Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 2, 2020
Article Number 204
Number of page(s) 16
DOI https://doi.org/10.1051/metal/2020014
Published online 30 March 2020

© EDP Sciences, 2020

1 Introduction

Chamfer technology is recognized as the best solution to prevent transverse corner cracks forming at the slab surface of micro-alloyed steel completely, but its application is very difficult [13]. The technical optimization of the mold has been firstly concerned in 1976, and various geometries of the mold have been researched [4]. Chamfer technology was not in successful progress for the industrial application from the late 1980s onwards. Insufficient cooling in narrow side mold, limit on the casting speed and the reduced working life of the mold restricted the application of chamfer technology. According to theoretical research results, combined with working conditions of slab caster, patented technologies and related equipment were developed [5,6]. The research has broken the technology bottlenecks of the technology industrial application, while ensuring the long-life operation of the chamfer mold.

That chamfering the corners of a continuous casting mold can increase the temperature field of a slab corner is a common idea when facing corner cracks problems. However, this idea of a chamfer mold technology can also modify the strain and stress field of the bloom at the solidification end because it changes the solidified shell at the corner edge of the bloom [7,8]. Reduction force of soft reduction technology mainly occurs in the centre area of a continuous casting bloom, which can effectively compensate the solidification shrinkage to improve the internal quality of continuous casting bloom and the deformation resistance decreases significantly. A chamfer technology of continuous casting bloom, combined with numerical simulation and industrial application of our team is applied to reduce the risk of internal cracks occurring in as-cast bloom. In addition, the chamfer geometries of bloom have been optimized to decrease and avoid the off-corner cracks in the soft reduction zones.

Chamfer technology has been used to investigate influences of slab corners configuration on the straight edge seam defects of rolled strip [9]. The chamfer narrow side mold was firstly developed and implemented on the flexible thin slab casting (FTSC) line in thin slab casting processes. A chamfer slab can raise the temperature near the edge, and make uniform the temperature distribution of chamfer slab, which will enlarge the homogeneous deformation region between the slab center and edge during the hot rolling process. A chamfer slab technology is an effective way to decrease the straight edge seam defects because the shape of the region near edge on chamfer slab before broadside rolled is more ideal. The rolling conditions can be improved and the amount of bulging can be decreased. Therefore, employing a chamfer mold with proper corner size can reduce the tension stress in the corner region and simultaneously avoid straight edge seam defects effectively during hot strip rolling. Chamfer technology [10] is also used in thin slab casting processes to reduce the occurrence of longitudinal surface crack of slabs. Longitudinal surface cracks in continuous casting process of slab are closely related with inadequate amount of narrow face taper. Therefore, chamfer technology can reduce the occurrence of longitudinal crack of slabs, straight edge seam defects of rolling strip, and prolong service life of narrow face mold.

Despite the importance of theoretical and applied research in chamfer technology, a comprehensive review of these studies has not yet been made. Consequently, the aim of this paper is to provide an in depth analysis of the available studies, categorise the employed approaches, and summarise the obtained results, which aims to provide theoretical and practical basis for large-scale application of chamfer technology in continuous casting process and thus effectively improve the quality of continuously cast products.

2 Cracks formation in the continuous casting of steel

2.1 Slab transverse corner cracks

The transverse corner cracks usually occur both on the loose side and fixed-side corners of micro-alloyed slabs [1114], and these cracks can transform into edge defects on the hot rolled sheet in the subsequent rolling process. The typical transverse corner cracks occurring in slab corners are presented in Figure 1. Scarfing operation has been used to remove transverse corner cracks from slab corners, however, longer and deeper cracks will still appear on the scrapped slab corners.

Transverse corner crack, one of the most common defects on slab surfaces, is associated with tensile stress concentration during the bending and straightening operation. Additional expenditure of scarfing operation has been required to remove these defects, and lead to delays in fulfilling orders.

thumbnail Fig. 1

Transverse corner cracks occurring in slab corners.

2.2 Longitudinal surface crack defects in as-cast thin slabs

The longitudinal surface crack defects are always located in the centre and quarter of the thin slabs, which can vary from a few centimeters up to a few meters [1518]. A macrograph of typical longitudinal surface crack is shown in Figure 2a. The optical micrographs of longitudinal surface cracks are often associated with surface depressions about 1 mm deep and occur almost equally on the fixed and loose side as shown in Figure 2b. Cracks are openings found on the thin slab surface, with variable length and depth.

Longitudinal surface cracks are affected by a network of transverse and longitudinal heterogeneities in the thickness of solidified shell. A homogeneous solidified shell helps to avoid the formation of longitudinal surface cracks, and reduce the risk of surface cracks. Some cracks on the failed surface are split apart and some near the surface are arranged along almost a straight line along the casting direction, which coincides with the orientation feature of longitudinal surface cracks.

Almost without exception, they are found to be difficult to eliminate during subsequent hot rolling and consequently they will constitute defects on rolled products, which are shown in Figure 3.

thumbnail Fig. 2

Longitudinal surface cracks of as-cast thin slab: (a) on the surface, (b) cross section.

thumbnail Fig. 3

Longitudinal cracks on the surface of strip.

2.3 Irregular deformed geometry near thin slab corners

Straight edge seam defects of rolled strip are sensitive to initial shape, so that the ideal deformation shape and surface quality of thin slab must meet strict requirements, especially for FTSC. However, the initial deformed geometry in corner regions of non-oriented silicon electrical steel thin slab was called “corner rotation” of solidified shell [1923]. A concave geometry of solidified shell was observed as shown in Figure 4. The stress, strain status and distribution of temperature between the center and the edge of thin slab are different during the rolling process. The thin slab corners gradually overturn to the top surface after rolling processes, and on the narrow side of thin slab can promote the movement of defects.

Generally, on hot rolling strip, a surface defects called the edge seam defect, as shown in Figure 5a, paralleling to the rolling direction arise from the wrinkles generated at the edges of the hot rolled non-oriented silicon strip. Then the defect samples are acquired forming the finished cold rolled sheet, and the defect occurring in the industry-produced sheet is shown in Figure 5b. All the defects are on a straight sliver line kept at a given distance from the edge of the sheet.

The defects, which remained on the surface after rolling, were analyzed in several directions and cross sections. Figure 6 shows a schematic illustration of side trimming which is close to both edges of the strip, at some distance from the edge.

Figure 7 shows the patterns of the defects shape and depth in cross section after hot rolling and pickling. The defects are openings found on the slab surface with variable length and depth. The cracks are not always linear lamination type defects. They are sometimes interrupted and further continued in zigzag. The cross sectional shape of defects was compared in Figure 7, the average depth of edge seam defects is about 480 μm. The opening direction of cracks is facing to the side of the industrial produced plate.

thumbnail Fig. 4

Macrograph of typical non-oriented silicon electrical steel thin slab.

thumbnail Fig. 5

Macrograph of typical edge seam defect on the products surface: (a) on the hot-rolled strip, (b) on the cold-rolled sheet.

thumbnail Fig. 6

Schematic illustration of sampling method.

thumbnail Fig. 7

Optical micrograph of defects shape and depth after hot rolling in cross section.

2.4 Industrial morphology of observed internal cracking in as-cast bloom

A high tendency of internal cracking in the continuously cast bloom appears exclusively in the midway region between the center and the bloom surface induced by the soft reduction process, as shown in Figure 8. Internal cracks of bloom in the longitudinal section form a certain angle with the bloom casting direction. In addition, the pores were always found in vicinity of internal crack [2427]. The observed result indicates that a substantial number of large voids and internal cracks are mainly distributed around the bloom internal regions.

Non-uniform distribution of alloying elements is one of the most important phenomena, which occur during steel solidification. Carbon element is concentrated on the boundaries of crack formation, as shown in Figure 9. The regions can act as crack initiators due to the stresses from solidification and thermal gradients. Furthermore, the brittle phases can form and provide a fracture path under tensile stress around the boundaries of crack formation. As tensile stresses occur at the boundaries of the formed cracks, these microcracks further propagate into larger dominant cracks.

thumbnail Fig. 8

Macrographs of internal defects in bloom longitudinal section.

thumbnail Fig. 9

EDS line scans across internal crack boundaries.

3 Design of chamfer mold and geometry of the narrow side of the mold

The purpose of this invention is to provide a kind of chamfer design for a mold with a funnel-shaped curved surface, which can effectively compensate the shrinkage of the solidified shell, enhance the solidification uniformity of the solidified shell and prolong the service life of the narrow side mold [2830].

In order to achieve the above purposes, the following inventions and technical solutions were provided, which are shown as follows:

  • working surface of narrow side mold comprises a middle flat and two side chamfer areas or a funnel-shaped curved surface;

  • width of narrow side mold is constant from the top to the bottom of mold;

  • double tapers structure of narrow face was adopted on the height direction of chamfer mold.

The design of the chamfer narrow face is schematically shown in Figure 10. During operation, the working face of the narrow side of the mold (1) comprises a middle flat area (2) and two chamfer areas (3, 4) which are arranged at two sides.

The multiple tapers of chamfer narrow side mold are shown in Figure 11. The chamfer angle of the mold is chamfer angle α of Figure 11. The contour line of the top edge of the middle area (2) is straight line, the contour lines of the two chamfer areas (3, 4) are concave curves, and the contour lines of bottom edge are composed of straight lines. In addition, the geometrical parameters (H, L, alpha) have been a functions of the height direction.

thumbnail Fig. 10

Schematic diagram of chamfer narrow face designing.

thumbnail Fig. 11

The cross section of the narrow side mold perpendicular to the height direction.

4 Results and discussion

4.1 Application of chamfer technology in continuous casting slab

The simulation results of the fluid flow, heat transfer and solidification within the mold show that the temperature of the slab corner is approximatively increased linearly with the increase of the chamfer angle, but the fluid flow is enhanced near the corner between the wide side and the narrow side of the slab. Figure 12 shows the photo of the chamfer mold applied in industrial production.

The results of the stress-strain Finite Element Analysis during the straightening process show that under the condition of the same chamfer length and straightening speed, the equivalent tangential strain of the slab corner is relatively minimal when the chamfer angle is 30° with the same cross section area.

Figure 13 shows the tensile stress in the casting direction during the straightening process of slabs, reducing the tensile stress of the slab corner can reduce the risk of corner crack effectively. The unit of stress is MPa in Figure 13. In addition, the horizontal axis is the width of the slab, and the vertical axis is the thickness of the slab. The comparison of the simulation results show that the maximum stress value at the edge is reduced from 16 MPa for a rectangular slab to 10 MPa for a chamfer slab. The chamfer slab reduces the tensile stress at the same straightening condition, and the maximum stress is reduced 37.5%.

The corner temperatures of the chamfer slabs and of the conventional slabs have been measured by infrared thermometer. The measurement results show that the temperature of the chamfer slab corner at the straightening segment is approximately 100 °C higher than the conventional slab. Slabs at straightening stage during continuous casting are shown in Figure 14.

Deformation shape of chamfer slab and rectangle slab after R1 rolling are shown in Figure 15. The corner temperature of chamfer slab is higher than that of rectangle slab, so the stress and strain status of the slab edge between the chamfer and rectangle slab are different during hot strip rolling [31]. The risk of straight seam was reduced by chamfer slab completely, due to a better temperature homogenization and ideal deformation shape.

Figure 16 is the configuration of an industrial chamfer slab. The results of industrial trials show that the transverse corner cracks for niobium, vanadium, titanium micro-alloyed steels produced by chamfer mold have significantly been reduced more than 95% compared with those in the traditional mold. The corners of the casting slab, which are produced by the chamfer mold technology, do not need scarfing. The successful application of the chamfer casting slab technology means that the problem of transverse corner cracks of micro-alloyed steel can be expected to be solved fundamentally. The casting speed of micro-alloyed steel slab by continuous casting was increased from 1.0 m/min to 1.2 m/min. Average used lifetime of chamfer narrow side mold is more than traditional right-angle narrow side mold.

thumbnail Fig. 12

The photo of the chamfer mold applied in industrial production.

thumbnail Fig. 13

Tensile stress in casting direction during straightening process of slabs: (a) rectangle slab, (b) chamfer slab.

thumbnail Fig. 14

Slabs at straightening stage during continuous casting: (a) chamfer slab, (b) rectangle slab.

thumbnail Fig. 15

Deformation shape after R1 rolling: (a) chamfer slab, (b) rectangle slab.

thumbnail Fig. 16

Configuration of industrial chamfer slab.

4.2 Application of chamfer technology in the continuous casting of blooms

The lowest temperature of the bloom cross section is located at bloom corner. The bloom corners resist and contribute to the effective deformation at the central area of the bloom in order to eliminate the center segregation and contribute to the deformation mainly occurring in the central area. The temperature field of the bloom corners can be drastically increased by optimizing the chamfer geometries, and the chamfer has more influence on the deformation and the reduction force. In addition, internal crack initiates when the strain accumulated between zero strength temperature (ZST) and zero ductility temperature (ZDT) exceeded a critical value [32]. In order to improve product quality and productivity of continuous casting bloom, an optimal design approach helped to find the optimized chamfer geometries. The chamfer geometries of continuous casting bloom were performed with different geometric features as illustrated in Figure 17.

The temperature distribution of the bloom has been calculated firstly, then the initial temperature fields were transferred to the 3D mechanical model, which was introduced in detail elsewhere [11]. Reduction rolls were regarded as discrete rigid bodies without deformation, and the bloom with a cross section of 280 mm × 325 mm was regarded as deformable bodies, the casting speed of the bloom was 0.7 m/min during the soft reduction process. In addition, the 3D bloom mechanical models have been developed using the software ABAQUS® under soft reduction conditions (Fig. 18). During the mechanical reduction between the reduction rolls and bloom surface, the friction coefficient was set as 0.3 [33], and the conversion factor of plastic heat generation was adopted as 0.9 [33]. A global coordinate system (z for the casting direction, x for transverse, y for vertical) was taken in this research.

The properties of as-cast bloom, such as density, thermal conductivity, specific heat capacity and thermal expansion coefficient, were taken from our previous papers [8,9]. In addition, a constitutive equation of high carbon steel [8,9] was adopted to describe the metal deformation behavior of the as-cast bloom. The applied constitutive equations is suitable to obtain accuracy of mechanical properties based on high temperature intervals in the 3D mechanical model.

In order to achieve a proper and reliable chamfer geometry, strategies for optimizing the chamfer geometry of the bloom cross section were investigated. The optimized chamfer angles are analyzed firstly.

The stress concentration distributions in a continuous casting bloom was calculated with the finite element method. The model was based on two different actual production bloom caster, one equipped with a small chamfer, another in a special steelworks with a chamfer with a single angle 45° and a length of 10 mm. In the model, the angle and the length of the chamfer were different. Figure 19 presents the effect of the chamfer angle on the stress concentration induced by a reduction operation. The correlation between stress states, von Mises equivalent stress and chamfer angle (α) were investigated under same reduction amount of 4 mm. The chamfer angles (α) are 30°, 40° and 60°, and the chamfer length (h) is a constant value of 50 mm. In order to characterize the distribution and the shape of internal cracks in continuous casting bloom, samples were taken from the brittle temperature range area of the bloom as it is shown on Figure 20.

Figure 21 shows the photographs of internal crack in the XZ face of specimens. It is apparent that the internal crack path is irregular and several internal cracks are located near the central part. Deformation of the continuous casting bloom leads to an increase of migrations, linearity and sensibility of grain boundaries within the brittle temperature region, which also indicates an increased tendency to the internal cracks. Crack propagation is retarded by relaxation of local stresses along grain boundaries of base metal owing to deformation taking place at the crack mouth. It is identified that initial cracks become dominant with the greatest length, at whatever depth, measured in the direction perpendicular to the stress axis. Abaqus gives stress S11 (sigma_xx, in the transverse direction) in Figure 22 and S33 (sigma_zz, in the casting direction) in Figure 24.

In Figure 21, with casting direction z horizontal and transverse direction x vertical, some of the segments of the crack open under the influence of S11 (transverse stress, blue arrows) and other due to S33 (red arrows, longitudinal stress).

Values of tensile stress are necessary to calculate the damage accumulation in the micro-domain. Hence, the maximum tensile stress determines the location of initial cracks. Maximum tensile stress is almost in the same area of the brittle temperature range under different chamfer angles as shown in Figure 22. Internal cracks of continuous casting bloom can be initiated when the stress concentration exceeds the critical fracture stress during soft reduction operation. Internal crack appears at the sites of maximum tensile stress near the boundary in the brittle temperature range [34,35] With the increase of chamfer angle (α), the maximum tensile stress gradually decreased in the brittle temperature range. The risk of internal cracks by stress concentration can be reduced. However, the area of stress concentration is decreased when the chamfer angle (α) is increased, which can be seen in Figure 23.

The stress generated by reduction operation is gradually decreased with the chamfer angle increasing, and tensile stress has changed to compressive stress when the chamfer angle is increased as shown in the Figure 24. The maximum stress in the brittle temperature range is dramatically decreased with chamfer angle (α) increasing of the chamfer geometries as shown in the Figure 25.

Application results of chamfer bloom during continuous casting process as shown in Figure 26, the temperature fields of the bloom corners rapidly increased due to the optimal length. The deformation zone of bloom occurs in the middle high temperature area, so that the deformation resistance of the bloom is significantly reduced.

Internal cracks in bloom cannot be completely eliminated by conventional reduction. However, internal cracks and shrinkage cavities are almost eliminated using chamfer bloom, showed on Figure 27. In the practice, the produced chamfer bloom ensures higher quality and productivity. In order to reduce the risk of internal cracks in the brittle temperature range, increasing chamfer angle and chamfer length are the most effective way to get the highest reduction efficiency. Thus, the chamfer bloom is suitable for the reduction technology because the deformation mainly occurs in the bloom center. And the internal quality of the bloom can be significantly improved.

thumbnail Fig. 17

Section of chamfer bloom during soft reduction.

thumbnail Fig. 18

3D finite element model and temperature distribution for deformation analysis.

thumbnail Fig. 19

Von Mises equivalent stress in the bloom with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

thumbnail Fig. 20

Illustration of the sample point and sample place in the bloom.

thumbnail Fig. 21

Local stress for initial brittle crack propagation in XZ face of steel.

thumbnail Fig. 22

Tensile stress in the brittle temperature range with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

thumbnail Fig. 23

Maximum tensile stress in brittle temperature range with different chamfer angles.

thumbnail Fig. 24

Stress along direction z in the brittle temperature region with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

thumbnail Fig. 25

Maximum shear stress in brittle temperature range with different chamfer angles.

thumbnail Fig. 26

Application results of chamfer bloom during continuous casting process.

thumbnail Fig. 27

Macrostructure photos of the middle carbon chamfer bloom under soft reduction: (a) transverse section, (b) longitudinal section.

4.3 Application of chamfer technology in continuous casting thin slab

Controlling the surface longitudinal crack formation is concomitant with the optimization of the contact situation between mold and solidified shell around the narrow face corner, which can be assured by adjusting taper of mold. The irregular deformed geometry near the thin slab corner and the straight edge seam defect formation on the surface near edge of the hot rolled strip was analyzed. A chamfer mold was developed and implemented successfully in the industrial production process. Figure 28 shows a schematic view of a funnel-mold for thin slab casting process. This particular funnel design has flat, parallel sections in the center of mold and near the narrow faces. The exact taper of narrow face mold in continuous thin slab casters is essential for optimized surface quality of the thin slab. The taper has a substantial effect on shrinkage process, shell growth and development of surface cracks. However, abrasive wear can take place during sliding contacts [36], mold wear can occur on the interface between thin slab and mold when thin slab moves down ward through the mold during casting.

Wear photographs of narrow face mold after several hundred heats are shown in Figure 29a and 29b, respectively. The appropriate amount of mold taper is an important design parameter, and this taper of narrow face mold is larger than that of wide face one.

At larger narrow face taper, the value of interfacial pressure between the thin slab and mold is enlarged so that the narrow face mold can be easily worn. The wear amounts of narrow side mold are more serious than wide side mold. The effective wear amount of narrow side mold operated after over 130∼150 heats was shown on Figure 30.

The effective wear amount of the narrow side mold is affected by both width and height position of the mold. There is a minimum wear amount at mid-width of mold. Then it increases monotonically near the corner of mold. As Figure 30a shows, in the transverse of narrow side mold, abrasion curves of narrow surface are mainly symmetry in the mold centerline. The wear amount and shape of narrow side mold are little and convex in the mold centerline, while they are large and concave on both sides. The greatest extent of wear takes always place at the bottom of the narrow side of the mold.

thumbnail Fig. 28

Schematic of flexible thin slab casting.

thumbnail Fig. 29

Photographs of the narrow face mold wear after several hundred heats: (a) top side, (b) bottom side.

thumbnail Fig. 30

Wear amount of narrow side mold: (a) surface plot of wear amount as a function of wide and height, (b) apparent wear parameter.

4.3.1 Insufficiency of the standard tapered mold approach

After narrow side mold wear, contact situations between mold and solidified shell change, as shown in Figure 31. Initial narrow side mold uniformly contacts solidified shell. With increasing narrow face taper, the crack susceptibility decreases along the region of wide face, because the narrow face taper toward the mid-width of mold compresses the solidifying shell along the wide face. Okamura and Yamamoto [37] showed that the increase of the narrow face taper might be expected to prevent the tensile stress of slab and to avoid cracks formation.

After narrow surface mold worn, although the middle region of narrow surface mold are still in contact with solidified shell, a big gap exists on both sides of narrow face mold. In addition, the middle of narrow side mold is remain contacted with the solidified shell closely.

thumbnail Fig. 31

Schematic diagram of contact situation between mold and solidified shell.

4.3.2 Introduction of a chamfer in the cross section

Considering the results of mold wear behavior, the mold taper was greatly reduced in the lower part of narrow side mold. In addition, the bottom wear of narrow side mold has an adverse impact on the narrow surface shape of thin slabs. When the narrow side additional taper is high and wear of narrow side mold is serious, the narrow face shape of the thin slabs will be seriously depressed. This is the reason for the formation of irregular deformed geometry near the corner and on the narrow side of thin slab. These observations resulted in countermeasures to enhance uniform shell formation in mold and prevent the longitudinal surface cracks on the thin slabs. Longitudinal surface crack defects in continuous casting process of thin slabs are related with inadequate amount of narrow face taper. After wearing of narrow side molds, contact situation and stress state between mold and solidified shell have been changed, as shown in Figure 32. Increasing the narrow face taper can be expected to avoid the longitudinal crack formation.

However, the corner region of worn mold can not contact well with solidified shell, a gap forms in the narrow face mold corner. In addition, the crack susceptibility increases along the region of wide side mold, because the narrow side mold corner cannot compress the solidified shell along the wide side mold. Therefore, the taper of worn narrow side mold cannot meet the requirements, which increases the occurrence of longitudinal surface cracks.

thumbnail Fig. 32

Stress states of solidified shell in mold.

4.3.3 Experimental results of chamfer technology

Figure 33 has shown the industrial application of chamfer mold, the longitudinal surface crack risk of the hypoperitectic steel has been greatly reduced by this chamfer technology.

Figure 34 shows configurations of chamfer thin slabs, which are produced by chamfer mold. A chamfer narrow side mold can effectively enhance the solidification uniformity of the solidified shell. Therefore, the longitudinal surface cracks of thin slabs are decreased by the chamfer mold during practical continuous casting.

In order to prevent and/or minimize the defects of straight edge seam, the chamfer thin slabs for hot rolling has been developed and discussed. The homogenization and optimization of the chamfer thin slabs are needed to reduce the stress concentration, there are two advantages using the chamfer thin slabs for rolling. On one hand, the distribution of temperature near the edge is ameliorated. On the other hand, deformed shape has been improved by the chamfer shape, since the distribution of temperature and ideal deformation shape of chamfer thin slabs. The risk of straight seam defect is greatly reduced. From the point of thin slab shape, the irregular edge shape of thin slab has been improved obviously, while the surface protrusions near the corner and on the narrow side of thin slab have been eliminated. Meanwhile, the reduction ratio of each pass is to be decreased, and the rolling force will be homogeneously distributed. The hot rolling conditions of thin slabs are to be homogenized and optimized for minimizing the defect in the edge side of the rolled thin slab.

In order to investigate the effect of irregular geometry near the thin slab corner on the temperature distribution, the FEM is used to solve the distribution of temperature and evolution field during the hot strip rolling [38,39]. By using chamfer thin slabs, the deformation of shape is improved as that the distribution of temperature near the edge is ameliorated. The temperature distributions of the rectangle and chamfer thin slab between after soaking furnace and right before hot rolling are indicated in Figure 35. The corner temperature of chamfer thin slab is higher than that of rectangle thin slab. So the stress and strain status of the thin slab edge between the chamfer and rectangle thin slab are different during hot strip rolling. This forming process of creases to straight seam defects during hot strip rolling is schematically shown in Figure 36. The detailed schematic diagram of the defected region beneath the surface indicates that the formation of the surface straight edge seam is closely related with the initial irregular state of thin slab.

The irregular deformed geometry near corners and on narrow side of thin slab can be intruded or extruded during hot rolling, and the surface of the edge seam becomes extended. The protrusions of the irregular surface can be folded during hot rolling to form surface edge seam.

The rectangle edges experience tensile stresses because of the movement of the edge, which occurs as a fairly large tensile stress. The initial bulging of rectangle thin slab is observed at the edges. The effect will be enhanced. Therefore, a tension stress is generated at the edges if ductility is low to make the length at the edge equal to that in the center [40]. The uniform temperature distribution of chamfer thin slab will enlarge homogeneous deformation between the thin slab center and edge during hot rolling, which will release the tensile strain of chamfer thin slabs. In addition, chamfer slab can improve the rolling conditions and decrease the amount of bulge. Therefore, employing a chamfer mold with proper corner size can reduce the tension stress in the corner region and simultaneously avoid straight edge seam defects effectively during hot plate rolling. This chamfer technology is used successfully in many Iron and Steel Companies; for instance, Shougang Jingtang United Iron & Steel Co., Ltd., HBIS Iron and Steel Group Co., Ltd., Chongqing Iron and Steel Co., Ltd., Benxi Iron and Steel (Group) Co., Ltd., Anshan Iron and Steel (Group) Co., Ltd., and Lianyuan Iron and Steel Co., Ltd.. In addition, the chamfer technology has been employed steadily in industrial production of typical niobium, vanadium, titanium micro-alloyed steels.

thumbnail Fig. 33

Industrial application of chamfer mold.

thumbnail Fig. 34

The configuration of chamfer thin slabs produced by chamfer mold.

thumbnail Fig. 35

The temperature of the rectangle and chamfer thin slab right before hot rolling.

thumbnail Fig. 36

Schematic diagram of the straight edge seam forming process.

5 Conclusions

The aim of this paper was to provide an overview of theoretical and applied research in chamfer technology of continuous casting process. Based on the analysis of the available literature, the following conclusions were drawn:

  • the transverse corner crack of micro-alloyed steel slab has been solved fundamentally by this chamfer technology, and it was significant to promote low-cost production of niobium, vanadium, and titanium micro-alloyed steel. The theoretical and applied research has broken the technology bottlenecks of the technology industrial application, while ensuring a long-life operation of the chamfer mold;

  • chamfer technology has been developed and implemented to investigate the effect of chamfer geometry on internal crack induced by a reduction operation of continuous casting bloom. The chamfer bloom avoids the edge solidified shell and the reduction force effectively concentrates in the bloom center. The bloom deformation zone occurs in the middle high temperature area, so the deformation resistance of the bloom is significantly reduced. In addition, adjusting the chamfer technology can decrease stress concentration to minimize the risk of off-corner subsurface cracks during the soft reduction operation;

  • chamfer technology for flexible thin slab casting process can effectively compensate the shrinkage of solidified shell, prevent the longitudinal surface crack of thin slabs and prolong the service life of the narrow side mold. The chamfer thin slab can raise the temperature near edge. As the result, the uniform temperature distribution of chamfer thin slab will enlarge homogeneous deformation between the thin slab center and edge during hot rolling, which was an effective way to decrease the straight edge seam defects;

  • in order to significantly improve the product efficiency and metal yield of chamfer technology, on-line thermal adjustment control system for chamfer mold and dynamic secondary cooling model for continuous casting process of chamfer technology are remaining and problems area.

Acknowledgments

The present work is financially supported by the National Key Research and Development Program of China No. 2017YFB1103700. The authors are grateful to senior engineer Jian Huang and senior engineer Jun Liu in Bengang Steel Plates Corp., Ltd. for their help to conduct the plant trial. The authors are also grateful to Dr. Xuebing Wang in National Engineering and Research Center for Continuous Casting Technology for discussing on the accuracy of stress analysis in casting direction during straightening process of slabs. Sincerely thanks are given to Dr.  Hongbiao Tao for his invaluable assistance during the experimental process of this paper.

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Cite this article as: Nanfu Zong, Yang Liu, Sida Ma, Weizhao Sun, Tao Jing, Hui Zhang, A review of chamfer technology in continuous casting process, Metall. Res. Technol. 117, 204 (2020)

All Figures

thumbnail Fig. 1

Transverse corner cracks occurring in slab corners.

In the text
thumbnail Fig. 2

Longitudinal surface cracks of as-cast thin slab: (a) on the surface, (b) cross section.

In the text
thumbnail Fig. 3

Longitudinal cracks on the surface of strip.

In the text
thumbnail Fig. 4

Macrograph of typical non-oriented silicon electrical steel thin slab.

In the text
thumbnail Fig. 5

Macrograph of typical edge seam defect on the products surface: (a) on the hot-rolled strip, (b) on the cold-rolled sheet.

In the text
thumbnail Fig. 6

Schematic illustration of sampling method.

In the text
thumbnail Fig. 7

Optical micrograph of defects shape and depth after hot rolling in cross section.

In the text
thumbnail Fig. 8

Macrographs of internal defects in bloom longitudinal section.

In the text
thumbnail Fig. 9

EDS line scans across internal crack boundaries.

In the text
thumbnail Fig. 10

Schematic diagram of chamfer narrow face designing.

In the text
thumbnail Fig. 11

The cross section of the narrow side mold perpendicular to the height direction.

In the text
thumbnail Fig. 12

The photo of the chamfer mold applied in industrial production.

In the text
thumbnail Fig. 13

Tensile stress in casting direction during straightening process of slabs: (a) rectangle slab, (b) chamfer slab.

In the text
thumbnail Fig. 14

Slabs at straightening stage during continuous casting: (a) chamfer slab, (b) rectangle slab.

In the text
thumbnail Fig. 15

Deformation shape after R1 rolling: (a) chamfer slab, (b) rectangle slab.

In the text
thumbnail Fig. 16

Configuration of industrial chamfer slab.

In the text
thumbnail Fig. 17

Section of chamfer bloom during soft reduction.

In the text
thumbnail Fig. 18

3D finite element model and temperature distribution for deformation analysis.

In the text
thumbnail Fig. 19

Von Mises equivalent stress in the bloom with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

In the text
thumbnail Fig. 20

Illustration of the sample point and sample place in the bloom.

In the text
thumbnail Fig. 21

Local stress for initial brittle crack propagation in XZ face of steel.

In the text
thumbnail Fig. 22

Tensile stress in the brittle temperature range with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

In the text
thumbnail Fig. 23

Maximum tensile stress in brittle temperature range with different chamfer angles.

In the text
thumbnail Fig. 24

Stress along direction z in the brittle temperature region with different chamfer angles: (a) actual design, (b) 30°, 50 mm, (c) 40°, 50 mm, (d) 45°, 50 mm, (e) 60°, 50 mm.

In the text
thumbnail Fig. 25

Maximum shear stress in brittle temperature range with different chamfer angles.

In the text
thumbnail Fig. 26

Application results of chamfer bloom during continuous casting process.

In the text
thumbnail Fig. 27

Macrostructure photos of the middle carbon chamfer bloom under soft reduction: (a) transverse section, (b) longitudinal section.

In the text
thumbnail Fig. 28

Schematic of flexible thin slab casting.

In the text
thumbnail Fig. 29

Photographs of the narrow face mold wear after several hundred heats: (a) top side, (b) bottom side.

In the text
thumbnail Fig. 30

Wear amount of narrow side mold: (a) surface plot of wear amount as a function of wide and height, (b) apparent wear parameter.

In the text
thumbnail Fig. 31

Schematic diagram of contact situation between mold and solidified shell.

In the text
thumbnail Fig. 32

Stress states of solidified shell in mold.

In the text
thumbnail Fig. 33

Industrial application of chamfer mold.

In the text
thumbnail Fig. 34

The configuration of chamfer thin slabs produced by chamfer mold.

In the text
thumbnail Fig. 35

The temperature of the rectangle and chamfer thin slab right before hot rolling.

In the text
thumbnail Fig. 36

Schematic diagram of the straight edge seam forming process.

In the text

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