSS SECONDARY RECRYSTALLIZATION IN GRAIN ORIENTED SILICON STEEL

Grain oriented silicon steel are mainly produced by two methods. One is the two-stage cold rolling method(l) and the other is the one -stage cold rolling method(2). The two -stage cold rolling method uses mainly MnS as an inhibitor, while the one-stage cold rolling method uses MnS and AIN as inhibitors. Hot rolled sheets containing MnS as an inhibitor and containing AIN and MnS as inhibitors were processed both by onestage and two stage cold rolling method and investigated the effect ofinhibitors on the texture evolution by grain growth.


1.INTRODUCTION
Grain oriented silicon steel are mainly produced by two methods. One is the two-stage cold rolling method(l) and the other is the one -stage cold rolling method(2). The two -stage cold rolling method uses mainly MnS as an inhibitor, while the one-stage cold rolling method uses MnS and AIN as inhibitors. Hot rolled sheets containing MnS as an inhibitor and containing AIN and MnS as inhibitors were processed both by one-stage and two stage cold rolling method and investigated the effect ofinhibitors on the texture evolution by grain growth.
2.EXPERIMENTAL PROCEDURE 2.3mm thick hot rolled specimens A1,A2 and B1,B2 were processed as shown in Table 1  The orientations ofsecondary recrystallized grains(A1,A2,B2) were measured by the back-reflection Laue method. The relationships between the primary annealed texture and secondary annealed texture was analyzed by the SHG method(4)explained briefly below.
Boundaries in P.9 orientation relationship are more mobile than other boundaries in Fe-3%Si steel containing A1N and MnS as inhibitors (5)  smallest in A1 and the largest in A2. The Goss orientation is not recognized in B1.Very large difference in texture between A1 and B1 after secondary GRAIN ORIENTED SILICON STEEL 681 recrystallization annealing can be mainly attributed to the difference of the inhibitor as texture and grain size before secondary recrystallization annealing are similar in both specimens.  orientation is distributed with the ideal Goss orientation as a peak as shown in Fig.2. Fig.3(a) to 3(c) show that A1 and B1, are similar in the distribution pattern of the Ic9 values.
The frequency of secondary recrystallized grains in the specimen A1 is relatively well correlated with ICY.9 values.

Secondary recrystallization should occur in B1 if the inhibitor intensity in B1
is same as that ofA1 as both specimens have almost the same Ic9 values. GRAIN ORIENTED SILICON STEEL 683 This clearly indicates the grain boundary migration characteristics are affected by the inhibitor intensity.
Distributions of Ic9 and PCNP.9 values in the primary recrystallization specimen B1 are shown in Fig.4. It shows that PCNP.9 values are in close relationship with the texture after grain growth(see Fig.2) and no relationships are found with ICY.9 values. This suggests that the PCNP9 values are operative on the texture evolved by grain growth when the inhibitor intensity is relatively weak( MnS alone). IfPcNE9 values are also operative in A2, then {334) orientations should be one ofthe major texture components after grain growth as PCNP.9 values of these orientations are high. However, the main texture component after grain growth is Goss orientation as shown in Figs. 3(a). This contradictory phenomena can be reasonably explained on the assumption that the r.1 boundary inhibition effect varies with inhibitor intensities. Fig.5(a),(b) show distributions of PCNP.9 and PCNP.1 values respectively in the primary matrix of the specimen A2. It shows that most of the orientations with PCNP.9 value of 30 or more exhibit PCNP.1 value of 100 or more. The PCN9 value near the Goss orientation alone belongs to a region of 30 or more, where the value of PcNP.1 is less than 100. This means that when the inhibitor intensity is above a certain level, nucleus orientations having high PCNI values are inhibited by the 1 boundaries to.grow by increased inhibitor effect of grain boundary migration by r.1 boundaries. Therefore, orientations having high PCN9 value s and low PcN.lvalues, namely an ideal Goss orientation in this case, can preferentially grow compared with other orientations. 684 J. HARASE ET AL This consideration supports the hypothesis that the grain boundary inhibition effect ofIl botmdm-ies varies dth the intensity ofthe inhibitr.

5.Conclusion
(1) In the one stage cold rolling method,Ic9 values are highest in the Goss orientation and PCN219 values are very high in the {334} < 9 13 3 > orientation in the primary matrix regardless of the materials used.
Goss secondary recrystal!ization takes place when both AIN and MnS are contained and no secondary recrystallization takes place and the texture with the major component of {334}<9 13 3 > evolves by secondary recrystallization annealing when MnS alone is contained.
(2) In the two stage cold rolling method, PCNP-9 values are the highest in the Goss orientation in the primary matrix and Goss secondary recrystallization takes place regardless ofthe materials used.
(3)It is concluded that Ic r& is operative in the one -stage cold rolling method and PCN2]9 is operative in the two-stage cold rolling method for the occurrence ofGoss secondary recrystallization. 6.D.G.brandonActa Metall.,14,1479Metall.,14, (1966 1. INTRODUCTION The mechanism for the recrystallization texture formation based on the orientated nucleation and selective growth theory has been widely accepted. Recently a nucleation mechanism through twinning has been proposed for the dynamic recrystailizatlon of A1 slngle crystals /1/. The authors found that ultra-low C Tl-added steels with 1.5 Si showed a very sharp so called a -fiber cold rolling texture (<110>//RD) and a very sharp near {554}<225> recrystallization texture at the center layer. The subgrain coalescence mechanism does not seem to be dominant, because there are very few common-orientation components between cold rolling and recrystalllzatlon textures. Then with a particular emphasis on a nucleation mechanism through twinning, the recrystalllzatlon texture formation in ultra-low C Tl-added steels with 1.5 S1 ls discussed.

EXPERIMENTAL PROCEDURE
Ultra-low C Tl-added steels containing O and 1.5% Si, as given in Table 1, were melted in vacuum. The 20 mm thick slabs were soaked at 1573 K for 1 h, alr-cooled to 1323 K, and then hot rolled to 4 mm thick above 1223 K. 685 686 N. MIZUI & K. LOI After hot ro11Ing the hot bands were Immediately quenched into water. They were thinned to 2 mm thick by machining and cold rolled to 0.5 mm by 75 reduction. Subsequently they were annealed at 1123 for 3 mln in a salt bath. Xray texture analysls was conducted for the central layers of the hot bands, cold rolled and annealed steels.  Figure 1 summarizes the {200} pole figures of the hot-rolled, cold-rolled and annealed specimens for both 0 and 1.5 Si steels. Although the hot rolling texture of a O SI steel was random, that of a 1.5X Si steel was a sharp ND//<100> and <111> texture, which was predicted from the full constrained Taylor model. The texture components developed by cold rolling in 0.0SI steel was both a and (<111>//ND) fibers, which are known as normal cold rolling texture of low C steels. While in the case of 1.5Si steel was a only a -fiber. Latter can be arisen from the hot band structure with elongated grains of 1.5SI steel, as calculated with the relaxed Taylor model by Van Houtte /2/. Also there were a large difference between the annealing of them. Those were the full fiber and a near {554}<225> textures, respectively.

DISCUSSION
It is well established that a near {554}<25> component has the orientation relationship {211}<011> in a fiber in the cold rolling texture by 35" rotation around <110> near normal direction. This orientation relationship results from high angle boundary migration during the grain growth. However the nucleation mechanism of {554}<25> is still unclear. Figure 3 shows the {200} pole figure, on which the areas with the intensity higher than 1.5 times of random level in both the cold rolling and ULTRA-LOW C Ti-ADDED STEEL WITH 1.57o Si recrystallization textures are depicted. Because of the lack of common components between both textures, the subgrain coalescence dose not see to be dominant in the present case. Thus in the following discussion, a new nucleation mechanism will be discussed in order to explain the present result.
Recently it was reported that nucleation occurs through multiple twinning in hl single crystals. In steels with the increase In SI and the decrese in C, the stacking foukt energy decreses. Therefore in 1.5 S1 Tl-added steel, the nucleation through twining can be expected.
Then with a particular emphasis on a nucleation mechanism through twinning, an analysls was done as in Flg.2.
(3) Twin orientations of Orientation R were calculated and checked whether they are in the a-fiber (Orientation T).
As to (112)[110], only 4 combinations can give orientation T in the a -fiber, as shown in Table 2 and Flg.3. There are three different rotation paths from orientation T to orientation R, because three different twin systems with the same twinning direction give the same Orientation R. However in the present result only one of them, that is h in Flg.4, seems to have occurred. Therefore another condition is required.
To consider the activation stress of every twin system, Schmid's factors were calculated for the Tucker stress state, that is, the compressive stress in normal direction and the tensile stress in roiling direction, as shown in Table 2. Then only two combinations are expected to occur. The combination B gives {5,3,16}<12,4,3> recrystalllzation component. In the case of combination B, the orientation T and the orientation G are mirror-symmetric to each other. Therefore the growth probabiiity should be much smaller than that of the combination h. 5. CONCLUSION Ultra-low C Ti-added steels with 1.5 Si were hot rolled in ferrltic region and water-quenched immediately. h 1.5 Si steels showed a sharp a -fiber texture after cold rolling and a near {554)<225> texture after subsequent annealing.
The {554}<225> component has the 35" rotation relationship around <110> axes near ND from the near {211} <011> cold rolling texture component. owever the lack of common components between cold rolling and recrystallization textures suggests that the subgraln coalescence was not the dominant process for the texture formation. Then a new mechanism for recrystalllzation texture formation was discussed by introducing a new nucleation model through twinning in the deformed matrix. It is concluded that sharp {554}<225> recrystallizatlon texture component is nucleated by twinning which occurs in near {100}<011> deformed matrix and grows into near {211}<011> deformed matrix by high angle boundary migration.