A method of hot rolling a corrosion resistant high temperature alloy

Abstract

This invention relates to a method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys, belonging to the field of hot-rolled coil rolling technology. The invention includes the following steps: 1) slab preparation; 2) slab heating during initial rolling; 3) initial rolling during initial rolling; 4) grinding; 5) intermediate slab heating; 6) high-pressure water descaling; 7) coil rolling; and 8) finished product coiling. This invention enables stable rolling at a final rolling temperature of not less than 980℃, solving the problem of unstable processing of NO6625 high-temperature alloy hot-rolled coils.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled coil rolling methods, specifically relating to a hot-rolled coil rolling method for rolling corrosion-resistant high-temperature alloys. Background Technology

[0002] The steam generator is one of the most critical pieces of equipment in a high-temperature gas-cooled reactor nuclear power system. Its function is to convert the heat from the nuclear reactor into steam at nearly 600°C, which drives the steam turbine generator set to generate electricity.

[0003] The core material of the steam generator is N06625 high-temperature alloy, a corrosion-resistant and oxidation-resistant nickel-chromium-molybdenum-niobium alloy with excellent strength and toughness below 1093℃. N06625 material is easy to cold work, but difficult to hot work. The key rolling difficulty lies in the small processing temperature window and the large deformation resistance. At 1100℃, the peak value is σe=350MPa, and hot working is almost impossible below 980℃. Due to the material's high temperature sensitivity and narrow processing range, the single-pass rolling force of N06625 coils needs to reach 4500t~5000t during the hot working stage, while ensuring that the final rolling temperature of the material is not lower than 980℃.

[0004] Currently, hot rolling in China mainly uses continuous rolling mills. Continuous rolling mills are characterized by large floor space and long roller conveyor transport time. The length of the rolling line is generally around 400m, and the distance between roughing and finishing rolling is generally over 60m. Although there are insulation covers in the intermediate transport roller conveyor area, the insulation effect is weak. On the other hand, continuous rolling mills have separate roughing and finishing mill designs. Roughing is mostly a reversible mill with a large roll diameter that is difficult to roll thin, so it is responsible for preparing the blank for finishing rolling. Finishing rolling generally consists of 5 to 8 stands, all of which are irreversible mills. The rolled piece passes through the mill in one go. The rolling force of a single stand in continuous rolling mills is generally designed to be within 4000t. Even with adjustments to the rolling process, it is still impossible to meet the rolling requirements of N06625 alloy. Summary of the Invention

[0005] This invention addresses the aforementioned problems and overcomes the shortcomings of existing technologies by providing a method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys. This invention enables stable rolling at a final rolling temperature of not less than 980℃, solving the problem of the inability to stably process NO6625 high-temperature alloy hot-rolled coils.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] This invention provides a method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys, characterized in that the method includes the following steps:

[0008] 1) Slab preparation: The slab surface is milled, the milling cutter edge is no more than 1mm, the slab surface is not allowed to have cracks or defects, the defect points are ground by using a flap wheel, the slab surface is free of grinding burns, the grinding direction is parallel to the rolling direction, the depth-to-width ratio is not less than 1:20, the slab is a regular rectangle, and there are no uncut tongue-shaped heads, the slab thickness difference from the plate is no more than 1mm, the width difference from the plate is no more than 3mm, the bulge of the long side and the short side is no more than 2mm, and the wedge shape is no more than 3mm;

[0009] 2) Slab opening heating: The slab size is 155 mm x 1100~2700 mm. A walking beam gas heating furnace is used for heating. The temperature is selected as 1150℃-1200℃ and the heating time is 200 min - 240 min.

[0010] 3) Hot rolling of billet: Use a 1780mm hot rolling mill, and roll in 3 passes with power added per pass controlled at 10%~15%, rolling speed 2 m / s~2.5m / s, initial rolling temperature not lower than 1050℃, and final rolling temperature not lower than 950℃.

[0011] 4) Grinding: Grind the entire surface of the intermediate billet to remove surface oxide scale and defects such as surface cracks;

[0012] 5) Intermediate billet heating: The billet size is 105 mm x 1100 mm x 4080 mm. It is heated in sections using a walking beam gas heating furnace. It is preheated at 1000℃ for 30 minutes, then heated to 1200-1250℃ within 30 minutes, and then held for 180-240 minutes.

[0013] 6) High-pressure water descaling: 220 bar to 330 bar water pressure is used to descale the slab, peel off the surface oxide scale, and avoid the generation of oxide scale pressing defects;

[0014] 7) Coil Rolling: The slab specifications are 105mm x 1100mm x 4080mm. After exiting the furnace, the slab reaches the mill entrance within 45 seconds. The initial rolling temperature is not lower than 1150℃. Rolling adopts a 9-pass flat rolling followed by a 4-pass hot coil rolling pattern, for a total of 13 passes. The thickness entering the hot coil furnace is 22mm, and the finished product output thickness is set at 6.0mm. The power increase for flat rolling passes is controlled at 14%~24%, and the power increase for hot coil rolling passes is controlled at no more than 23%. The overall processing rate of flat rolling and hot coil rolling adopts a parabolic arrangement of "low-high-low". The power increase for the last pass is controlled between 13%~15%. The flat rolling speed is controlled at 2.5m / s, and the hot coil rolling speed is controlled at 3.5m / s. ~5m / s, ensure the final rolling temperature is not lower than 980℃, adjust the roll cooling to 20% of the normal flow rate, set the coiling furnace temperature to 1200℃, and the coil temperature to 980℃. Before the strip roll enters the coiling furnace, use flying shears to cut the head and tail of the mother strip, and set the speed difference between the upper and lower rolls so that the strip head is in a downward-curved state.

[0015] 8) Finished product curling: Use a curling machine in position control mode to curl the roll material. The curling temperature is not lower than 750℃ and the curling tension is controlled between 170 KN and 200 KN.

[0016] Furthermore, in step 2, the heating temperature is selected to be 50-100°C lower than the conventional heating temperature to reduce energy consumption.

[0017] Furthermore, in step 3, the flat rolling process uses a small reduction and low power to obtain a flat plate shape, which facilitates subsequent surface treatment.

[0018] Furthermore, in step 6, the slab should be quickly passed through the descaling device, and after descaling, the surface temperature drop of the slab should be controlled within 50°C.

[0019] Furthermore, in step 7, the roll type is a concave roll with a concavity of 0.75mm in the radial direction, which, together with the work roll bending roll, enables controllable convexity of the sheet material.

[0020] Furthermore, in step 7, during the entire rolling process, except for descaling, all other processes are carried out without water. In order to reduce temperature drop during the flat rolling stage, a large reduction method is used for rolling in the first 3 passes.

[0021] Furthermore, in step 7, the speed of the upper roller is greater than that of the lower roller.

[0022] Furthermore, in step 7, the power increase per pass in the furnace is controlled at 13%~21%.

[0023] The beneficial effects of the present invention.

[0024] This invention selects a hot-rolled coil mill process route. As it is a single-stand mill integrating roughing and finishing rolling, compared to a two-stand mill, it eliminates the need for transport to the finishing stage after flat rolling, achieving seamless transition between roughing and finishing and reducing temperature drop caused by roller conveyor transport. Furthermore, compared to continuous rolling mills, the integration of roughing and finishing rolling allows a single stand to withstand greater rolling forces, with a single rolling pass force reaching 5500t. Two heated coiling furnaces are longitudinally placed at the inlet and outlet of the mill. During hot-rolled coil rolling, the temperature drop during strip thinning is reduced. Therefore, through technical means, while ensuring stable rolling and preventing equipment damage, the entire rolling process is maintained within the process window by adjusting the pass processing rate and controlling the rolling rhythm, thus achieving successful material rolling.

[0025] Furthermore, thanks to its short single-stand transport distance and the ability to provide heating in a coiling furnace, stable rolling with a final rolling temperature of no less than 980℃ can be achieved by rationally matching the coiling furnace temperature and optimizing the distribution of processing passes, thus solving the problem of hot working NO6625 high-temperature alloy. Attached Figure Description

[0026] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0027] Figure 1 This is a flowchart of the processing steps of the present invention. Detailed Implementation

[0028] Referring to the accompanying drawings, this embodiment provides a method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys, particularly for high-temperature alloys of grade NO6625, comprising the following steps:

[0029] 1) Slab preparation: The slab surface is milled, the milling cutter edge is no more than 1mm, and no cracks or defects are allowed on the slab surface. Defect points are ground using a flap wheel. There are no grinding burns on the slab surface, the grinding direction is parallel to the rolling direction, the depth-to-width ratio is no less than 1:20, the slab is a regular rectangle, and no uncut tongue-shaped ends are allowed. The slab thickness difference from the plate is no more than 1mm, the width difference from the plate is no more than 3mm, the bulge on the long and short sides is no more than 2mm, and the wedge shape is no more than 3mm.

[0030] Slab preparation is carried out to enhance rolling stability and prevent lateral bending and defect introduction.

[0031] 2) Slab opening heating: The slab size is 155 mm x 1100~2700 mm. A walking beam gas heating furnace is used for heating. The temperature is selected as 1150℃-1200℃ and the heating time is 200 min - 240 min.

[0032] Since this process involves billet rolling, the power consumption is very small. Therefore, the key point of this process is the thorough burning of the material. The heating temperature can be 50-100℃ lower than the conventional heating temperature to reduce energy consumption.

[0033] 3) Hot rolling of billet: A 1780mm hot rolling mill is used, and rolling is carried out in 3 passes. The power added per pass is controlled at 10%~15%, the rolling speed is 2 m / s~2.5m / s, the initial rolling temperature is not lower than 1050℃, and the final rolling temperature is not lower than 950℃.

[0034] Since the straightening machine on the hot rolling line is rarely configured for thicknesses greater than 100mm, the key control point of this process is the plate shape. Flat rolling uses a method of small reduction and small power to obtain a flat plate shape, which facilitates subsequent surface treatment.

[0035] The main purpose of this process is to select a suitable base material for coil rolling. If the intermediate billet is too thick, the final rolling temperature of the finished coil will be insufficient, making it impossible to process. If the intermediate billet is too thin, the yield will be low and the economic benefits will be poor.

[0036] 4) Grinding: Grind the entire surface of the intermediate billet to remove surface oxide scale and defects such as surface cracks.

[0037] The purpose of this process is to remove surface defects.

[0038] 5) Intermediate billet heating: The billet size is 105 mm x 1100 mm x 4080 mm. It is heated in sections using a walking beam gas heating furnace. It is preheated at 1000℃ for 30 minutes, then heated to 1200-1250℃ within 30 minutes, and then held for 180-240 minutes.

[0039] 6) High-pressure water descaling: 220 bar to 330 bar water pressure is used to descale the slab, peel off the surface oxide scale, and avoid the generation of oxide scale defects.

[0040] The slab should pass through the descaling device quickly. After descaling, the surface temperature drop of the slab should be controlled within 50°C.

[0041] 7) Coil rolling: The slab specifications are 105mm x 1100mm x 4080mm. After the slab exits the furnace, it reaches the mill entrance within 45 seconds. The initial rolling temperature is not lower than 1150℃. Therefore, the rolling speed and transport speed need to be adjusted. The roller conveyor speed is calculated based on the length from the walking beam furnace to the mill to achieve rapid transport.

[0042] After the slab arrives at the rolling mill, a 1780mm hot roll mill is used. The rolling process adopts a pattern of 9 passes of flat rolling plus 4 passes of hot roll rolling, for a total of 13 passes.

[0043] To prevent excessive temperature drop, the thickness of the strip entering the coiling furnace should be as thick as possible. However, the thickness of the finished product and the number of passes required to reach the finished product thickness stage must also be considered. Fewer passes in the coiling mode will be more beneficial for rolling. Therefore, the thickness entering the coiling furnace is selected as 22mm, and the thickness of the finished product output is set as 6.0mm.

[0044] The power increase for flat rolling passes should be controlled at 14%~24%, and the power increase for hot rolling passes should be controlled at no more than 23%, preferably between 13%~21%. This is because although the rolled piece can be reheated in the hot rolling furnace in hot rolling mode, the reheating effect is far less than the cooling effect. As the thickness of the strip decreases, the surface area increases, and the cooling is more significant, especially at the beginning and end of the strip. Due to the high deformation resistance of NO6625 alloy, a large power increase may cause mill alarms and rolling deviation. This needs to be carefully considered when setting the process.

[0045] The overall processing rate of flat rolling and hot rolling is arranged in a parabolic pattern of "low-high-low", and the power of the last pass is controlled between 13% and 15% to obtain a good strip shape.

[0046] The rolling speed for flat rolling is controlled at 2.5 m / s, and the rolling speed for hot rolling is controlled at 3.5 m / s to 5 m / s. The final rolling temperature is guaranteed to be no less than 980℃. When selecting the rolling speed, attention should be paid to the deformation rate and the change in transmission current, and the speed should not exceed the equipment limit.

[0047] The preferred roll type is a concave roll with a concavity of 0.75 mm in the radial direction. This, combined with the work roll bending roll, allows for controllable convexity of the sheet metal and improves rolling stability.

[0048] The roll cooling flow rate is adjusted to 20% of the normal flow rate. During the entire rolling process, except for descaling, all other processes are carried out without water. In order to reduce the temperature drop during the flat rolling stage, a large reduction method is used for rolling in the first three passes to obtain deformation heat to fill the temperature drop. However, the maximum power increase should not exceed 25% to prevent the rolling from causing the roll head to curl and damage the equipment.

[0049] Temperature control in the coiling furnace plays a crucial role in the rolling of coils. High furnace temperature will cause overheating at the edge of the coil, resulting in double-sided ripples after rolling. Low coiling furnace temperature will not provide effective heat compensation. After several adjustments, the coiling furnace temperature was set at 1200℃ and the coil temperature at 980℃. This ensured that the material would not experience excessively high temperatures on both sides and that effective heat compensation could be achieved.

[0050] To avoid steel piling during strip threading, the head and tail of the strip are cut with a flying shear before the coil is rolled in the furnace. The cutting length can be adjusted according to the actual rolling situation. The head and tail do not need to be cut into regular rectangles after cutting. It is better to leave a small amount of arc so that the strip head can still be rolled even if it bends laterally, thus improving the rolling yield.

[0051] On the other hand, due to the high strength of NO6625, the strip head will curl up during the threading process and will not slide upward against the swing guide plate. Therefore, in the furnace coiling mode, it is necessary to increase the rolling speed of the upper roller and set the speed difference between the upper and lower rollers so that the speed difference between the upper and lower rollers is greater than that of the lower roller, so that the strip head is in a downward-curving state, which greatly reduces the upward curling of the head. At the same time, the laser detection device in front of the coiling furnace needs to be compensated for the data so that the coil is coiled immediately after the threading is completed, to avoid the strip head hitting the coil and damaging the equipment.

[0052] 8) Finished product curling: Use a curling machine in position control mode to curl the roll material. The curling temperature is not lower than 750℃ and the curling tension is controlled between 170 KN and 200 KN.

[0053] Through the above steps, the process mode of intermediate slab opening is adopted, the total processing rate of the rolled piece is controlled, and in the coil rolling mode, the processing rate of different passes is reasonably matched in conjunction with the cooling of the rolled piece. This fully utilizes the capacity of the rolling mill while reducing the peak rolling force and the rolling mill load. In the furnace coil threading stage, the laser signal is pre-compensated to avoid steel piling during threading, ensuring the stability and controllability of the entire rolling process. Secondly, the intermediate slab is ground to remove harmful surface defects, and after heating, high-pressure water descaling is used to remove the oxide scale generated during slab heating. Combined with the small processing rate of less than 18% in the last two passes of furnace coil finishing rolling, the surface quality and shape of the rolled piece are effectively controlled. Finally, by increasing the rolling and roller conveyor speeds, making full use of the deformation heat in the flat rolling stage and the furnace heating in the finishing rolling stage, the temperature drop of the rolled piece is minimized, allowing the rolled piece to complete the rolling at a high temperature. From a process perspective, this work solves the problem of difficult hot working of NO6625 high-temperature alloy due to its small processing temperature window and high deformation resistance.

[0054] It is understood that the above specific description of the present invention is only for illustrating the present invention and is not limited to the technical solutions described in the embodiments of the present invention. Those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention to achieve the same technical effect; as long as the use needs are met, they are all within the protection scope of the present invention.

Claims

1. A method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys, characterized in that, The method includes the following steps: 1) Slab preparation: The slab surface is milled, the milling cutter edge is no more than 1mm, the slab surface is not allowed to have cracks or defects, the defect points are ground by using a flap wheel, the slab surface is free of grinding burns, the grinding direction is parallel to the rolling direction, the depth-to-width ratio is not less than 1:20, the slab is a regular rectangle, and there are no uncut tongue-shaped heads, the slab thickness difference from the plate is no more than 1mm, the width difference from the plate is no more than 3mm, the bulge of the long side and the short side is no more than 2mm, and the wedge shape is no more than 3mm; 2) Slab opening heating: The slab size is 155 mm x 1100~2700 mm. A walking beam gas heating furnace is used for heating. The temperature is selected as 1150℃-1200℃ and the heating time is 200 min - 240 min. 3) Hot rolling of billet: Use a 1780mm hot rolling mill, and roll in 3 passes with power added per pass controlled at 10%~15%, rolling speed 2 m / s~2.5m / s, initial rolling temperature not lower than 1050℃, and final rolling temperature not lower than 950℃. 4) Grinding: Grind the entire surface of the intermediate billet to remove surface oxide scale and defects such as surface cracks; 5) Intermediate billet heating: The billet size is 105 mm x 1100 mm x 4080 mm. It is heated in stages using a walking beam gas heating furnace. It is preheated at 1000℃ for 30 minutes, then heated to 1200℃~1250℃ within 30 minutes, and then held for 180-240 minutes. 6) High-pressure water descaling: 220 bar to 330 bar water pressure is used to descale the slab, peel off the surface oxide scale, and avoid the generation of oxide scale pressing defects; 7) Coil Rolling: The slab specifications are 105mm x 1100mm x 4080mm. After exiting the furnace, the slab reaches the mill entrance within 45 seconds. The initial rolling temperature is not lower than 1150℃. Rolling adopts a 9-pass flat rolling followed by a 4-pass hot coil rolling pattern, for a total of 13 passes. The thickness entering the hot coil furnace is 22mm, and the finished product output thickness is set at 6.0mm. The power increase for flat rolling passes is controlled at 14%~24%, and the power increase for hot coil rolling passes is controlled at no more than 23%. The overall processing rate of flat rolling and hot coil rolling adopts a parabolic arrangement of "low-high-low". The power increase for the last pass is controlled between 13%~15%. The flat rolling speed is controlled at 2.5m / s, and the hot coil rolling speed is controlled at 3.5m / s. ~5m / s, ensure the final rolling temperature is not lower than 980℃, adjust the roll cooling to 20% of the normal flow rate, set the coiling furnace temperature to 1200℃, and the coil temperature to 980℃. Before the strip roll enters the coiling furnace, use flying shears to cut the head and tail of the mother strip, and set the speed difference between the upper and lower rolls so that the strip head is in a downward-curved state. 8) Finished product curling: Use a curling machine in position control mode to curl the roll material. The curling temperature is not lower than 750℃ and the curling tension is controlled between 170 KN and 200 KN.

2. The method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 1, characterized in that, In step 2, the heating temperature is selected to be 50-100℃ lower than the conventional heating temperature to reduce energy consumption.

3. The method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 2, characterized in that, In step 3, the flat rolling process uses a small reduction and low power to obtain a flat plate shape, which facilitates subsequent surface treatment.

4. The method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 1, characterized in that, In step 6, the slab should pass through the descaling device quickly, and the surface temperature drop of the slab after descaling should be controlled within 50°C.

5. A method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 4, characterized in that, In step 7, the roll type is a concave roll with a concavity of 0.75mm in the radial direction. In conjunction with the work roll bending roll, the convexity of the sheet material can be controlled.

6. The method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 1, characterized in that, In step 7, except for descaling, all other processes are carried out without water. In order to reduce temperature drop during the flat rolling stage, a large reduction method is used for rolling in the first three passes.

7. The method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 1, characterized in that, In step 7, the speed of the upper roller is greater than that of the lower roller.

8. A method for rolling hot-rolled coils of corrosion-resistant high-temperature alloys according to claim 1, characterized in that, In step 7, the power increase per pass of the furnace is controlled at 13%~21%.

Images