Study on Formation Mechanism of Edge Cracks and Targeted Improvement in Hot-Rolled Sheets of Grain-Oriented Electrical Steel

Abstract

Edge cracks in hot-rolled sheets of industrial grain-oriented electrical steel significantly affect the yield rate and pose substantial challenges to cold rolling fabrication. Eliminating such structural defects through hot rolling requires a thorough understanding of their formation mechanism. This study investigates the formation mechanism of edge cracks in hot-rolled sheets, which are characterized by coarse strip-like grains with typical thicknesses ranging from 20 μm to 100 μm. Coarse, strip-shaped grains have low fracture stress, which is the cause of edge cracks. They originate from abnormally developed columnar grains in continuous casting slabs after reheating, which is unavoidable in industrial large-scale production. Inadequate fragmentation and insufficient recrystallization during rough rolling result in residual coarse grains of intermediate slabs, and their preferential deformation and outward protrusion lead to the formation of grooves. In the subsequent finishing rolling process, deformed coarse grains near the grooves undergo further elongation, developing into distinct strip-like structures. Based on the above mechanistic understanding, the edge microstructure under various rolling parameters was investigated, and targeted improvement measures for edge cracks were proposed. It is concluded that the edge quality can be significantly enhanced through increasing the total width reduction, additional rough rolling passes, and the implementation of edge heating during rough rolling. Quantitative analysis demonstrates that increasing the rolling passes from D to E significantly reduces the fraction of band structure from 64% to 48% and the average width of elongated grains from 43.5 μm to 38.4 μm.

1. Introduction

Grain-oriented electrical steel (CGO) is a special iron-silicon alloy with excellent electromagnetic conversion characteristics. The high magnetic induction intensity along the rolling direction makes it a crucial material for manufacturing transformer cores [1,2]. Edge cracking is a ubiquitous quality defect in CGO hot-rolled sheets. The defect usually occurs at one or both sides of a hot-rolled sheet edge, macroscopically characterized by delamination along the thickness direction. In mild cases, the cracks exhibit burrs intermittently distributed along the rolling direction, while in severe cases, the cracks exhibit serrations continuously distributed along the rolling direction. The severity of edge cracks in hot-rolled sheets directly affects the edge trimming amount required before cold rolling. For hot-rolled sheets with severe edge cracking, even after trimming, they are still prone to suffering from breakage during subsequent cold rolling and annealing processes, which significantly disrupts production rhythm. Therefore, mitigating the severity of edge cracking in CGO hot-rolled sheets is crucial for improving the yield rate and ensuring stable production.

To achieve high electrical resistivity and low iron loss, CGO and high-grade non-oriented electrical steel (NOES) require a high content of Si and Al, which significantly increases the difficulty of subsequent industrial rolling processes [3]. To mitigate rolling challenges, extensive research has been conducted on the hot-rolling and cold-rolling processes of electrical steel. Recent decades have witnessed significant progress in understanding and controlling edge cracking in NOES, particularly driven by the rapid development of new energy vehicle technologies. For instance, Chen et al. [4] investigated the formation mechanism of edge cracks in hot-rolled 1.4 wt% Si non-oriented silicon steel sheets, and found that cracks occurred when either the residence time of the slab in the reheating furnace exceeded 160 min or when the charging temperature was below 500 °C. They attributed the formation of edge cracks to high-temperature oxidation and decarburization during reheating. Similarly, Wu et al. [5] investigated the edge-cracking mechanism in hot-rolled high-grade NOES with 3.2 wt% Si content. The coarse grains at the slab edge deform and extrude along the width direction, thus overflowing onto the lower surface of the hot-rolled sheet. Therefore, the original edge boundary is enveloped by the extruded metal, creating crack initiation sites.

In contrast to NOES, investigations of the formation mechanism of edge cracks in CGO hot-rolled sheets are far less in-depth. The applicability of the aforementioned edge cracking mechanisms in CGO and their adaptation to industrial rolling conditions remain unclear. The higher reheating temperature required for CGO (necessary to achieve complete solid solution of inhibitor elements) increases slab susceptibility to severe overheating, significantly aggravating the risk of edge cracking during subsequent hot rolling. Current industrial approaches for mitigating edge cracking in CGO hot-rolled sheets include adjusting the rolling reduction ratio in roughing mill stands to refine the microstructure so that edge cracking along the width direction can be mitigated [6], edge-rolling at the F0 finishing mill stands to control edge shape [7,8], pre-rolling of slabs [9], increasing carbon content to regulate dendritic structure formation during continuous casting [10], applying edge heaters to prevent overcooling in edge regions [11], and adding Mo to suppress grain boundary oxidation [12]. These methods are typically employed in combination to reduce edge cracking severity. However, systematic studies on the evolution mechanism of edge cracking and its control strategies are much less penetrated, and no unified explanation has been established to date.

This study aims to analyze the formation and evolution mechanism of edge cracking in CGO hot-rolled sheets, so as to provide critical theoretical guidance for the industrial-scale production of CGO hot-rolled sheets with superior edge quality. In particular, in previous studies, little attention is paid to the effect of edge rolling during rough rolling on edge cracks. In this study, the edge microstructure and texture of hot-rolled sheets with different edge-rolling schemes are systematically investigated, employing metallographic microscopy and scanning electron microscopy equipped with electron backscatter diffraction (EBSD) systems. Then, targeted strategies for optimization to edge rolling based on actual industrial production conditions is explored, which provides a reference for grain-oriented electrical steel manufacturing.

2. Materials and Experimental Methods

In this work, the experimental material selected includes industrial CGO hot-rolled sheets produced by Ningbo Iron & Steel Co., Ltd. (Ningbo, China) with chemical composition, as presented in

Table 1. Chemical composition of samples (wt.%).

C	Si	Mn	P	S	Als	N	Cu
0.030~0.042	3.05~3.30	0.19~0.22	≤0.016	0.004~0.009	0.016~0.024	0.0075~0.0108	0.48~0.52

. The production process is illustrated in Figure 1a, involving 230 mm-thick as-cast slabs, 40 mm-thick intermediate slabs, and 2.3 mm-thick hot-rolled sheets.

In order to understand the solidified structure evolution of the as-cast slab, the slab samples obtained from the continuous caster were heated to temperatures ranging from 1200 °C to 1300 °C for 180 min using a muffle furnace in the laboratory, as illustrated in Figure 1b. For the observation of the structure characteristics of slabs before and after reheating treatment, the slabs were milled flat, polished, and then subjected to hot acid etching (HCl).

To further investigate the microstructural evolution of slabs during hot rolling and the effect of rolling process parameters on the edge microstructure of hot-rolled sheets, industrial-scale intermediate slabs and CGO hot-rolled sheets under varying rough rolling parameters were characterized. As shown in

Table 2. Rough rolling parameters of selected hot-rolled sheet.

	Total Reduction in Rough Rolling	Passes of Rough Rolling
Experimental group 1	A	D
Experimental group 2	B	D
Experimental group 3	C	D
Experimental group 4	C	E

, the total width reduction (i.e., the difference between the width of the slab and the hot-rolled sheets) gradually increases from A to C, and the rough rolling pass gradually increases from D to E. The microstructures near the crack region were examined using the optical microscopy (OM, LEICA DMI 5000 M, Leica Microsystems, Wetzlar, Germany). The grain orientation near the edge crack is determined by electron backscatter diffraction (EBSD, Nova Nano SEM400, FEI, Hillsboro, OR, USA).

3. Formation Mechanism of Edge Crack

3.1. Macroscopic Characteristics and Microstructure of Edge Crack

Figure 2a shows the typical macroscopic morphology of edge cracks in hot-rolled sheets. It can be clearly seen that the cracks do not occur in the whole length of a coil, but take place intermittently. After being uncoiled, the edge of the defect-free hot-rolled sheet is smooth and straight (Figure 2b), while the edge of the hot-rolled sheet with a defect is serrated, and the cracking direction is along the TD direction (Figure 2c). The cracks, with width of up to 12 mm, manifest as distinct V-shaped notches accompanied by significant outward protrusions of lower metal surfaces, resulting in pronounced delamination in the thickness direction. Notably, the width of outward protrusion is basically the same as the crack depth.

The microscopic analysis of the edge microstructure of a hot-rolled sheet with a defect and a defect-free hot-rolled sheet is performed in order to explore the formation mechanism of an edge crack. Figure 2d schematically illustrates the sampling locations and the observation planes. The observation plane ① is the surface used for the analysis of crack initiation sites and the microstructure of the crack-affected zone, as well as the edge microstructure of the defect-free hot-rolled sheet. Since the surfaces of the large and open cracks were heavily oxidized at hot-rolling temperatures, they cannot be directly used for fractographic analysis. Before observation, the sample requires embedding, grinding to a certain depth, and polishing, so as to maintain the original microstructure, while removing the surface oxide layer. The observation plane ② is a standard cross-section used for the analysis of microstructural inhomogeneity in the thickness and transverse direction of the edge, and the normal metallographic sample preparation method is required.

Figure 3a,c presents the edge microstructure of the defect-free sheet observed from observation planes ① and ②. The microstructure primarily consists of deformed fiber grains, with elongated ferrite and pearlite distributed along the rolling direction, exhibiting relatively uniform phase distribution overall. Figure 3b,d display the microstructure of the edge cracking region in the hot-rolled sheet, observed from observation planes ① and ②. Figure 3b reveals significant microstructural inhomogeneity near the crack. The crack is surrounded by flow-deformed coarse ferrite, which is separated by micro-cracks and voids. Figure 3d demonstrates pronounced through-thickness heterogeneity. It can be found that there are coarse strip-like grains along the transverse direction, which may correspond to the coarse ferrite in Figure 3b, and their width ranges from 20 μm to 100 μm, and the length ranges from 10 mm to 15 mm. Between these coarse, strip-like grains, deformed fiber grains and pearlite are sandwiched. Pearlite is the key to phase transformation strengthening. As a strengthening phase, the size and distribution of pearlite determine the effectiveness of the phase transformation strengthening effect. Only fine pearlite dispersed in ferrite can ensure strength and toughness. In this study, coarse strip-like ferrite resulted in uneven pearlite distribution, failing to achieve phase transformation strengthening effects, with insufficient strength and toughness at the edges.

Figure 4a shows the band contrast (BC) map of the cross-section microstructure near the crack region. The bright areas in the image correspond to smaller strain/stress accumulation, while the darker areas correspond to larger strain/stress accumulation. Plastic deformation is primarily accomplished through slip. Figure 4b presents the distribution of the Schmid factor (SF). The larger SF value corresponds to a greater probability of slip system activation; therefore, the red color (i.e., large SF) means a grain with soft crystallographic orientation, and the blue color means hard orientation. It can be observed that the strip-like grains possess a relatively large SF, which suggests that plastic deformation is more likely to happen with these grains. Figure 4c shows geometrically necessary dislocation (GND) density, which can reflect the dislocations required to accommodate plastic deformation in crystalline materials, ensuring compatibility of the crystal lattice [13]. It can be found that the density of dislocations is lower in strip-like grains, while there are many more dislocations in the deformed fiber grains, which is consistent with the literature [5]. The grain textures are shown in Figure 4d. The coarse strip-like grain primarily exhibits rotated cube texture {100} <011>, whereas the textures of deformed fiber grains are predominantly {111} <011> and {100} <011>. Studies have proven that the rotated cube texture {100} in hot-rolled sheets is inherited from the {100} columnar grains present in the as-cast slab of grain-oriented electric steel.

The uneven distribution of GND is the inevitable result of non-uniform deformation [14]. The smaller GND density in strip-like grains enveloped by deformed fiber grains indicates a smaller resistance to the dislocation movement in strip-like grains during deformation, such as less grain boundary obstruction. Therefore, dislocations are easy to slip out of the strip-like grains rather than accumulate, and the deformation coordination between adjacent grains is better. Combined with the typical characteristics of large SF values in strip-like grains, it can be inferred that the strip-like grains undergo preferential deformation. This is consistent with the study pertinent to mixed grain [15] and plastic deformation [16,17]. Combined with Figure 3b, the elongated coarse grains (i.e., strip-like grains) in hot-rolled sheets are likely deformation products of coarse grains in slabs. The edge cracks are predominantly located within elongated coarse grains with the crack propagation direction perpendicular to the rolling direction, indicating that crack initiation is more likely to occur within the elongated coarse grains [18,19] and is attributed to the fracture of these elongated coarse grains. Compared with elongated fine grains (i.e., deformed fiber grain), elongated coarse grains, especially those millimeter-grade grains in the crack region of Figure 3, are more prone to fracture due to their lower yield strength and higher crack initiation probability [20,21].

3.2. Evolution of Coarse Grain at Slab Edge During Reheating and Rough Rolling

To investigate the origin of coarse grain in hot-rolled sheets, samples cut from as-cast slabs were reheated to 1200~1300 °C for 3 h in high-pure N₂ using a laboratory muffle furnace, so as to replicate reheating schedule of industrial as-cast slab.

Figure 5a presents the cross-section macrostructure of cutting slabs before reheating, revealing a significant edge bulge that deviates from the ideal rectangular profile. The slab edge face and broad face envelop columnar grains, while the inner region consists of equiaxed grains. Figure 5b,c shows the microstructural evolution of the slab edge with rising reheating temperatures. Notably, abnormal grain growth of columnar grains is observed after reheating, with maximum columnar grain widths reaching 8 mm at 1300 °C. With increasing reheating temperature, the columnar grains exhibit significant width expansion along the normal direction, whereas the equiaxed grain size remains essentially unchanged, consistent with previous reports by Landauer et al. [22] and Wu et al. [5].

As evident in Figure 5c, although columnar grains at the edge side surfaces and broad faces both present abnormal growth with increasing temperature, the abnormal growth is markedly more pronounced on the side surfaces. Wu et al. [5] attribute this abnormal growth behavior to the substantial accumulation of strain energy at the bulging edge during continuous casting, which provides an enhanced driving force for the preferential growth of columnar grain. In the conventional industrial production of CGO, continuous casting slabs are hot-charged into a 500 °C heating furnace without stress relief treatment; therefore, abnormally coarse grains inevitably occur after reheating. Critically, if the hot-rolling measures are not appropriate, they would evolve into mixed grain structures. Such a defect would not only inherit to the hot-rolled sheet, but also to the cold-rolling stage, ultimately resulting in edge cracks in cold-rolled products [23].

Through on-site industrial production tracking, significant differences were identified between the edge morphology of the intermediate slab corresponding to the defect-free hot-rolled sheet and that corresponding to the hot-rolled sheet with a serious edge crack (Figure 6). For intermediate slabs corresponding to the defect-free sheet, the macroscopic edge profile exhibits a smooth arc shape (Figure 6a). Microstructural analysis (Figure 6b) reveals fine and uniform grains, with an average grain size of approximately ASTM 7.0. In contrast, the intermediate slabs corresponding to hot-rolled sheets with edge cracks display irregular, non-arcuate edge profiles featuring multiple grooves and cracks, with pronounced wavy undulations along their propagation paths (Figure 6). The edge microstructure of intermediate slabs with such defects predominantly consists of millimeter-grade coarse grains (Figure 6d), which would undergo both transverse widening and longitudinal stretching in the subsequent finishing rolling and result in delamination in thickness direction and crack initiation.

Based on the above analysis, it can be reasonably concluded that the inadequate fragmentation and insufficient recrystallization of coarse grains at the edge constitute primary contributing factors to edge cracking in CGO hot-rolled sheets. Generally, hot rolling can hardly cause cracks at the edge of a hot-rolled sheet when the slab has a uniform casting microstructure. However, due to the accumulation of strain energy at the bulging edge (Figure 7a), the slab edge region develops abnormally coarse columnar grains (Figure 7b). Therefore, the coordinated deformation mechanism for the adjacent grains will be disrupted during hot rolling [24], and the coarse grains preferentially deform and protrude outward abnormally (Figure 7c), leading to the formation of a groove at the edge of intermediate slabs. During the subsequent finishing rolling, these coarse grains near the grooves are further elongated and deformed into coarse strip-like grains (Figure 7d), which exhibit significantly lower fracture strength compared to the fine grains, so they are more prone to cracking near the groove. Notably, the edge regions adjacent to the cracks experience localized stress relaxation, thus preventing further cracking along the edge, which explains why edge cracks typically exhibit an intermittent distribution pattern.

4. Effect of Hot-Rolling Process on Edge Microstructure of Hot-Rolled Sheet

The typical characteristic of the edge crack region in hot-rolled CGO originates from coarse grains at the slab edge. In industrial production, vertical roll edging during rough rolling can reduce the coarse grain size, and the total width reduction is a critical process variable that determines the edge microstructure of the hot-rolled sheet. Figure 8, Figure 9 and Figure 10 present the edge microstructure of the hot-rolled sheet under total width reductions in A, B, and C, respectively, at the rough rolling pass number of D.

Figure 8 illustrates the cross-section microstructure of the crack region in a hot-rolled sheet under total width reduction in A. Notably, mixed grain is highly pronounced, with coarse strip-like grains (thickness of 200 μm) dominating the region within 2.5 mm from the edge (Figure 8a). Intergranular gaps are observed between these coarse structures, which is consistent with the macroscopic edge defect shown in Figure 2. From Figure 8a–d, the proportion of strip-like grain and its thickness gradually decrease from the edge to the inside. In summary, the total width reduction in A results in the insufficient refinement of coarse grains and a high risk of edge cracking of the hot-rolled sheet.

Figure 9 illustrates the cross-section microstructure of the cracked region in a hot-rolled sheet under the total width reduction in B. The edge region displays a tearing structure characterized by the pronounced flow deformation and internal porosity (Figure 9a). Within 10 mm from the edge, the core region consists of deformed fiber grains (Figure 9b–d), while mixtures of coarse grains and fine grains are observed near the surface (Figure 9e). From the edge to the inside, there is a gradual decrease in the proportion of coarse grains with insufficient fragmentation near the surface, accompanied by an increase in surface recrystallized grains, which may be attributed to the temperature drop near the edge. In comparison to the edge microstructure under total width reduction in A, there is a significant reduction in the proportion of coarse grains, but the flow deformation still exists.

Figure 10 illustrates the cross-section microstructure of the crack region in a hot-rolled sheet under a total width reduction in C. It is evident that the dominant microstructure consists of deformed fiber grains. However, there are residual coarse grains near one rolling surface within 7.5 mm from the edge. In comparison to the microstructure shown in Figure 9, a further reduction in coarse grains at the edge is observed, and the flow deformation vanishes. Nevertheless, the residual coarse grains indicate that the process parameters require further optimization.

In summary, applying a large total width reduction during rough rolling is beneficial for the improvement of edge quality and reduces the probability of edge cracking. On the one hand, the fragmentation of coarse grains at slab edges is enhanced, thereby reducing the proportion of mixed grains and improving microstructural homogeneity; on the other hand, increased stress and strain at the edge are beneficial for enhancing the driving force of dynamic recrystallization and refining the microstructure of the hot-rolled sheet edge. Similarly, increasing the edge temperature can achieve this purpose.

4.1. Effect of Rough Rolling Pass on Microstructure of Hot-Rolled Sheet

Although increasing the width reduction can effectively improve edge quality and reduce the risk of edge cracking, as aforementioned, residual coarse grains still exist at the edges even if the width reduction is increased to a critical value of C (further increasing width reduction may lead to cobble risks), as shown in Figure 10. This indicates that relying solely on large width reduction cannot fully resolve edge defect issues. Therefore, additional measures must be incorporated to further refine microstructure of hot-rolled sheet edges and reduce the proportion of mixed grain.

Figure 11a presents the cross-section microstructure of the hot-rolled sheet edge in experimental group 4 (i.e., total width reduction in C and rolling pass E). The edge region primarily consists of deformed fiber grains, revealing microstructural homogeneity along the width direction. This indicates that when the rolling pass increases from D to E, the coarse grains near the edge are fragmented sufficiently during hot rolling. Consequently, the stress–strain distribution at the edge becomes more uniform, substantially reducing the risk of edge cracking. For further evaluation of banding severity, the longitudinal-section microstructure of the hot-rolled sheet from experimental groups 3 and 4 is characterized (Figure 11b,c). Quantitative analysis demonstrates that increasing the rolling passes from D to E significantly reduces the fraction of band structure from 64% to 48% and the average width of elongated grains from 43.5 μm to 38.4 μm, accompanied by a corresponding increase in the fraction of recrystallized grain near the surface. For hot-rolled sheet edges in experimental group 4, the yield strength and tensile strength increase to 481 MPa and 627 MPa, respectively (

Table 3. Mechanical properties of hot-rolled sheet edge under different optimized processes.

	Yield Strength/Mpa	Tensile Strength/Mpa
Experimental group 1	438	557
Experimental group 2	460	584
Experimental group 3	477	601
Experimental group 4	481	627

), and the enhanced edge ductility can efficiently reduce cracking susceptibility in the hot-rolled sheet. In conclusion, increasing the number of rough rolling passes can effectively mitigate banding severity, thereby improving processability for subsequent cold rolling operations.

4.2. Improvement Measures for Edge Crack of Hot-Rolled Sheet

Based on the aforementioned formation mechanism of edge cracking, mitigating edge cracking severity requires precisely controlling the edge microstructure, which can be implemented by suppressing the abnormal growth of columnar grains at the edge and promoting refinement and recrystallization of coarse grains.

To suppress the abnormal growth of columnar grains, it is critical to minimize the slab’s residence time in the high-temperature zone on the premise of ensuring the tapping temperature and section temperature difference. On one hand, grain growth can be effectively suppressed, facilitating grain fragmentation and refinement during subsequent hot rolling. On the other hand, decarburization and oxidation degree are reduced; otherwise, serious decarburization would hinder edge grain recrystallization under an oxidizing atmosphere in the reheating furnace and the internal oxidation of silicon at grain boundaries, increasing the risk of intergranular cracking.

To facilitate the recrystallization of coarse grains, the optimization of the rough rolling process is recommended. As discussed above, adopting total large width reduction and increasing rough rolling passes can effectively promote the fragmentation and refinement of coarse grains at the edge while enhancing the recrystallization driving force, consequently reducing the edge cracking risk. Additionally, increasing the edge temperature through induction heating before finishing rolling to provide appropriate temperature compensation within specific edge regions can further promote edge recrystallization.

5. Conclusions

Edge cracking in hot-rolled sheets is a critical problem in manufacturing grain-oriented electrical steel (CGO). In this study, the edge microstructure of CGO hot-rolled sheets with edge cracks was investigated, and the formation mechanism of edge cracks was analyzed. The main conclusions are summarized as follows:

(1)

Hot-rolled sheets with edge cracks display inhomogeneous microstructures consisting of coarse strip-like grains and deformed fiber grains. The former exhibits a low dislocation density and high Schmid factor, along with numerous microcracks and voids. The latter is sandwiched between coarse strip-like grains and demonstrates a higher dislocation density and a lower Schmid factor.

(2)

The bulge-induced internal stress at the slab edge promotes the abnormal growth of columnar grains during high-temperature reheating. The coarse grains would deform preferentially and further elongate during hot rolling, ultimately developing into elongated coarse grains and edge cracks.

(3)

Increasing total width reduction and increasing rough rolling passes can enhance recrystallized grain proportion and refine the strip-like grain. Combining these measures can effectively eliminate edge defects.