Onset Of Secondary Recrystallization Research Articles

The role of grain colony on secondary recrystallization in grain-oriented electrical steel: New insights from an original tracking experiment

Secondary recrystallization is responsible for the sharp Goss texture ({110}<001>) of grain-oriented electrical steels. Many investigations have focused on establishing the relations between the sharpness of final Goss textures and primary recrystallization textures measured by the X-ray diffraction. However, the contributions of micro-texture characteristics to secondary recrystallization have not yet been well understood. Here, we report that the grain colonies composed of grains with similar orientations in the primary recrystallization matrix had a particular role on the development of secondary recrystallization via an original tracking experiment. We observe that the γ-grain (<111>//normal direction, ND) colonies facilitate the retention of primary recrystallization matrix grains (≤25µm) having 20-45° disorientation with {110}<001> and relatively large difference in the associated frequency with precise Goss orientation and that having deviation angle of 15°, promoting the abnormal growth of low-deviation-angle Goss grains especially after the onset of secondary recrystallization and leading to sharp final Goss texture. By contrast, the abnormal growth of high-deviation-angle Goss grains may be also promoted after the initiation of secondary recrystallization in case of randomly-distributed γ-grains in primary recrystallization matrix and thus resulted in deteriorated magnetic properties. In this way, we reveal the role of grain colony on secondary recrystallization and the underlying mechanism for the effect of primary recrystallization textures on the sharpness of final Goss texture. We also observe that the formation of grain colonies is mainly related to the initial solidification microstructure, processing route as well as the deformation and recrystallization features of γ-grains.

A Comparative Study of Microstructure and Texture Evolution in Cu-Bearing and Nb-Bearing Grain Oriented Silicon Steels

Here, the authors elucidate the microstructure and texture evolution in ultra-low carbon grain oriented silicon steels with (Cu,Mn)S and Nb(C,N) as inhibitors. Both Cu- and Nb-bearing steels exhibit microstructural and texture gradients across the thickness. The surface layer is characterized by recrystallized equiaxed grains, while the subsurface layer is dominated by strong Goss texture, and the center layer is composed of elongated deformed bands with strong α-fiber including {100} ∼{223} texture. In contrast to the Cu-bearing hot rolled sheet, the deformed center zone of Nb-hot rolled sheet is broadened and indicate a new {111} component, while the neighboring Goss zone, which act as the nuclei for secondary recrystallization is reduced. Also, the fraction of Goss texture in Nb-primary recrystallized sheets is lower than Cu-primary sheets in a series of specimens subjected to identical annealing temperature prior to secondary recrystallization because of inheriting texture of hot rolled sheets. On the other hand, compared to the (Cu,Mn)S particles, the Nb(C,N) particles significantly hinder the primary recrystallized grain growth. The onset of secondary recrystallization for Cu-bearing steel is 950 °C, while the growth of Nb-primary grains continue to be strongly inhibited at 975 °C. After high-temperature annealing, a high magnetic induction (B8 is ≈1. 8 T) is obtained in Cu-bearing steel. In contrast, in Nb-bearing steel, there is no secondary recrystallization or the non-Goss grain ({113} ) is abnormal growth, and the magnetic induction B8 is ≈1.48–1.5 T. The lower magnetic induction of Nb-bearing silicon steel is related to the lower fraction of primary Goss texture and the stronger pinning effect of Nb(C,N) inhibitors.

Secondary recrystallization and magnetostriction in binary Fe81Ga19 thin sheets

Strong Goss texture ({110}〈001〉) was successfully developed by secondary recrystallization in Fe81Ga19 sheets without conventional application of inhibitor or surface energy effect as grain-oriented silicon steel. Goss grains cover 90% surface area of annealed sheet, and the magnetostriction coefficient reaches up to 262 ppm. The primary recrystallization microstructure and texture provide the prerequisites for nucleation and abnormal grain growth of secondary Goss grains. The dispersedly distributed Goss grains do not exhibit an evident advantage in grain size and number, and the normal grain growth of matrix grains can be inhibited by low-angle grain boundaries before onset of secondary recrystallization. The existence of low-angle grain boundaries is proposed to be an important factor for the occurrence of secondary recrystallization in binary Fe-Ga sheets based on grain boundary character distribution analysis.

On the use of copper-based substrates for YBCO coated conductors

It is well known that the recrystallization texture of heavily cold-rolled pure copperis almost completely cubic. However, one of the main drawbacks concerning the use of purecopper cube-textured substrates for YBCO coated conductor is the reduced secondaryrecrystallization temperature. The onset of secondary recrystallization (i.e., the occurrence ofabnormal grains with unpredictable orientation) in pure copper substrate was observed withinthe typical temperature range required for buffer layer and YBCO processing (600–850 °C). Toavoid the formation of abnormal grains the effect of both grain size adjustment (GSA) andrecrystallization annealing was analyzed. The combined use of a small initial grain size and arecrystallization two-step annealing (TSA) drastically reduced the presence of abnormal grainsin pure copper tapes.Another way to overcome the limitation imposed by the formation of abnormal grains is todeposit a buffer layer at temperatures where secondary recrystallization does not occur. Forexample, La2Zr2O7 (LZO) film with a high degree of epitaxy was grown by metal-organicdecomposition (MOD) at 1000 °C on pure copper substrate. In several samples the substrateunderwent secondary recrystallization. Our experiments indicate that the motion of grainboundaries occurring during secondary recrystallization process does not affect the quality ofLZO film.

Onset of Secondary Recrystallization in High Purity 3.3%Si Steel

Onset of Secondary Recrystallization in High Purity 3.3%Si Steel

Effect of Reheat Conditions on Microstructure Evolution and Precipitation Behavior in High Strength Linepipe Steel

The effects of slab reheat temperature and soaking time are studied to characterize austenite grain growth, microstructure homogeneity and dissolution of precipitates in linepipe X80 grade steel. It is shown that the uniformity of austenite microstructure strongly depends on the slab reheat temperature and soaking time. With increasing reheat temperature an abnormal growth of individual grains is observed that stems from gradual dissolution of microalloy carbonitrides. As the result, individual grain boundaries become unpinned and mobile thus "nucleating" secondary recrystallization. The highest reheat temperature at which the dissolution kinetics of precipitates is still slow enough to prevent the onset of secondary recrystallization within long soaking times is 1160°C. The as reheated austenite microstructure and the character of austenite grain size distribution are inherited throughout the entire roughing rolling sequence and even further downstream to the finishing rolling entry. The effects of reheat soaking time on shear fracture area and impact toughness are also described.

Effect of Reheating on Grain Size Evolution in High Strength Linepipe Steel

The effects of slab reheat temperature and soaking time are studied to characterize austenite grain growth, microstructure homogeneity and dissolution of precipitates in linepipe X80 grade steel. It is shown that the uniformity of austenite microstructure strongly depends on the slab reheat temperature and soaking time. With increasing reheat temperature an abnormal growth of individual grains is observed that stems from gradual dissolution of microalloy carbonitrides. As the result, individual grain boundaries become unpinned and mobile thus "nucleating" secondary recrystallization. The highest reheat temperature at which the dissolution kinetics of precipitates is still slow enough to prevent the onset of secondary recrystallization within long soaking times is 1160°C. The as reheated austenite microstructure and the character of austenite grain size distribution are inherited throughout the entire roughing rolling sequence and even further downstream to the finishing rolling entry.

The onset temperature of secondary recrystallization and the sharpness of Goss secondary recrystallization texture in the nitrided Fe–3%Si alloy

Primary recrystallized Fe–3%Si specimens were nitrided and annealed intermittently with a heating rate of 15°C/h in 100% N 2 atmosphere. The magnetic induction B 8 was measured after each annealing. The onset of secondary recrystallization was detected by the rapid rise of B 8. The maximum B 8 obtained was about 1.94 T when the onset temperature of the secondary recrystallization ( T cr) was around 1075°C. The same primary specimens were coated with MgO and annealed with the same heating rate in 5% H 2–N 2. The initial grain size and nitrogen content for the maximum B 8 were quite different in this annealing. The difference in the optimum grain size and nitrogen content for obtaining the highest B 8 in both annealing methods was explained by assuming that T cr determines the B 8 in both annealing methods.

窒化したFe-3%Si合金のGoss集合組織形成に及ぼす二次再結晶温度の影響

Primary recrystallized Fe-3%Si specimens were nitrided and annealed with the heating rate of 15°C/h in 100% N2 atmosphere. With an increase in the heating temperature by 10°C, the heating was interrupted and the specimen was rapidly cooled and the magnetic induction B8 was measured and rapidly heated again to the heating temperature. Onset of secondary recrystallization was detected by the rapid rise of B8. B8 after secondary recrystallization was closely related with the onset temperature of secondary recrystallization (Tcr). The maximum B8 obtained was about 1.94 T when the Tcr was around 1075°C and the B8 decreased with increasing in the Tcr. The Tcr was closely related with the primary grain size and it was about 1075°C when the grain size was about 15 μm. The Tcr increased with increasing primary grain size and it was about 1100°C when the primary grain size was about 22 μm.The same primary specimens were coated with MgO and annealed with the same heating rate in 5%H2-N2. The maximum B8 obtained was almost the same as the above annealing condition; however, the primary grain size for obtaining the maximum B8 was different in this annealing. The difference in the optimum grain size for obtaining the maximum B8 in both annealing methods was explained on the assumption that the maximum B8 is obtained when the Tcr is about 1075°C in both annealing methods.

窒化したFe-3%Si合金の2次再結晶に伴う磁束密度の変化

87.4% cold rolled and primary recrystallized Fe-3%Si specimens containing Al were nitrided after primary recrystallization. They were then annealed intermittently with the heating rate of 15°C/hr in a infrared radiation furnace in 100%N2 atmosphere. Magnetic induction B8 were measured after each annealing. The onset of secondary recrystallization was detected by the rapid rise of B8 value. The secondary recrystallization temperatures were higher when the initial grain sizes were larger or the nitrogen contents after nitriding treatment were higher. The secondary recrystallization with a sharp (110)[001] orientation took place at temperatures between 1070 and 1085°C in the specimens with different nitrogen contents and different initial grain sizes.The existence of an optimum temperature range for the selective growth of the (110)[001] oriented grain in secondary recrystallization suggests that the Σ9 boundary becomes most mobile in the temperature range under the given inhibitor intensity.

Instability of Secondary Recrystallization Caused by Excessive Cold-Rolling Reduction in Fe-3%Si Alloy

Secondary recrystallization of an Fe-3%Si alloy containing AIN and MnS as inhibitors with heavy cold rolling before primary recrystallization annealing was investigated with special reference to poor secondary recrystallization. Excessive cold-rolling reduction produces smaller primary recrystallized grains with higher intensity of {100} oriented grains and higher possibility for {100} oriented grains associated with Σ5 coincidence boundaries. Specific grain growth of the {100} primary recrystallized grain before the onset of secondary recrystallization is considered to cause the poor secondary recrystallization by reducing the driving force of secondary recrystallization. It may be essential for this unstable secondary recrystallization that the two equivalent orientations of the {100} oriented grains which increase with increasing cold-rolling reduction have the Σ5 coincidence orientation relationship with each other.

Fe-3%Si合金の高冷延圧下率での2次再結晶不安定化機構

Secondary recrystallization of an Fe-3%Si alloy containing AIN and MnS as inhibitors with heavy cold rolling before primary recrystallization annealing was investigated with special reference to poor secondary recrystallization. Excessive cold-rolling reduction produces smaller primary recrystallized grains with higher intensity of {100} oriented grains and higher possibility for {100} oriented grains associated with Σ5 coincidence boundaries. Specific grain growth of the {100} primary recrystallized grain before the onset of secondary recrystallization is considered to cause the poor secondary recrystallization by reducing the driving force of secondary recrystallization. It may be essential for this unstable secondary recrystallization that the two equivalent orientations of the {100} oriented grains which increase with increasing cold-rolling reduction have the Σ5 coincidence orientation relationship with each other.

Coincidence grain boundary and (100) [001] secondary recrystallization in Fe-3% Si

The mechanism of the (100) [001] texture evolution by grain growth was investigated by X-ray diffraction and back-reflection Laue diffraction method. The main results obtained are as follows. (1) (100) [001] secondary recrystallization texture evolved from a primary texture having a major orientation of {236} 〈5 15 9⦔ and including precipitates of AIN and MnS. The deviation angle of secondary grains from an ideal (100) [001] orientation increased with elevating annealing temperature. The evolution of a small amount of secondary grains other than (100) [001] orientation are also observed. They are (100) [001] (at 1150°C) and (100) [001] (at 1100°C and 1200°C). (2) The mechanism of the evolution of the (100) [001] texture by secondary recrystallization was found to be due to the highest frequency of Σ7 coincidence boundaries in relation to (100) [001] orientation in the matrix texture before the onset of secondary recrystallization. The evolution of the (100) [001] oriented secondary grains at the latter stage of the secondary recrystallization annealing was considered to be due to the second highest frequency of Σ7 coincidence boundaries in relation to (100) [011] orientation. The increase in the scatter of the orientation of (100) [001] secondaries from an ideal (100)[001] orientation was considered to be due to the decrease in the frequency of Σ7 coincidence boundaries with increasing deviation angle from an ideal (100) [001] orientation. (3) Present results supports the hypothesis proposed by the present authors that grains having the highest frequency of coincidence boundaries in the matrix can grow largest in the grain growth process.