FORGING METHOD FOR HIGH-EFFICIENCY CLOSING OF POROUS DEFECTS IN STEEL INGOTS OR BILLETS

Abstract

 The present invention relates to the field of forging, and more particularly to a forging method which can heal the void in ingots or billets effectively. It can be used in open die forging process of ingots and billets of all shapes. Wide anvil radial forging method is used during forging, which uses two flat plates as upper and lower anvils. For ingots or billets with height-diameter ratio less than 2, the reduction ratio during wide anvil radial forging is 20% to 25%; for ingots or billets with height-diameter ratio larger than 2, the reduction ratio is 20% to 40%. This invention can heal the voids in the ingots or billets, especially the central porosities, which cannot be done by traditional forging methods. Moreover, this invention can also heal the voids in large height-diameter ratio ingots or billets using small reduction ratio.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of forging, and more particularly to a forging method which can heal the void in ingots or billets effectively. It can be used in open die forging process of ingots and billets of all shapes.

BACKGROUND OF THE INVENTION

[0002] Forging is an important process in metal processing. It can crash the casting structure, refine the grain size, homogenize microstructure, and heal the defects like shrinkage cavities and porosities, which are very important for improving the quality of forged products.

[0003] In the ingots, shrinkage cavities and porosities are formed during casting and solidification process. During this process, the molten metal contacting the mold solidifies first, and molten metal in the center of the ingot solidifies last due to the slow heat transfer. So shrinkage cavities and porosities are formed in the center of the ingot, and lie along the axis of the ingot. For large ingots, the defects are severe. These defects must be healed by effective forging method, or they will cause catastrophic failure and significant economic loss. Similarly, for continuous casting billets, there is no hot top which can fill the shrinkage cavities and porosities, so severe defects lie along the center line of the billet which is hard to eliminate using traditional forging methods. So these billets are only used for forging tubes, which limits the scope of its application.

[0004] Figure 1 shows the sectioning result of a 100t ingot for nuclear low pressure rotor. The shrinkage cavities lie along the axis of the ingot. A finite element model is made using the shape of the ingot and the voids, and upsetting process of this ingot is simulated, as Figure 2 shows. The reduction ratio of upsetting is 50%, the initial voids (as shown in left part of Figure 2) cannot be completely closed after the upsetting when the reduction ratio is 50% (as shown in the right part of Figure 2). This is because of the shape and distribution of the voids. Therefore, during forging process, upsetting cannot heal the central porosities effectively. The porosities should be healed during cogging process.

[0005] Many cogging methods like WHF, FM and JTS methods are proposed to concentrate the strain on the central part of the billet. It has been realized using a wider anvil can heal the central porosities more effectively, but the anvil width in these methods are still not enough to heal the central porosities completely. As shown in Figure 3, finite element simulation is used to get the strain distribution after cogging of a billet using WHF method. The strain is unevenly distributed along the axis of the billet, and between the two presses there is a dead metal zone where the strain is very small. So these forging methods cannot heal the defects in the billets effectively.

[0006] Upsetting process cannot be used on large height-diameter ratio billets, like continuous casting billets or ingots with height-diameter ratio greater than 2, so only cogging process can be used. When only cogging can be used, the forging ratio is relatively small. The strain on the axis of the billet is small and it is difficult to close the central porosities. If forging ratio is increased, the diameter of the

SUMMARY OF THE INVENTION

[0007] This invention provides a forging method which can heal the voids in the ingots or billets effectively, and solving the problem that the current forging methods cannot heal the voids, especially the central porosities in the ingots.

[0008] This invention also provides a small reduction forging method which can close the voids in large height-diameter ratio billets effectively, and solving the problem that the current forging methods cannot close the voids, especially the central porosities in large height-diameter ratio billets.

[0009] The technical solutions of this invention are:

During the forging method which can heal the voids in the ingots or billets effectively, two flat plates are used as upper and lower anvils to press the ingots or billets along the radial direction, which is called wide anvil radial forging. For ingots or billets with height-diameter ratio less than 2, the reduction ratio during wide anvil radial forging is 20% to 25%; for ingots or billets with height-diameter ratio larger than 2, the reduction ratio during wide anvil radial forging is 20% to 40%.

[0010] In this invention, a forging method which can heal the voids in the ingots or billets effectively comprises the following steps:

1) For ingots or billets with height-diameter ratio of 1.2∼2, pre-upsetting should be used. The billet should be forged to height-diameter ratio of 0.8~1.1; for ingots or billets with height diameter ratio smaller than 1.2, no pre-upsetting is required;

2) Two flat plates are used as upper and lower anvils and the billet is pressed along the radial direction. The reduction ratio is 20%∼25%. This process is called wide anvil radial forging;

3) After wide anvil radial forging, the billet is rotated 90° and cogging is used until the height-diameter ratio of the billet is 1.8∼2.2;

4) Reheat the billet and upset it until the height-diameter ratio is 0.6~0.7; another cogging process is used to forge the billet to the final shape.

[0011] In step 2), the pressing direction of wide anvil radial forging is the radial direction, which is the same with the pressing direction of traditional cogging process.

[0012] In step 2), the reduction ratio of wide anvil radial forging refers to the pressing depth of the upper plate is 20% ∼ 25% of the original height or diameter of the ingot or billet. The reduction ratio is calculated as follows:

[0013] Wherein, ΔH is the pressing depth of the upper plate during the forging process; H is the original height of the ingot or billet along the pressing direction. If the ingot or billet is a cylinder and the pressing is along the radial direction, its diameter is taken as H.

[0014] In step 2), after wide anvil radial forging, the billet is reheated to the forging temperature to reduce the force required for forging and heal the voids which are already closed.

[0015] In step 3), WHF method or FM method is used for cogging, the reduction ratio of each press is 20%∼25%. After two passes, the billet is rotated 90° and another pass is processed. After these processes the height-diameter ratio of the billet is 1.8∼2.2

[0016] In step 3), after the cogging process, the pressing direction of wide anvil radial forging is marked. And in step 4), the first pass of cogging is pressed along the marked direction, which can enhance the effect of void closing.

[0017] In step 4), the wide anvil radial forging can be used again during the cogging process, and the pressing direction should be the same with the previous one in step 2). Using this method two times along the same direction can make the void fully closed along this direction, making the forging method more effective.

[0018] In step 4), if the wide anvil radial forging is used again, WHF or FM method should be used first. For the first pass of cogging, the pressing direction should be the same with the pressing direction of wide anvil radial forging in step 2). Then the billet is rotated 90° and the second pass is processed. The pressing depth is 20%∼25% for each pass. After these two passes the height-diameter ratio of the billet is 0.8∼1.1, and wide anvil radial forging can be used again. Then WHF or FM method can be used to forge the billet to the final shape.

[0019] In step 4), the billet can be reheated before the wide anvil radial forging to reduce forging force.

[0020] Moreover, in this invention, a forging method which can heal the voids in large height-diameter ratio ingots or billets using small reduction ratio comprises the following steps:

1) For billets with height-diameter ratio bigger than 2, upsetting cannot be used. But wide anvil radial forging can be used. Two flat plates are used as upper and lower anvils and the billet is pressed along the radial direction. The reduction ratio is 20%∼40%.

2) After wide anvil radial forging, the billet is reheated to the forging temperature;

3) Rotate the billet 90°, WHF or FM method is used to forge the billet to the final shape.

[0021] In step 1), two big plates are used as upper and lower anvils, and the pressing direction is the radial direction, which is the same as the pressing direction of normal cogging process.

[0022] In step 1), the reduction ratio is calculated as follows:

[0023] Wherein, ΔH is the pressing depth of the upper plate during the forging process, H is the original height of the ingot or billet along the pressing direction, and if the ingot or billet is a cylinder and the pressing is along the radial direction, its diameter is taken as H.

[0024] In step 2), after wide anvil radial forging, the ingot or billet is reheated to forging temperature and holding for more than 2 hours, which can reduce the forging force and heal the voids which are already closed.

[0025] In step 2), after wide anvil radial forging, the shape of the ingot or billet is flat-square.

[0026] In step 3), after wide anvil radial forging, the billet is rotated 90°, WHF or FM method is used for two passes, and the reduction ratio for each press is 20%; after the two passes, the billet should be forged to the final shape.

[0027] The beneficial effects of the present invention are:

1. The present invention proposes a forging method for effectively healing internal voids in ingots or billets, comprising the steps of: 1) pre-upsetting the ingot; 2) use two flat plates as upper and lower anvil and press the billet along the radial direction; 3) rotate the billet 90° and use traditional WHF or FM cogging method to forge it; 4) reheat the billet, use upsetting and cogging again to forge the billet to the final shape. Compared with traditional cogging methods like WHF or FM method, this method can greatly increase the strain in the center of the ingot, heal the voids in the ingot, and greatly increase the possibility of passing ultrasonic test after forging.

2. The present invention proposes a forging method for effectively healing internal voids in ingots or billets. This method can produce high quality forged products using less heating and forging passes. So it can reduce equipment occupancy, improve production efficiency; reduce energy consumption, all of which are helpful for reducing production cost.

3. The present invention proposes a forging method which can heal the voids in large height-diameter ratio ingots or billets using small reduction ratio, comprising the steps of: 1) two flat plates are used as upper and lower anvils and the billet is pressed along the radial direction; 2) reheat the billet to the forging temperature; 3) rotate the billet 90°, WHF or FM method is used to forge the billet to the final shape. Compared with traditional cogging method like WHF or FM method, this method can greatly increase the strain in the center of the billet, heal the voids in the billet, and greatly increase the possibility of passing ultrasonic test after forging.

4. The present invention proposes a forging method which can heal the voids in large height-diameter ratio ingots or billets using small reduction ratio. Compared with traditional forging method, the pressing depth of this method is smaller, so using this method can produce qualified forging products with larger diameter, which greatly increase application scope of the product.

[0028] In summary, finite element simulation is used to investigate the strain distribution and void closing behavior when traditional forging method is used. Based on this, a forging method for effectively healing internal voids in ingots or billets is proposed. This forging method can be used for open-die forging of ingots or billets of all shapes, and especially helpful for ingots or billets with severe central porosities. This method can heal the voids in the center of the billet and ensure the forged product can pass the ultrasonic test. For large height-diameter ratio billets, this method can produce qualified forging products with larger diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the sectioning result of a 100t ingot for nuclear low pressure rotor, which have severe internal defects.

[0030] The left part of Figure 2 shows the initial shape of the central porosities of the ingot, and the right part is the shape of the porosities after upsetting with 50% reduction ratio, which is obtained using finite element simulation. The dimension in the figures is millimeter.

[0031] Figure 3 is the strain distribution of a large height-diameter ratio billet after WHF cogging process, which is obtained using finite element simulation.

[0032] Figures 4a-e is the schematic diagrams of the forging process, wherein:

Figure 4a shows the diagram of the ingot;

Figure 4b shows the upsetting process;

Figure 4c shows the wide anvil radial forging process;

Figure 4d shows WHF cogging method (plane A in the horizontal direction);

Figure 4e shows WHF cogging method (plane A in the vertical direction).

Figure 5 shows a simplified model of the real voids which have similar size and shape.

Figure 6a shows the strain distribution after wide anvil radial forging in Example 1.

Figure 6b shows the shape of the voids after wide anvil radial forging in Example 1.

Figure 7a shows the strain distribution after WHF cogging process in Comparative Example 1.

Figure 7b shows the shape of the void after WHF cogging process in Comparative Example 1.

Figure 8a shows the strain distribution after WHF cogging process in Comparative Example 2.

Figure 8b shows the shape of the voids after WHF cogging process in Comparative Example 2.

[0033] In these figures: 1 - ingot; 2 - billet; 3 - upper die for upsetting; 4 - lower die for upsetting; 5 - upper plate for wide anvil radial forging; 6 - lower plate for wide anvil radial forging; 7 - upper flat anvil for WHF cogging process; 8 - lower flat anvil for WHF cogging process; 9 - clamp; B - void position.

[0034] Figure 9-11 is schematic diagrams of the forging process of large height-diameter ratio billet, wherein:

Figure 9 is the diagram of large height-diameter ratio billet.

Figures 10a and 10b show the wide anvil radial forging process. Figure 10a is the front view, and Figure 10b is the left view.

Figures 11a and Figure 11b show WHF cogging process. Figure 11a is the front view, Figure 11b is the left view.

Figure 12a is the cross section of a continuous casting billet. Figure 12b is the local amplification of the center of the billet in Figure 12a.

Figure 13 is the strain distribution after wide anvil radial forging in Example 2 which is obtained using finite element simulation.

[0035] In these figures: 11 - large height-diameter ratio billet (ingot or continuous casting billet, etc.); 12 - billet; 13 - upper plate for wide anvil radial forging; 14 - lower plate for wide anvil radial forging; 15 - upper flat anvil for WHF cogging process; 16 - upper flat anvil for WHF cogging process.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] During the forging method which can heal the voids in the ingots or billets effectively, two flat plates are used as upper and lower anvils to press the ingots or billets along the radial direction, which is called wide anvil radial forging. For ingots or billets with height-diameter ratio less than 2, the reduction ratio during wide anvil radial forging is 20% to 25%; for ingots or billets with height-diameter ratio larger than 2, the reduction ratio during wide anvil radial forging is 20% to 40%.

[0037] Now, this invention will be described in detail, referring to the drawings and examples.

[0038] In this invention, a forging method which can heal the voids in the ingots or billets effectively comprises the following steps:

1) Figure 4a shows the diagram of the ingot 1. As Figure 4b shows, if ingot 1 should be upsetted depends on its height-diameter ratio. For ingots or billets with height-diameter ratio of 1.2∼2, pre-upsetting should be used. The billet 2 should be put on lower die 4, and upper die 3 should be used to upset the billet to height-diameter ratio of 0.8∼1.1. After pre-upsetting, the ingot is ready for the following cogging process. For ingots or billets with height diameter ratio smaller than 1.2, no pre-upsetting is required;

2) As Figure 4c shows, the billet should be put on lower plate 6, and the axis should be horizontal. In this figure, the vertical plane is defined as plane A. Upper plate 5 should be used to press this billet along the radial direction, which is the pressing direction of traditional cogging method. In this invention, this process is called wide anvil radial forging.

[0039] During wide anvil radial forging, the reduction ratio is 20%∼25%, which means the pressing depth of upper plate 5 is 20% ∼ 25% of the original height or diameter of the ingot or billet. The reduction ratio is calculated as follows:

[0040] Wherein, ΔH is the pressing depth of the upper plate 5 during the forging process, H is the original height of the ingot or billet along the pressing direction, and if the ingot or billet is a cylinder and the pressing is along the radial direction, its diameter is taken as H.

[0041] After wide anvil radial forging, the billet is reheated to the forging temperature to reduce the force required for forging and heal the voids which are already closed.

3) WHF or FM method is used for cogging. As Figure 4d shows, WHF method is used on billet 2. The billet is rotated 90° and now the plane A is horizontal. Put the billet 2 on lower flat anvil 8, and use upper flat anvil 7 to press on corresponding position. The reduction ratio is 20%∼25%. Then move the billet 2 to the left in Figure 4d, repeat this pressing process until the anvil reaches the end of the billet. This is called one pass. The second pass should be pressed in the same direction. Then the billet is rotated 90° and another pass is processed. After three passes, the height-diameter ratio of the billet is 1.8∼2.2, as Figure 4e shows. After the cogging process, the pressing direction of wide anvil radial forging is marked. And in step 4), the first pass of cogging is pressed along the marked direction, which can enhance the effect of void closing.

4) Reheat the billet, and use upsetting and cogging again. First the billet is reheated to the forging temperature. As Figure 4b shows, put the billet 2 on lower die 4 and use upper die 3 to upset the billet to height-diameter ratio of 0.5~0.7. Then as Figure 4e shows, use cogging method to press the billet along the wide anvil radial forging direction (which means plane A is in vertical direction). Rotate the billet 90° and the second pass is processed. During this pass the plane A is in horizontal direction as Figure 4d shows. The reduction ratio of these two passes is 20%∼25%. After these two passes the height-diameter ratio of billet 2 is 0.8∼1.1. As Figure 4c shows, now use wide anvil radial forging again. The billet can be reheated before wide anvil radial forging to reduce forging force. The pressing direction is the same as that of the first wide anvil radial forging in step 2), so the voids can be fully closed after large deformation in one direction. As Figure 4d shows, then the billet should be forged to the final shape using traditional cogging method.

[0042] For ingots with good internal quality, traditional cogging can be used to replace the second wide anvil radial forging in step 4). But the pressing direction of the first cogging pass should be same as that of the first wide anvil forging.

[0043] In this invention, the billet temperature of pre-upsetting, wide anvil radial forging and cogging should depend on the requirement of the material of the billet.

Example 1

[0044] In this example, the original shape of the ingot is cylinder; the original size is Φ2230×2370mm; the height-diameter ratio is 1.063; the weight of the ingot is approximately 100 tons and the material is 6Cr2MnMoV The ingot is heated to 1200°C before forging. As shown in Figure 5, a simplified void model is made based on sectioning results of the 100t ingot. The size, shape and location of this void model are similar to the real voids. In this model, the void is cylindrical shape of Φ12.14mm×90mm, with fillet radius of 5mm at each cylinder edge. The height-diameter ratio of the ingot is smaller than 1.2, so wide anvil radial forging is directly used and the reduction ratio is 20%. Figures 6a shows the strain distribution after wide anvil radial forging, and the result shows the strain concentrate on the center of the billet, and can be 0.5 or more. This is very helpful for healing the central porosities. Figure 6b shows the void can be fully closed after wide anvil radial forging. And after the subsequent heating process, the closed surfaces of the void can be healed and will not open again during the following forging processes. So the central porosities can be completely healed during forging, and greatly increase the possibility of passing ultrasonic test.

Comparative Example 1

[0045] This comparative example use WHF cogging method to forge the ingot for one pass. The width of upper and lower flat anvils are 1200mm, other conditions are the same as Example 1. The same simplified void model is also made in the center of the ingot. Figure 7a shows the strain distribution after cogging. The width of the flat anvil is small, so there is a dead metal zone between two pressing positions where the strain are relatively small, which is not enough for the void to close. Figure 7b shows the void shape after one pass cogging. Comparing with Figure 5, the shape of the void is only changed slightly.

Comparative Example 2

[0046] This comparative example use WHF cogging method to forge the ingot for one pass. Comparing with Comparative Example 1, the starting position of the die is shifted for half of its width (600mm), and other conditions are the same. Figure 8a shows the strain distribution after cogging. From the figure it can be seen that the void is in the high strain zone, which is helpful for the void to close. Figure 8b shows the void shape after one pass cogging. Comparing with Figure 5, the shape of the void is changed, but the strain is still not enough for the void to close.

[0047] From Comparative Examples 1 and 2, it can be seen that one pass cogging is not enough to make the central porosities of the ingot fully closed. Even if two passes are pressed in the same direction, the die will be shifted for half of its width during the second pass, and the effect for closing voids is not much better than Comparative Example 2. So traditional cogging methods cannot close central porosities effectively.

[0048] The method proposed in this invention can heal the voids, especially the central porosities in the ingots or billet. This cannot be done by traditional forging methods. Compared with traditional cogging method like WHF or FM method, this method can greatly increase the strain in the center of the ingot, heal the voids in the ingot, and greatly increase the possibility of passing ultrasonic test after forging.

[0049] Moreover, in this invention, a forging method which can heal the voids in large height-diameter ratio ingots or billets using small reduction ratio comprises the following steps:

1) As Figure 9 shows, large height-diameter ratio billets (like ingots or continuous casting billets) are billets with height-diameter ratio bigger than 2 (normally ≤10). As Figure 10a and 10b shows, put the billet 12 on the lower plate 14, and the axis of the billet 12 should be horizontal. Use the upper plate to press the billet along the radial direction. This process is called wide anvil radial forging. In this figure the vertical plane is defined as plane A.

[0050] During wide anvil radial forging, the reduction ratio is 20%∼40%, which means the pressing depth of upper plate 13 is 20%∼40% of the original height or diameter of billet 12. The reduction ratio is calculated as follows:

[0051] Wherein, ΔH is the pressing depth of the upper plate 13 during the forging process, H is the original height of the ingot or billet along the pressing direction, and if the ingot or billet is a cylinder and the pressing is along the radial direction, its diameter is taken as H.

2) After wide anvil radial forging, the ingot or billet is reheated to forging temperature and holding for more than 2 hours (normally 2-20 hours), which can reduce the forging force required and heal the voids which are already closed.

3) Traditional WHF method or FM method is used for cogging. As Figure 11a and 11b shows, WHF method is used on billet 12. The billet is rotated 90° and now the plane A is horizontal. Put the billet 12 on lower flat anvil 16, and use upper flat anvil 15 to press on corresponding position. The reduction ratio is 20%. Then move the billet 12 to the left in Figure 11b, repeat this pressing process until the anvil reaches the end of the billet. This is called one pass. The second pass should be pressed in the same direction. After these two passes, the billet will be forged to the final shape using traditional cogging method.

Example 2

[0052] In this example, 9 continuous casting billets are used as large height-diameter ratio billet. The original shape of the billet is Φ600mm×2000mm, and the height-diameter ratio is 3.33. The material is S45C+B, and the weight is approximately 4.4 tons. The heating temperature before forging is 1200 °C. As shown in Figures 12a and 12b, there are severe shrinkage cavities throughout the center of the billet. Upsetting cannot be used on this billet because of its large height-diameter ratio. Firstly, Wide anvil radial forging is used to forge the billet to flat-square shape, and then the billet is heated again for 3 hours. Figure 13 shows the strain distribution after wide anvil radial forging, and the result shows the strain concentrate on the center of the billet, which is bigger than 0.4. In traditional WHF forging method, as Figure 3 shows, the largest strain on the axis of the billet is only 0.28. This shows wide anvil radial forging can concentrate the strain on the center of the billet, and make the strain evenly distributed along the axis of the billet. This is very helpful for healing the central porosities. After the heating, the billet is forged to Φ385mm using WHF cogging method. After forging, the billet is inspected using ultrasonic test based on GB/T 6402-1991 grade 2, which is strict for continuous casting billet. And the qualified rate of the 9 billets is 100%.

Comparative Example

[0053] In this example, 9 continuous casting billets used are the same batch of the billets used in Example 2. 600mm width KD anvil (upper flat anvil and lower V-shape anvil) are used to forge the billet to Φ385mm. The width of the KD anvil is limited, so between the two presses there is a dead metal zone where the strain is very small. After forging, the billet is inspected using ultrasonic test based on GB/T 6402-1991 grade 2. And the qualified rate of the 9 billets is 55.6%.

Example 3

[0054] In this example, an ingot is used as large height-diameter ratio billet. The original shape of the ingot is Φ1000mm×3000mm, and the height-diameter ratio is 3. The material is S45, and the weight is approximately 18 tons. The heating temperature before forging is 1200 °C. There are severe shrinkage cavities in the center of the ingot. Upsetting cannot be used on this ingot because of its large height-diameter ratio. Firstly, Wide anvil radial forging is used to forge the billet to flat-square shape, and then the billet is heated again for 5 hours. After heating, the billet is rotated 90° and WHF cogging method is used for two passes. Then the billet is forged to Φ600mm. After forging, the billet is inspected using ultrasonic test based on GB/T 6402-1991 grade 2. Testing result shows the billet is qualified.

[0055] These examples show that the forging method proposed in this invention can heal the voids in large height-diameter ratio ingots or billets using small reduction ratio. Comparing with traditional forging method, the pressing depth of this method is smaller, so using this method can produce qualified forging products with larger diameter, which greatly increase application scope of the product.

Claims

1. A method of forging which can heal the voids in ingots and billets effectively comprising: a wide-anvil radial forging wherein two flat plates are used as upper and lower anvil and the billet is pressed along the radial direction, the reduction ratio being comprised between 20% and 25% for ingots or billets with height-diameter ratio smaller than 2, and the reduction ratio being comprised between 20% and 40% for ingots or billets with height diameter ratio larger than 2,.

2. The method according to claim 1, characterized in that said method comprises the steps of:

1) Pre upsetting ingots or billets with height diameter ratio comprised between 1.2 and 2, said ingots or billets being forged to height diameter ratio comprised between 0.8 and 1.1; no pre upsetting being required for ingots or billets with height diameter ratio smaller than 1.2;

2) Wide anvil radial forging wherein two flat plates are used as upper and lower anvil and the ingots or billets are pressed along the radial direction, the reduction ratio being comprised between 20% and 25%.

3) Rotating the ingots or billets of 90° and cogging being used until the height-diameter ratio of the ingots or billets is comprised between 1.8 and 2.2;

4) Reheating the ingots or billets and upsetting them until the height-diameter ratio is comprised between 0.6 and 0.7; another cogging being used to forge the billet to the final shape.

3. The method according to claim 2, characterized in that it comprises the step of, after the wide anvil radial forging in step 2), the billet is reheated to the forging temperature to reduce the force required and heal the voids which are already closed.

4. The method defined in claim 2, characterized in that it comprises: in step 3), WHF method or FM method is used for cogging, and the reduction ratio of each press is comprised between 20% and 25%, the billet being rotated of 90° after two passes, and another pass being processed, the height-diameter ratio of the billet being comprised between 1.8 and 2.2 after this process.

5. The method according to anyone of claims 2 and 4, characterized in that in step 3), after the cogging process, the direction of wide anvil radial forging is marked, and in step 4), the first pass of cogging is pressed along this direction, which can enhance the effect of void closing.

6. The method according to claim 2, characterized in that it comprises in step 4), using again the wide anvil radial forging, the pressing direction being the same as the previous one in step 2).

7. The method according to anyone of claims 2 and 6, characterized in that it comprises: in step 4), if the wide anvil radial forging is used again, using first WHF or FM method, the pressing direction for the first pass of cogging being the same as the pressing direction of wide anvil radial forging in step 2), the billet being then rotated of 90° and the second pass being processed, the reduction ratio being comprised between 20% and 25% for each pass, the height-diameter ratio of the billet being comprised between 0.8 and 1.1 after the two passes, and wide anvil radial forging being used again, WHF or FM method being then used to forge the billet to the final shape.

8. The method defined according to claim 7, characterized in that the billet is reheated to reduce to forging force required before the wide anvil radial forging in step 4).

9. The method according to claim 1, characterized in that it comprises the steps of:

1) For billets with height-diameter ratio bigger than 2, using two flat plates as upper and lower anvil and the billet being pressed along the radial direction, the reduction ratio being comprised between 20% and 40%.

2) Reheating the billet to the forging temperature after wide anvil radial forging;

3) Rotating the billet of 90°, WHF or FM method being used to forge the billet to the final shape.

10. The method defined according to claim 9, characterized in that it comprises: in step 2), after the wide anvil radial forging, reheating the billet to the forging temperature, holding for more than 2 hours to reduce the force required and healing the voids which are already closed.

11. The method defined according to claim 9, characterized in that in step 2), after wide anvil radial forging, the shape of the ingot or billet is flat-square.

12. The method defined in claim 9, characterized in that it comprises: in step 3), after wide anvil radial forging, rotating the billet of 90°, WHF or FM method being used for two passes, and the reduction ratio for each press being 20%; the billet being forged to the final shape after the two passes.

Drawing