Method for improving the corner cracks of a press roll and a cast strand

Abstract

The application provides a method for improving the corner crack of a casting billet and a press-down roller, which comprises the following steps: setting a side edge on the main body of the press-down roller; and making the inner side of the side edge, which is in contact with the casting billet, a smooth transition surface, so that the tensile deformation of the corner area of the casting billet is changed into extrusion deformation in the process of press-down, and the original driving force of the crack of the corner area of the casting billet is fundamentally eliminated, thereby avoiding the generation of the press-down crack; and the smooth transition surface adopts a circular arc or a continuously changing arc transition, so as to ensure that the contact surface between the side edge and the casting billet does not generate stress concentration in the process of press-down deformation, and further avoids the generation of the press-down crack.

Description

Technical Field

[0001] This invention relates to the field of continuous casting processing technology, and more specifically, to a method for improving the corner cracks of the pressure roll and the pressure roll of the billet. Background Technology

[0002] Reduction technology is an effective technique for improving segregation, porosity, and shrinkage cavities in continuously cast billets, and its research and application have developed rapidly in recent years. Reduction technology includes light reduction, a combination of light and heavy reduction, and heavy reduction. Light reduction involves progressive reduction using multiple rollers, typically requiring 3-6 rollers, with a total reduction of approximately 8-16 mm, depending on the billet cross-section, steel grade, and solidification process. The combination of light and heavy reduction essentially builds upon light reduction by adding one or two larger reductions on subsequent rollers or rollers near the end of solidification, such as a single roller reduction of 5-10 mm. Heavy reduction, compared to the previous two methods, specifically refers to achieving a large reduction using 1-2 pairs of rollers, such as a single roller reduction of up to 30 mm. Furthermore, the reduction location is relatively wide from a process perspective, starting from the center with a solidity of 0.3 and extending to a certain area after solidification.

[0003] As described above, the pressing technique improves the internal quality of the cast billet by deforming it through the pressing of rollers. The pressing amount of a single pressing roller ranges from less than 1 mm to more than 20 mm. However, pressing has a negative effect, leading to corner cracks. These cracks manifest as micro-transverse or longitudinal cracks. They can occur simultaneously with oscillation marks or outside the oscillation mark area. Cracks generally occur within 15 mm of the corner, present both under pressing and without pressing, and some are even located directly at the corner tip. Comparison reveals that without pressing, corner cracks are absent or minimal, while they are highly likely to exist after pressing. Therefore, it can be confirmed that these are cracks caused by pressing, specifically surface corner cracks resulting from pressing.

[0004] Currently, practical experience shows that while rolled bars and wire rods do not necessarily suffer from quality issues due to pressure cracking at the corners, they certainly pose potential risks. Furthermore, rolling larger bars will inevitably lead to problems if the rolling ratio is insufficient. Current solutions primarily involve adjusting the water flow in the two cooling zones after the secondary cooling stage to maximize the overall billet temperature and thus minimize pressure cracking at the corners. However, this approach has not proven effective in practice. Summary of the Invention

[0005] In view of the above problems, the purpose of this invention is to provide an improvement method for pressing rolls and pressing corner cracks in billets, so as to solve the problem that the existing technology, which adjusts the water volume of the two cooling zones after secondary cooling to increase the temperature of the entire billet as much as possible, thereby reducing pressing cracks in the corner area, cannot fundamentally and completely solve the problem of pressing cracks in the corner area.

[0006] The present invention provides a pressing roller, including a pressing roller body, on which a side edge is provided; the inner surface of the side edge in contact with the casting billet is a smooth transition surface, and the smooth transition surface adopts a circular arc or a continuously changing arc transition.

[0007] Furthermore, a preferred embodiment is that the inner surface of the side edge is designed according to the contour of the corner of the billet before pressing or according to the contour of the corner of the billet after pressing.

[0008] Furthermore, a preferred embodiment is that the method for designing the inner surface of the side edge based on the contour of the corner of the billet before pressing includes the following steps:

[0009] Let point A be the intersection of the inner surface of the side edge and the inner arc surface of the billet before pressing, and point B be the intersection of the inner surface of the side edge and the side surface of the billet before pressing.

[0010] An arc transition is used between the intersection point A and the intersection point B, and the radius of the arc is set as radius R;

[0011] The value of the radius R is determined based on the distance between the side edge and the corner of the billet and the pressing amount of the pressing roll body;

[0012] The inner surface of the side edge is designed based on the value of the radius R, the position of the intersection point A, and the position of the intersection point B.

[0013] Furthermore, a preferred embodiment is that the radius R is greater than or equal to 15 mm.

[0014] Furthermore, a preferred embodiment is that the method for designing the inner surface of the side edge based on the contour of the corner of the pressed-down billet includes the following steps:

[0015] The point on the inner arc surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point M; the point on the side surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point N.

[0016] Using point M as a base, extend vertically upwards, and the intersection of the vertically upward extension line of point M with the inner arc surface of the billet before pressing is taken as point M'; using point N as a base, extend vertically upwards to the same distance as from point M to point M', to obtain point N'.

[0017] Points M' and N' are used as two points on the transition curve of the inner surface of the side edge, and the inner surface of the side edge is designed based on the two points on the transition curve.

[0018] Furthermore, a preferred approach is to use a quadratic or cubic curve method to smoothly transition the transition curve of the inner surface of the side edge, in the process of designing the inner surface of the side edge based on the two points of the transition curve, with the M' and N' points as the two points of the transition curve, so as to eliminate stress concentration caused by deformation during the transition curve process.

[0019] Furthermore, a preferred embodiment is that designing the inner surface of the side edge based on the transition curve of the M' point and the N' point as the two points of the transition curve includes:

[0020] A point C is arbitrarily set between point M', point N', and the corner of the billet before pressing;

[0021] The inner surface of the side edge is obtained by designing the curve of the inner surface of the side edge through the points M', N', and C.

[0022] Furthermore, a preferred embodiment is that designing the inner surface of the side edge based on the transition curve of the M' point and the N' point as the two points of the transition curve includes:

[0023] The maximum vertical length from the corner of the billet before pressing down to points M' and N' is taken as the radius R';

[0024] The inner surface of the side edge is designed based on the position of point M', the position of point N', and the value of radius R'.

[0025] Furthermore, a preferred embodiment is that the preset distance is greater than or equal to 15mm.

[0026] This invention provides a method for improving corner cracks in cast billets, which involves using a pressing roller as described above to press down the cast billet; wherein:

[0027] The pressing rollers are arranged correspondingly on both the inner and outer arcs of the billet; or, the pressing rollers are arranged correspondingly on the inner arc of the billet, and flat rollers are arranged on the outer arc of the billet.

[0028] As can be seen from the above technical solution, the pressing roller and the method for improving corner cracks in the billet provided by the present invention, by setting a side edge on the main body of the pressing roller; the inner surface of the side edge in contact with the billet is a smooth transition surface, so that the corner area is changed from tensile deformation of the billet to extrusion deformation during the pressing process, fundamentally eliminating the driving force for cracks in the corner area of ​​the billet, thereby avoiding the generation of pressing cracks; the smooth transition surface adopts a circular arc or a continuously changing arc transition to ensure that the contact surface between the side edge and the billet does not generate stress concentration during the pressing deformation process, further avoiding the generation of pressing cracks.

[0029] To achieve the foregoing and related objectives, one or more aspects of the invention include the features that will be described in detail below. The following description and accompanying drawings illustrate certain exemplary aspects of the invention. However, these aspects indicate only a few of the various ways in which the principles of the invention can be used. Furthermore, the invention is intended to encompass all such aspects and their equivalents. Attached Figure Description

[0030] Other objects and results of the invention will become more apparent and readily understood with reference to the following description taken in conjunction with the accompanying drawings. In the drawings:

[0031] Figure 1 This is a schematic diagram of the inner side surface design of the side edge of the pressure roller according to an embodiment of the present invention;

[0032] Figure 2 A schematic diagram of the inner side surface design of the side edge of the pressure roller according to another embodiment of the present invention;

[0033] Figure 3 This is a schematic diagram of the inner surface curve of the side edge designed according to two points of the transition curve according to an embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram of the inner surface design of the side edge in Embodiment 1 of the present invention. Detailed Implementation

[0035] In the following description, numerous specific details are set forth for illustrative purposes and to provide a thorough understanding of one or more embodiments. However, it will be apparent that these embodiments may also be implemented without these specific details.

[0036] In view of the aforementioned existing technology, the method of adjusting the water volume of the two cooling zones after secondary cooling to increase the temperature of the entire billet as much as possible, thereby reducing the corner cracks, cannot fundamentally solve the problem of corner cracks. Therefore, an improvement method for corner cracks in the billet by adjusting the pressure roller is proposed.

[0037] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0038] To illustrate the improvement method for corner cracks in the pressure roll and the billet pressure roll provided by the present invention, Figure 1 The inner surface design of the side edge of the pressure roller according to an embodiment of the present invention is shown; Figure 2 The inner side surface design of the side edge of the pressure roller according to another embodiment of the present invention is shown; Figure 34 shows the curve of the inner surface of the side edge designed according to the two points of the transition curve according to an embodiment of the present invention; 5 shows the inner surface design of the side edge in embodiment 1 of the present invention.

[0039] like Figures 1 to 4 As shown in the figure, the pressing roller provided by the present invention includes a pressing roller body, and a side edge is provided on the pressing roller body; the inner surface of the side edge that contacts the casting billet is a smooth transition surface, and the smooth transition surface adopts a circular arc or a continuously changing arc transition.

[0040] By setting a side edge on the main body of the pressing roller, and the inner surface of the side edge in contact with the billet being a smooth transition surface, the corner area is changed from tensile deformation of the billet to extrusion deformation during the pressing process. This fundamentally eliminates the driving force for cracks to occur in the corner area of ​​the billet, thereby avoiding the generation of pressing cracks. The smooth transition surface adopts an arc or a continuously changing arc transition to ensure that no stress concentration occurs on the contact surface between the side edge and the billet during the pressing deformation process, further avoiding the generation of pressing cracks.

[0041] As a preferred embodiment of the present invention, the inner surface of the side edge is designed according to the contour of the corner of the billet before pressing or according to the contour of the corner of the billet after pressing. The design of the inner surface of the side edge of the pressing roller can be based on the contour of the corner of the billet before pressing or according to the contour of the corner of the billet after pressing.

[0042] As a preferred embodiment of the present invention, the method for designing the inner surface of the side edge based on the contour of the corner of the cast billet before pressing includes the following steps:

[0043] Let point A be the intersection of the inner surface of the side edge and the inner arc surface of the billet before pressing, and point B be the intersection of the inner surface of the side edge and the side surface of the billet before pressing.

[0044] A circular arc is used to transition between intersection point A and intersection point B, and the radius of the arc is set as radius R;

[0045] The value of radius R is determined based on the distance between the side edge and the corner of the billet and the amount of pressing down of the pressing roll body;

[0046] Design the inner surface of the side edge based on the value of radius R, the position of intersection point A, and the position of intersection point B.

[0047] As a preferred embodiment of the present invention, the radius R is greater than or equal to 15 mm.

[0048] like Figure 1As shown, before pressing, the intersection points of the inner surface of the side edge and the billet contour are intersection points A and B. Intersection point A is the intersection point of the side edge and the inner arc surface of the billet, and intersection point B is the intersection point of the side edge and the side surface of the billet. The inner surface of the side edge uses a rounded transition between intersection points A and B, with a radius defined as R. Considering that pressing cracks mostly occur in the corner area, within 15mm of the corner, R is greater than or equal to 15mm. Simultaneously considering the issue of pressing deformation and the effects of different pressing amounts, the optimal range for R is greater than or equal to 20mm.

[0049] Based on the method of designing the inner side surface of the side edge according to the contour of the corner of the billet before pressing, there is a special design that only ensures that the corner area brings extrusion. Specifically, before pressing, the distance from intersection point A to the corner is less than 10mm, and at the same time, the distance from intersection point B to the corner is greater than or equal to 15mm before pressing, and it is not on the side of the billet, but at a distance from the side. This distance is greater than the width expansion on one side after the billet is pressed. That is, intersection point B still does not contact the side of the billet after the billet is pressed. In this way, the curve of the inner side surface of the side edge is constructed between intersection point A and intersection point B.

[0050] As a preferred embodiment of the present invention, the method for designing the inner surface of the side edge based on the contour of the corner of the pressed-down billet includes the following steps:

[0051] The point on the inner arc surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point M; the point on the side surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point N.

[0052] Using point M as a base, extend vertically upwards, and take the intersection of the vertically upward extension line of point M and the inner arc surface of the billet before pressing as point M'; using point N as a base, extend vertically upwards to the same distance as from point M to point M', and obtain point N'.

[0053] Points M' and N' are taken as two points on the transition curve of the inner surface of the side edge, and the inner surface of the side edge is designed based on the two points on the transition curve.

[0054] As a preferred embodiment of the present invention, when M' and N' are taken as two points on the transition curve of the inner surface of the side edge, the inner surface of the side edge is designed based on the two points of the transition curve.

[0055] The transition curve of the inner side surface is smoothed by using a quadratic or cubic curve method to eliminate stress concentration caused by deformation during the transition curve process.

[0056] As a preferred embodiment of the present invention, points M' and N' are designated as the two points of the transition curve of the inner surface of the side edge, and the inner surface of the side edge is designed based on these two points of the transition curve, including:

[0057] Arbitrarily set a point C between point M', point N' and the corner of the billet before pressing;

[0058] The inner surface of the side edge is obtained by designing the curve of the inner surface of the side edge through points M', N' and C.

[0059] As a preferred embodiment of the present invention, points M' and N' are designated as the two points of the transition curve of the inner surface of the side edge, and the inner surface of the side edge is designed based on these two points of the transition curve, including:

[0060] The maximum vertical length from the corner of the billet before pressing down to points M' and N' is taken as the radius R';

[0061] The inner surface of the side edge is designed based on the positions of point M', point N', and the value of radius R'.

[0062] As a preferred embodiment of the present invention, the preset distance is greater than or equal to 15mm.

[0063] like Figure 2 As shown in the figure, the dashed frame represents the outline of the billet before pressing, and the thick solid frame represents the outline of the billet after pressing deformation. In practice, pressing cracks mainly occur in the area within 15mm of the corner of the billet after pressing deformation. Figure 2 Using points M and N as a basis (where M is the position where the distance from the corner of the inner arc surface of the billet after pressing is greater than or equal to 15mm, and N is the position where the distance from the corner of the side surface of the billet after pressing is greater than or equal to 15mm), the point where M intersects the inner arc of the billet contour before pressing is found vertically upwards and is taken as point M'. Point N is obtained by moving point N vertically upwards by the same distance from M to M'. In this way, after pressing, point N' naturally moves to point N, ensuring that the side surface is within 15mm or more after pressing. Therefore, points M' and N' constitute the two points of the optimal transition curve of the inner side surface. The curve uses a quadratic or cubic curve method for smooth transition to eliminate stress concentration caused by deformation during the transition curve process. The quadratic or cubic curve method is a technique known to those skilled in the art, and therefore will not be elaborated here.

[0064] Obviously, from Figure 2 As can be seen, for the curve between points M' and N', the deformation is gentler from point M' to the corner, resulting in a smaller force squeezing the corner during the pressing process and a smaller resistance during billet pulling. Therefore, there are various designs for the curve between points M' and N'. The curve design method in patent ZL201410848703X previously proposed by the inventors of this invention can be used to design various smooth curves between points M' and N'. Since patent ZL201410848703X is a disclosed technology, it will not be elaborated here.

[0065] The method of designing the inner side surface of the side edge based on the contour of the corner of the billet after pressing involves factors such as the M' and N' points, the pressing amount performed by the pressing roll, and the width extension of the billet. Extensive empirical data on pressing is required to design the optimal transition curve between these factors. Compared to the method of designing the inner side surface of the side edge based on the contour of the corner of the billet before pressing, the method of designing the inner side surface of the side edge based on the contour of the corner of the billet after pressing yields a transition curve with minimal pressing force and minimal billet pulling resistance, thus promoting stable production.

[0066] Meanwhile, regarding the method of designing the inner surface of the side edge based on the contour of the corner of the cast billet after pressing, after determining points M' and N', the maximum value of the vertical length from the corner of the cast billet before pressing to points M' and N' is taken as the radius R'. Then, the inner surface of the side edge is designed based on the position of point M', the position of point N', and the value of radius R'. At this time, the transition curve of the inner surface of the side edge is a circular arc.

[0067] like Figure 3 As shown, with Figure 2 The points M' and N' formed in the diagram create a coordinate system. Numerous curves can be drawn from point M' to point N', resulting in countless design curves for the inner side surface of the pressure roller. Any design curve convex towards the corner can achieve the effect of squeezing the corner during the pressing process. However, different inner side surface curve designs result in completely different pressing forces and billet pulling resistance, offering considerable design flexibility. This embodiment provides a curve design method for a specified point: a point C is arbitrarily set between points M', N', and the corner point before billet pressing. An inner side curve of the pressure roller can be designed using points M', N', and C. After setting point C, the three points can be constructed using the curve design method described in the inventor's previous patent ZL201410848703X to create a smooth curve between them. Since patent ZL201410848703X is prior art, it will not be elaborated upon here. Figure 3 The paper presents the design results of two different C-points, namely C1 and C2 curves, which obviously affect the pressure and drawing resistance. Of course, the M'N'C1 curve design is obviously better.

[0068] The method for improving corner cracks in cast billets provided by this invention uses a pressing roller as described above to press down the cast billet; wherein:

[0069] Pressing rollers are arranged on both the inner and outer arcs of the billet; or, pressing rollers are arranged on the inner arc of the billet, and flat rollers are arranged on the outer arc of the billet.

[0070] For continuous casting, the pressing rollers provided in this embodiment can be arranged on both the inner and outer arcs of the billet. However, due to the special nature of continuous casting, only the upper roller actively exerts force while the lower roller passively acts during pressing. This results in the pressing deformation not evenly bisecting the pressing amount on the inner and outer arc surfaces of the billet. If the same side edge design is used, the final deformation may be inconsistent. Therefore, the process can be designed according to the design method of the corner contour of the billet after pressing, and specific designs can be made for the specific deformation of the inner and outer arc surfaces of the billet after pressing, forming different inner surface curves of the side edges corresponding to the inner and outer arcs of the billet, thereby ensuring the pressing effect. At the same time, the side edge design provided in this embodiment can be applied only to the inner arc surface of the billet, while the outer arc surface of the billet uses a flat roller design. This can address the situation where pressing cracks only occur in the corner area of ​​the inner arc surface.

[0071] To better illustrate the nozzle arrangement method for secondary cooling of continuously cast billets provided by this invention, the following example is provided:

[0072] Example 1

[0073] Taking the continuous casting production of small square billets in a certain factory as an example, the cross-section is 160X160mm. Using a full-arc casting machine with an arc radius of 10m, after extensive pressing practice, it was found that after a single pressing roller reduces the billet by 10mm, the width extension is within 4mm, and the extension on each side is within 2mm. Taking a single side as an example, after a 10mm reduction, the extension is 2mm. Therefore, the billet before pressing is 160X160mm, and after a 10mm reduction, the billet becomes 150X164mm.

[0074] like Figure 4 As shown, if OM is defined as the X-axis, and M is the positive direction of X, ON is the Y-axis, and N is the positive direction of Y, and OM=ON=15mm is taken as the distance of the corner crack within about 15mm of the corner, then the coordinates of several feature points can be obtained as M(15,0), N(0,15), M'(25,0), N'(10,15), and the corner coordinates of the billet before pressing are (25,12). Thus, based on the coordinates of M' and N', the arc equation of M'N' can be obtained, which gives the optimal curve of the inner side of the side edge of the pressing roller. After pressing down 10mm, point M' moves to point M, and point N' moves to point N, which just meets the design requirement that the billet is in a state of extrusion within 15mm of the corner.

[0075] Meanwhile, the above working conditions can also be designed in the following way: Find the center 0' with points M' and N', and O'M'=O'N'=15mm. The M'N' curve can be designed according to the arc. In this way, after pressing down 10mm, point M' moves to point M and point N' moves to point N, which just meets the design requirement that the billet is in the extrusion state within 15mm of the corner.

[0076] It should be noted that this embodiment is merely a detailed description of the method for improving corner cracks of the pressing roll and the billet provided by the present invention in practical applications, and does not limit the technical solution provided by the present invention.

[0077] As can be seen from the above specific embodiments, the pressing roller and the method for improving corner cracks in the billet provided by the present invention, by setting a side edge on the main body of the pressing roller; the inner surface of the side edge in contact with the billet is a smooth transition surface, so that the corner area changes from tensile deformation of the billet to extrusion deformation during the pressing process, fundamentally eliminating the driving force for cracks in the corner area of ​​the billet, thereby avoiding the generation of pressing cracks; the smooth transition surface adopts a circular arc or a continuously changing arc transition to ensure that the contact surface between the side edge and the billet does not generate stress concentration during the pressing deformation process, further avoiding the generation of pressing cracks.

[0078] The method for improving the pressing roll and the corner crack of the billet according to the present invention has been described above by way of example with reference to the accompanying drawings. However, those skilled in the art should understand that various modifications can be made to the method for improving the pressing roll and the corner crack of the billet according to the present invention without departing from the scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the contents of the appended claims.

Claims

1. A pressing roller, comprising a pressing roller body, characterized in that, A side edge is provided on the main body of the pressing roller; the inner surface of the side edge that contacts the billet is a smooth transition surface, so that the corner area changes from the tensile deformation of the billet to the extrusion deformation during the pressing process; the smooth transition surface adopts a circular arc or a continuously changing arc transition; The inner surface of the side edge is designed according to the contour of the corner of the billet before pressing or according to the contour of the corner of the billet after pressing. The method for designing the inner surface of the side edge based on the contour of the corner of the billet before pressing includes the following steps: Let point A be the intersection of the inner surface of the side edge and the inner arc surface of the billet before pressing, and point B be the intersection of the inner surface of the side edge and the side surface of the billet before pressing. An arc transition is used between the intersection point A and the intersection point B, and the radius of the arc is set as radius R; The value of the radius R is determined based on the distance between the side edge and the corner of the billet and the pressing amount of the pressing roller body; wherein the radius R is greater than or equal to 15mm; The inner surface of the side edge is designed based on the value of the radius R, the position of the intersection point A, and the position of the intersection point B.

2. The roll as claimed in claim 1, characterized in that The method for designing the inner surface of the side edge based on the contour of the corner of the pressed-down billet includes the following steps: The point on the inner arc surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point M; the point on the side surface of the pressed-down billet, at a preset distance from the corner of the billet, is designated as point N. Taking point M as the base, extend vertically upwards, and take the intersection of the vertically upward extension line of point M and the inner arc surface of the billet before pressing as point M'. Using point N as a base, extend vertically upwards to the same distance as the distance from point M to point M', to obtain point N'. Points M' and N' are used as two points on the transition curve of the inner surface of the side edge, and the inner surface of the side edge is designed based on the two points on the transition curve.

3. The roll as claimed in claim 2, characterized in that In the process of designing the inner surface of the side edge based on the transition curve where points M' and N' are used as two points on the inner surface of the side edge, The transition curve of the inner surface of the side edge is smoothed by using a quadratic or cubic curve method to eliminate stress concentration caused by deformation during the transition curve process.

4. The roll as claimed in claim 2, wherein The step of using points M' and N' as the two points of the transition curve for the inner surface of the side edge, and designing the inner surface of the side edge based on the two points of the transition curve, includes: A point C is arbitrarily set between point M', point N', and the corner of the billet before pressing; The inner surface of the side edge is obtained by designing the curve of the inner surface of the side edge through the points M', N', and C.

5. The roll as claimed in claim 2, wherein The step of using points M' and N' as the two points of the transition curve for the inner surface of the side edge, and designing the inner surface of the side edge based on the two points of the transition curve, includes: The maximum vertical length from the corner of the billet before pressing down to points M' and N' is taken as the radius R'; The inner surface of the side edge is designed based on the position of point M', the position of point N', and the value of radius R'.

6. The pressing roller according to claim 2, characterized in that, The preset distance is greater than or equal to 15mm.

7. A method of improving the corner cracks of a cast strand, characterized by The billet is pressed down using the pressing roller as described in any one of claims 1-6; wherein: The pressing rollers are arranged correspondingly on both the inner and outer arcs of the billet; or, the pressing rollers are arranged correspondingly on the inner arc of the billet, and flat rollers are arranged on the outer arc of the billet.

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