Microstructure and texture development during annealing in UF340

UF340 is a new type of cold-rolled fine grain high-strength steel with excellent formability developed by Shougang Co., Ltd Company. In this paper, the formation mechanism of recrystallisation texture of UF340 during annealing at 720 °C were systematically investigated. The as-received cold rolled sheet displayed a pronounced α-fiber texture together with a weak γ-fiber texture. With increasing annealing time, recrystallization occurred in the elongated grains, γ-fiber texture grains generated and grew up gradually. The texture component of the original α-fiber grains shift toward {112} <110>, {023} <100>, {223}<352> and finally {111} <112>. High r-values of UF340 in all directions were achieved by the development of the remarkable γ-fiber texture. In addition, the r¯ value of the annealed UF340 was as high as 1.96, and the Δr value was only 0.2. As a result, excellent deep drawability and relatively high strength of UF340 were realized simultaneously, making it one of the ideal materials for automobile outer panels.


Introduction
Nowadays, with the intensification of market competition, complexity in the design of automobile bodies is gradually increasing, and the demands for materials with high formability is becoming more and more urgent [1,2].To fulfill these requirements, Beijing Shougang Co., Ltd.developed a new type of high-strength ultrafine grain steels with excellent deep drawing properties, high dent resistance and good surface property at room temperature, which have been successfully commercialized as Uni-Fish (UF) series.
Mechanical properties of polycrystalline metals are definitely related to their mcrostructures, like grain size, dislocation distribution, and crystal orientation [3][4][5].Among the different steel grades, interstitial free (IF) steel finds an important position in the automotive industry owing to its good deep drawability, which is achieved through a strong normal direction (ND) fiber recrystallization texture formed during the annealing process [6][7][8].For the UF steel, annealing process also plays a critical role in the formation of the final microstructure and quality of the products.Therefore, characterization and understanding the recrystallization behavior and the associated texture evolution during annealing process are essential to the product research and development.
In this study, a typical UF steel product UF340 (with tensile strength of 340 MPa) was used to investigate the recrystallization behavior and the evolution of microstructure, in particular, texture during the annealing progress.The tensile properties were tested.The microstructure and crystallographic texture of the material before and after annealing process were measured on the RD-ND (rolling direction-normal direction) plane using an Apreo 2 S scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD) system.From this observation plane, the shape change of cold-rolled grains during annealing process can be better observed.The specimens examined by EBSD were prepared by mechanical polishing and subsequently vibratory polishing on Vibromet 2. The analysis of the EBSD data was performed using AZtecCrystal software.

Microstructure evolution
The microstructure of the as-received (cold rolled) UF340 sample is shown in figure 1(a), which is characterized by thin lamellar bands elongated along the RD with the width of about 3-15 μm.The maximum aspect ratio of the grains is measured as 23.6, and the mean value is 6.6, indicating that the cold rolling process introduces a large amount of deformation, which provides sufficient stored energy and nucleation sites for subsequent recrystallization [9]. Figure 1(b) shows the φ2 = 45˚ orientation distribution function (ODF) section of the as-received sample.There is a strong α-fiber texture with the maximum intensity of 17.59 near {112}<110>, which means that the <110> direction of most grains is parallel to the RD.At the same time, a weak γ-fiber texture and rotated cube orientation {001}<110> can also be found in the sample.The microstructural evolution of UF340 annealed at 720 ℃ for different time is illustrated in figure 2. The average grain sizes of them are measured as 7.73 μm, 8.01 μm, 8.70 μm and 8.86 μm, respectively, which are smaller than ordinary IF steels (more than 10 μm) [10][11][12].The average aspect ratios of the <110>//RD oriented grains in these samples are calculated as 4.46, 3.85, 2.85 and 2.52, respectively.It can be seen in figure 2 that the elongated linear grains decrease obviously, the grains tend to be equiaxed and grow up gradually with increasing annealing time.The <111>//ND oriented grains grow up significantly, while the shape of them remains equiaxed during annealing progress.The area fraction of γ-fiber texture increases from 39.15% (8 s) to 42.9% (15 s), 48.04% (30 s) and 48.97% (54 s).The γ-fiber texture is a typical annealing texture in IF steel, however, its formation mechanism in UF steel still needs further investigation.For this purpose, the grains are divided into low aspect ratio part, i.e. grains whose aspect ratio lower than 2, and high aspect ratio part, the remaining grains whose aspect ratio belongs to the higher part of the data distribution.The φ2 = 45˚ ODF sections of high aspect ratio and low aspect ratio parts are calculated and shown in figure 3  It is obviously that the grains with high aspect ratio gradually changed from α-fiber texture to γ-fiber texture with increasing annealing time.The texture components of them shift toward {112} <110>, {023} <100>, {223}<352> and finally {111} <112>, which is different from IF steel ({112} <110> → {223} <110> →{111} <110>) [10].The grains with low aspect ratio maintain γ-fiber texture, mainly {111} <112> component, during the whole time.
Crystal orientation map of the sample annealed for 15 s are illustrated in figure 4(a).The {111} <110> and {111} <112> grains nucleate and grow up at the grain boundaries of the elongated α-fiber grains.The misorientation profiles from point A to point B in figure 4(a) is given in figure 4(b).A characteristic orientation gradient can be found within the deformed {112} <110> grain, indicating that enough stored energy for recrystallization nucleation and the driving force for interfacial migration required for grain growth can be provided here.During the annealing progress of UF340, on the one hand, the elongated grains in the cold rolled material transform from α-fiber texture to γ-fiber texture, and the grains tend to be equiaxed.On the other hand, recrystallized γ-fiber texture grains nucleate and gradually grow up.Finally, a relatively fine and uniform γ-fiber texture structure is formed in UF340.

Mechanical properties
Texture is an important characteristic for determining the material's formability.After recrystallization during annealing progress, UF340 obtains excellent deep drawing properties.The yield strength (YS), ultimate tensile strength (UTS), plastic strain ratio (r value) and the strain harden exponent (n value) are tested at 0°, 45° and 90° to the RD, and the data are given in table 2. The as-annealed UF340 has relatively high strength and low yield ratio.The ̅ value is calculated as 1.96, and the ∆r value is as low as 0.2.The UF340 steel has finer grains and higher r and n values, showing better deep drawing properties.

Conclusion
A new type of high-strength fine grain steel UF340 was used to investigate the microstructure evolution during annealing process and the tensile properties after annealing.Conclusions are summarized as follows: (1) The elongated α-fiber texture grains in the cold rolled sample shifted to {112} <110>, {023} <100>, {223}<352> and finally {111} <112>, and elongated grains gradually transformed into equiaxed ones.
(2) The γ-fiber texture grains generated at the grain boundaries of α-fiber texture grains and grew up gradually, the shape of which remained equiaxed.
(3) A strong γ-fiber texture was obtained during annealing process.The ̅ and the ∆r values of the

Figure 1 .
Figure 1.(a) Optical image and (b) φ2 = 45˚ ODF section of the cold rolled sample.

Figure 3 .
Figure 3. φ2 = 45˚ ODF sections of (a) high aspect ratio parts and (b) low aspect ratio parts in the UF340 samples annealed for 8 s, 15 s, 30 s and 54 s.

Table 1 .
Chemical compositions in mass % of UF340.

Table 2 .
Tensile properties of the annealed UF340.