Preparation method of large-diameter thin-wall Al-Mg-Si aluminum alloy circular pipe

Abstract

The invention discloses a preparation method of a large-diameter thin-wall Al-Mg-Si aluminum alloy round pipe, which comprises the following steps: burdening, smelting, refining, casting, ingot casting homogenization heat treatment, preheating, extruding, online quenching, stretching and straightening, saw cutting, inner wall polishing and annealing, and is characterized in that the heat preservation temperature of heat treatment in the annealing process is 390 + / -5 DEG C, the heat preservation time is 3-6 hours, and air cooling is carried out along with a furnace after heat preservation is finished; a row of bottoming round pipes with the outer diameter of 100-200 mm and the wall thickness of 10-40 mm are placed on a bottom frame of the annealing furnace, and the large-diameter thin-wall round pipes are evenly distributed and placed on the bottoming round pipes during furnace charging. A layer of bottoming round pipe is additionally arranged on the bottom layer of an annealing furnace material rack, large-diameter thin-wall aluminum alloy round pipes and the bottoming round pipe are uniformly arranged in the unified direction during furnace charging, the length of the bottoming round pipe is larger than or equal to that of the large-diameter thin-wall aluminum alloy round pipes, and the contact mode of the bottoming round pipe and the large-diameter thin-wall aluminum alloy round pipes is changed into surface contact from point contact. The stress area of the bottom of the aluminum alloy circular pipe is greatly increased, and the aluminum alloy circular pipe is prevented from being pressed to be concave.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum alloy processing, and in particular to a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube. Background Art

[0002] Large-sized thin-walled aluminum alloy round tubes are often used in gas-insulated metal-enclosed switchgear, which uses sulfur hexafluoride (SF6) gas with excellent insulation and arc-extinguishing properties as the insulating and arc-extinguishing medium, and seals all high-voltage electrical components in a grounded metal cylinder. Due to the special and harsh use environment, there are high requirements for the internal and external surface quality and welding quality of the round tube material. The inner wall of the round tube must be smooth, and defects such as dents, uneven lines, and mold marks are not allowed. The profile surface of Al-Mg-Si aluminum alloy produced by extrusion will inevitably have some adsorbed particles and mold marks, and because the round tube has a round shape, the bottom has a small contact area with the load-bearing object, and it is very easy to be dented at the contact position, forming a product defect.

[0003] In addition, the production of round tubes by forward extrusion will involve welding. The metal is divided into multiple strands before entering the die hole of the die, and then merged in the welding chamber of the die hole. Under the action of high temperature, high pressure, and high vacuum, the metal is welded at the same time as it merges, and then flows out of the die hole to form a round tube. How to ensure the quality of the weld is the main technical problem of combined die extrusion. If the weld position of the round tube is poorly welded, it is easy to crack and be scrapped during the service of the product. Summary of the invention

[0004] In view of the above shortcomings, the present invention provides a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube to solve the problem of inner cavity concavity caused by extrusion of a large-diameter round tube.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] A method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube comprises the following steps:

[0007] (1) Ingredients: According to the alloy composition and mass percentage, raw and auxiliary materials such as aluminum ingots and master alloys are prepared;

[0008] (2) Melting and refining: putting the raw and auxiliary materials into a smelting furnace for melting and refining to obtain a melt of the desired ingot chemical composition;

[0009] (3) Casting: Casting the refined melt to obtain a round ingot required for extrusion production;

[0010] (4) homogenization heat treatment, homogenization heat treatment of the ingot;

[0011] (5) preheating the mold, ingot, and extrusion barrel, extruding the preheated ingot on an extruder to obtain a round tube profile, and performing online quenching and stretching straightening on the profile after extrusion out of the mold, and then sawing and interrupting to obtain a semi-finished product;

[0012] (6) Grinding the inner wall: Use a grinder to grind the inner wall of the round tube to eliminate various surface quality defects of the inner wall.

[0013] (7) Annealing: The round tube is subjected to annealing heat treatment to obtain a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, wherein the annealing heat treatment has an insulation temperature of 390±5°C and an insulation time of 3-6h. After the insulation is completed, the tube is air-cooled with the furnace. A row of bottom-laying round tubes with an outer diameter of 100-200mm and a wall thickness of 10-40mm is placed on the bottom frame of the annealing furnace. When loading the furnace, the large-diameter thin-walled round tubes are evenly arranged and placed on the bottom-laying round tubes.

[0014] Preferably, there is a certain gap between the bottom-laying circular tubes, and the gap between adjacent bottom-laying circular tubes is 50-200 mm.

[0015] Preferably, the chemical composition and mass percentage of the aluminum alloy material ingredients are: Si: 0.65%-0.75%, Fe: 0.15-0.25%, Cu: ≤0.12%, Mn: 0.20%-0.35%, Mg: 0.50-0.65%, Cr: 0.12%-0.20%, Zn: ≤0.20%, Ti: ≤0.05%, Zr: ≤0.05%, and the rest are Al and inevitable impurities, each inevitable element is less than 0.05%, and the total amount of inevitable elements is less than 0.10%.

[0016] Preferably, during stretching, stretching pads are placed in the inner cavities of the head end and the tail end of the aluminum alloy round tube respectively, and the stretching rate is 0.5-2.5%.

[0017] Preferably, the metal holding temperature during the ingot homogenization heat treatment is 570±10°C, and the metal holding time is 7-14h.

[0018] Preferably, the ingot is heated by electromagnetic induction gradient, the heating temperature of the ingot head is 500-520°C, and the temperature gradient decreases by 10°C at an interval of 300mm; the mold heating temperature is 500°C±10°C, the extrusion barrel heating temperature is 430°C±10°C, the extrusion product speed is 2.5-6.0m / min, the profile online quenching cooling inlet temperature is ≥500°C, and the profile online quenching cooling outlet temperature is ≤50°C.

[0019] Preferably, the outer diameter of the aluminum alloy round tube is φ300-600 mm, the wall thickness of the round tube is 5-10 mm, and the finished product length is 6-12 m.

[0020] Preferably, the ingredients are put into a smelting furnace for smelting at a smelting temperature of 720-760°C; after smelting, the ingredients are refined at a refining temperature of 710-730°C and a refining chlorine gas flow rate of 0.1-0.3Nm 3 / h, use electromagnetic stirring device to stir, remove gas and slag, let it stand for 30-45 minutes after refining before casting.

[0021] Preferably, the casting speed is 25-40 mm / min, the casting temperature is 690-710°C, and the casting cooling water temperature is 28-35°C.

[0022] Preferably, water-cooled semi-continuous casting is used for casting, with a casting temperature of 700-730°C, a casting cooling water temperature of 28-35°C, a casting stable material temperature of 690-710°C, a casting speed of 25-40 mm / min, and a casting cooling water flow rate of 380-420 m 3 / h, the obtained round ingot has a diameter of 580mm, and the outer shell of the ingot is removed by a car skin machine to obtain a round ingot with a diameter of 566mm. The length of a single round ingot is 7000-8000mm. After the head and tail wastes are cut off, the ingot is cut into ingot blanks with a length of 1500-1800mm.

[0023] Preferably, an automatic grinder is used to finely grind the inner cavity of the large-diameter thin-walled circular tube, and the surface roughness of the inner wall of the circular tube after grinding is less than or equal to Ra0.5 μm.

[0024] Preferably, after cutting off the head and tail waste, a sample is taken from the head end of the round tube to detect the quality of the weld, the weld position is determined after the sample is alkaline-etched, and the welding condition of the weld position is detected.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The large-diameter thin-walled Al-Mg-Si aluminum alloy round tube prepared by the present invention has a thin wall thickness, low hardness, small bottom contact area and poor bearing capacity, especially after annealing, the surface hardness is reduced from 6HW to less than 3HW, and the round tube becomes soft. Since the hardness of the aluminum alloy round tube is reduced after heating during the annealing process, it is affected by the deadweight of the aluminum alloy round tube. When its bottom contacts the annealing furnace rack, the outer surface is very likely to produce plastic concavity, forming a raised bulge in the inner cavity of the aluminum alloy round tube. The present invention adds a layer of bottom-laying round tube to the bottom layer of the annealing furnace rack. When the furnace is loaded, the large-diameter thin-walled aluminum alloy round tube and the bottom-laying round tube are uniformly arranged in a unified direction. The length of the bottom-laying round tube is greater than or equal to the length of the large-diameter thin-walled aluminum alloy round tube. The contact mode of the two is changed from "point contact" to "surface contact", which greatly increases the force-bearing area at the bottom of the aluminum alloy round tube and avoids the aluminum alloy round tube from being concave.

[0027] The present invention controls the ingot heating temperature, mold temperature, extrusion barrel temperature, and extrusion speed, selects appropriate process parameters on the premise of ensuring the welding quality and surface quality of the aluminum alloy round tube, and ensures product quality and production efficiency. The welding quality is determined by factors such as pressure, time and flow rate. In theory, the higher the temperature, the better the welding, but it is affected by the speed. At the same time, it is necessary to consider the product cross-section and the temperature drop requirements of online quenching, and select a suitable speed on the premise of ensuring mechanical properties. The present invention avoids poor welding by adopting reasonable parameters of the ingot heating temperature, mold temperature, extrusion barrel temperature, and extrusion speed to obtain excellent welding quality and surface quality of the aluminum alloy round tube, and avoid easy cracking and scrapping during service.

[0028] There is a certain gap between the bottom round tubes, so that the furnace gas can flow freely during heating, and the heating temperature rises more evenly, avoiding the local soft zone performance failure of the product and avoiding the generation of coarse grains.

[0029] The present invention places stretching pads in the inner cavities of the head end and the tail end of the round tube respectively when stretching and straightening the round tube, thereby reducing the deformation at both ends, improving the dimensional accuracy, and effectively improving the yield rate of the round tube.

[0030] The present invention adopts a specific casting temperature, and the ingot temperature is relatively high, which can effectively reduce the deformation resistance of the metal and facilitate extrusion molding, so as to avoid blocking or damage to the extrusion tool when using a longer ingot and a larger extrusion coefficient; at the same time, the extrusion process is used to produce a work hardening effect, so that the round tube obtains a certain strength and avoids being dented by the cooling bed belt spacers during the production process; high-temperature extrusion is used during extrusion, which is conducive to extrusion molding, thereby ensuring the diameter and wall thickness dimensional accuracy of the round tube.

[0031] The present invention optimizes the process parameters of smelting, refining and casting to obtain a clean aluminum alloy melt with uniform composition and a hydrogen content of ≤0.12ml / 100Al, effectively reducing the presence of hydrogen embrittlement and various casting structural defects, and obtaining an excellent ingot for extrusion processing.

[0032] The present invention adopts an automated grinder to grind the inner wall of the entire round tube to eliminate surface quality defects such as pitting, mold marks, scratches, etc. that may appear on the inner wall of the round tube, avoid tip discharge of the round tube during service, and improve the safety of the product.

[0033] The present invention provides a preparation method of a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube. By selecting appropriate ingot casting process, extrusion process, heat treatment process and inner cavity grinding process, and adopting reasonable auxiliary tooling, the present invention effectively solves the problems of inner cavity concavity and poor welding of the extruded large-diameter thin-walled aluminum alloy extruded round tube through a combination of multiple improvements. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments are briefly introduced below.

[0035] Figure 1 Schematic diagram of large-diameter thin-walled Al-Mg-Si aluminum alloy round tubes prepared in embodiments and comparative examples of the present invention;

[0036] Figure 2 It is a schematic diagram of the placement of large-diameter thin-walled aluminum alloy round tubes and bottom-laying round tubes during annealing in Examples 1 to 3 of the present invention;

[0037] Figure 3 This is a schematic diagram of the placement of large-diameter thin-walled aluminum alloy round tubes and bottom-laying round tubes during annealing in Example 4 of the present invention. DETAILED DESCRIPTION

[0038] The technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] In the description of the present invention, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they cannot be understood as limitations on the present invention.

[0040] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0041] Please refer to Figure 1The outer diameter of the target aluminum alloy round tube is 508mm, the wall thickness is 8mm, and the alloy state is 6005A-H112. Industrial grade aluminum ingots with a purity of 99.70% or more are used as raw materials, and related raw and auxiliary materials and intermediate alloys are used for smelting and casting. The chemical composition and mass percentage of the alloys of each embodiment and comparative example are shown in the following table:

[0042] Table 1 Alloy chemical composition and mass percentage (wt%) of each embodiment and comparative example

[0043]

[0044] Example 1

[0045] This preferred embodiment provides a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, and the specific steps are as follows:

[0046] (1) Ingredients: Ingredients are prepared according to the corresponding chemical compositions and mass percentages in Table 1.

[0047] (2) Melting and refining: Put aluminum ingots, magnesium ingots, zinc ingots and master alloys and other raw and auxiliary materials into a dome hydraulic tilting melting furnace for melting, fully stir and slag removal. The melting furnace is kept at a temperature of 720-740°C and the melting time is 6 hours; the refining temperature is 710-730°C and the refining chlorine flow rate is 0.1-0.3Nm 3 / h, use electromagnetic stirring device to stir, remove gas and slag, let it stand for 30-45 minutes after refining before casting.

[0048] (3) Casting: Water-cooled semi-continuous casting is used for casting, with a casting temperature of 690-700°C, a casting speed of 27 mm / min, a casting cooling water temperature of 28-34°C, and a casting cooling water flow rate of 390 m 3 / h, the diameter of the obtained round ingot is 580mm, and the final car body is a round ingot with a diameter of 566mm. The length of the round ingot is 7000mm, and the ingot is cut into ingot blanks with a length of 1800mm after the head and tail waste are cut off.

[0049] (4) Ingot homogenization heat treatment: The metal insulation temperature of the ingot homogenization heat treatment is 570±10℃, and the metal insulation time is 14h.

[0050] (5) Preheating: The ingot is placed in an induction furnace for heating and insulation. The heating temperature is 510°C. A gradient heating method is adopted. The temperature at the front end of the ingot decreases every 300 mm. There are 6 heating zones with a temperature gradient of 10°C, i.e. 510°C-500°C-490°C-480°C-470°C. The insulation time is 15-20 min. The mold heating temperature is 500°C±10°C, and the extrusion barrel heating temperature is 430°C±10°C.

[0051] (6) Extrusion: The preheated ingot is placed in an extrusion barrel of an extruder for extrusion to obtain a round tube profile. The extrusion speed is 5.0 m / min, and the profile inlet temperature for online quenching and cooling is 525°C.

[0052] (7) Online quenching: After extrusion molding, online quenching is carried out, using strong air cooling. The total capacity of the supporting motor is 5.5KW, and the quenching air volume is 70%, which quickly cools the profile to below 50°C.

[0053] (8) Stretching: The quenched round tube profile is stretched and straightened with a stretching rate of 1.6%. During stretching, stretching pads are placed in the inner cavity of the head and tail ends of the round tube respectively.

[0054] (9) Use an automated grinder to grind the inner wall of the entire round tube.

[0055] (10) Annealing: Annealing temperature is 390±5℃, and the insulation time is 4h. After the insulation is completed, the furnace is air-cooled. A row of bottom-laying round tubes with an outer diameter of 150mm and a wall thickness of 20mm are placed on the bottom frame of the annealing furnace. When loading the furnace, large-diameter thin-walled aluminum alloy round tubes are evenly arranged and placed on the bottom-laying round tubes. There is a certain gap between the bottom-laying round tubes, and the gap between adjacent bottom-laying round tubes is 60mm. The bottom-laying round tubes are fixed, such as by wedges, to prevent the bottom-laying round tubes from rolling. Please refer to Figure 2 During implementation, a row of bottom-laying round tubes 30 are placed on the annealing furnace chassis 20, and large-diameter thin-walled aluminum alloy round tubes 10 are evenly arranged on the bottom-laying round tubes. The large-diameter thin-walled aluminum alloy round tubes 10 and the bottom-laying round tubes 30 are evenly arranged in the same direction. The length of the bottom-laying round tubes 30 is greater than or equal to the length of the large-diameter thin-walled aluminum alloy round tubes 10. The contact mode between the two changes from "point contact" to "surface contact", which greatly increases the force-bearing area at the bottom of the aluminum alloy round tubes and avoids the aluminum alloy round tubes from being pressed. There is a certain gap between the bottom-laying round tubes, so that the furnace gas can flow freely during heating, and the heating temperature rises more evenly, avoiding the local soft band performance of the product and avoiding the generation of coarse grains.

[0056] The large-diameter thin-walled round tube prepared in this embodiment has no dent defect on the surface after annealing and the inner wall surface roughness is Ra0.4 μm, and the welding is good.

[0057] Example 2

[0058] This preferred embodiment provides a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, and the specific steps are as follows:

[0059] (1) Ingredients: Ingredients are prepared according to the corresponding chemical compositions and mass percentages in Table 1.

[0060] (2) Melting and refining: Put aluminum ingots, magnesium ingots, zinc ingots and master alloys and other raw and auxiliary materials into a dome hydraulic tilting melting furnace for melting, fully stir and slag removal. The melting furnace insulation temperature is 720-740℃, the melting time is 7 hours; the refining temperature is 710-730℃, and the refining chlorine flow rate is 0.1-0.3Nm 3 / h, use electromagnetic stirring device to stir, remove gas and slag, let it stand for 30-45 minutes after refining before casting.

[0061] (3) Casting: Water-cooled semi-continuous casting is used for casting, with a casting temperature of 690-700°C, a casting speed of 30 mm / min, a casting cooling water temperature of 29-35°C, and a casting cooling water flow rate of 410 m 3 / h, the diameter of the obtained round ingot is 580mm, and the final car body is a round ingot with a diameter of 566mm. The length of the round ingot is 7000mm, and the ingot is cut into ingot blanks with a length of 1800mm after the head and tail waste are cut off.

[0062] (4) Ingot homogenization heat treatment: The metal insulation temperature of the ingot homogenization heat treatment is 570±10℃, and the metal insulation time is 14h.

[0063] (5) Preheating: The ingot is placed in an induction furnace for heating and insulation. The heating temperature is 500°C. A gradient heating method is adopted. The temperature of the front end of the ingot decreases every 300 mm. There are 6 heating zones in total. The temperature gradient is 10°C, i.e. 500°C-490°C-480°C-470°C-460°C. The insulation time is 15-20 min. The mold heating temperature is 500°C±10°C, and the extrusion barrel heating temperature is 430°C±10°C.

[0064] (6) Extrusion: The preheated ingot is placed in an extrusion barrel of an extruder for extrusion to obtain a round tube profile. The extrusion speed is 6.0 m / min, and the profile inlet temperature for online quenching and cooling is 525°C.

[0065] (7) Online quenching: After extrusion molding, online quenching is carried out, using strong air cooling. The total capacity of the supporting motor is 5.5KW, and the quenching air volume is 60%, which quickly cools the profile to below 50°C.

[0066] (8) Stretching: The quenched round tube profile is stretched and straightened with a stretching rate of 1.6%. During stretching, stretching pads are placed in the inner cavities of the head and tail ends of the round tube respectively.

[0067] (9) Use an automated grinder to grind the inner wall of the entire round tube.

[0068] (10) Annealing: Annealing temperature is 390±5℃, and the insulation time is 3.9h. After the insulation is completed, the furnace is air-cooled. A row of bottom-laying round tubes with an outer diameter of 180mm and a wall thickness of 40mm are placed on the bottom frame of the annealing furnace. When loading the furnace, large-diameter thin-walled aluminum alloy round tubes are evenly arranged and placed on the bottom-laying round tubes. There is a certain gap between the bottom-laying round tubes, and the gap between adjacent bottom-laying round tubes is 80mm. The bottom-laying round tubes are fixed, such as by wedges, to prevent the bottom-laying round tubes from rolling. Please refer to Figure 2 During implementation, a row of bottom-laying round tubes 30 are placed on the annealing furnace chassis 20, and large-diameter thin-walled aluminum alloy round tubes 10 are evenly arranged on the bottom-laying round tubes. The large-diameter thin-walled aluminum alloy round tubes 10 and the bottom-laying round tubes 30 are evenly arranged in the same direction. The length of the bottom-laying round tubes 30 is greater than or equal to the length of the large-diameter thin-walled aluminum alloy round tubes 10. The contact mode between the two changes from "point contact" to "surface contact", which greatly increases the force-bearing area at the bottom of the aluminum alloy round tubes and avoids the aluminum alloy round tubes from being pressed. There is a certain gap between the bottom-laying round tubes, so that the furnace gas can flow freely during heating, and the heating temperature rises more evenly, avoiding the local soft band performance of the product and avoiding the generation of coarse grains.

[0069] The large-diameter thin-walled round tube prepared in this embodiment has no dent defect on the surface after annealing; many fine pits appear on the inner wall surface of the round tube, and the roughness of the inner wall after grinding is Ra0.45μm, and the welding is good.

[0070] Example 3

[0071] This preferred embodiment provides a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, and the specific steps are as follows:

[0072] (1) Ingredients: Ingredients are prepared according to the corresponding chemical compositions and mass percentages in Table 1.

[0073] (2) Melting and refining: Put aluminum ingots, magnesium ingots, zinc ingots and master alloys and other raw and auxiliary materials into a dome hydraulic tilting melting furnace for melting, fully stir and slag removal. The melting furnace insulation temperature is 720-740℃, the melting time is 7 hours; the refining temperature is 710-730℃, and the refining chlorine flow rate is 0.1-0.3Nm 3 / h, use electromagnetic stirring device to stir, remove gas and slag, let it stand for 30-45 minutes after refining before casting.

[0074] (3) Casting: Water-cooled semi-continuous casting is used for casting, with a casting temperature of 690-710°C, a casting speed of 30 mm / min, a casting cooling water temperature of 27-34°C, and a casting cooling water flow rate of 420 m 3 / h, the diameter of the obtained round ingot is 580mm, and the final car body is a round ingot with a diameter of 566mm. The length of the round ingot is 7000mm, and the ingot is cut into ingot blanks with a length of 1800mm after the head and tail waste are cut off.

[0075] (4) Ingot homogenization heat treatment: The metal insulation temperature of the ingot homogenization heat treatment is 570±10℃, and the metal insulation time is 14h.

[0076] (5) Preheating: The ingot is placed in an induction furnace for heating and insulation. The heating temperature is 520°C. A gradient heating method is adopted. The temperature of the front end of the ingot decreases every 300 mm. There are 6 heating zones with a temperature gradient of 10°C, i.e. 520°C-510°C-500°C-490°C-480°C. The insulation time is 15-20 min. The mold heating temperature is 500°C±10°C, and the extrusion barrel heating temperature is 430°C±10°C.

[0077] (6) Extrusion: The preheated ingot is placed in an extrusion barrel of an extruder for extrusion to obtain a round tube profile. The extrusion speed is 2.5 m / min, and the profile is subjected to online quenching and cooling. The inlet temperature is 525°C.

[0078] (7) Online quenching: After extrusion molding, online quenching is carried out, using strong air cooling. The total capacity of the supporting motor is 5.5KW, and the quenching air volume is 60%, which quickly cools the profile to below 50°C.

[0079] (8) Stretching: The quenched round tube profile is stretched and straightened with a stretching rate of 1.6%. During stretching, stretching pads are placed in the inner cavities of the head and tail ends of the round tube respectively.

[0080] (9) Use an automated grinder to grind the inner wall of the entire round tube.

[0081] (10) Annealing: Annealing temperature is 390±5℃, and the insulation time is 3.9h. After the insulation is completed, the furnace is air-cooled. A row of bottom-laying round tubes with an outer diameter of 130mm and a wall thickness of 20mm are placed on the bottom frame of the annealing furnace. When loading the furnace, large-diameter thin-walled aluminum alloy round tubes are evenly arranged and placed on the bottom-laying round tubes. There is a certain gap between the bottom-laying round tubes, and the gap between adjacent bottom-laying round tubes is 60mm. The bottom-laying round tubes are fixed, such as by wedges, to prevent the bottom-laying round tubes from rolling. Please refer to Figure 2During implementation, a row of bottom-laying round tubes 30 are placed on the annealing furnace chassis 20, and large-diameter thin-walled aluminum alloy round tubes 10 are evenly arranged on the bottom-laying round tubes. The large-diameter thin-walled aluminum alloy round tubes 10 and the bottom-laying round tubes 30 are evenly arranged in the same direction. The length of the bottom-laying round tubes 30 is greater than or equal to the length of the large-diameter thin-walled aluminum alloy round tubes 10. The contact mode between the two changes from "point contact" to "surface contact", which greatly increases the force-bearing area at the bottom of the aluminum alloy round tubes and avoids the aluminum alloy round tubes from being pressed. There is a certain gap between the bottom-laying round tubes, so that the furnace gas can flow freely during heating, and the heating temperature rises more evenly, avoiding the local soft band performance of the product and avoiding the generation of coarse grains.

[0082] The large-diameter thin-walled round tube prepared in this embodiment has no dent defect on the surface after annealing and the inner wall surface roughness is Ra0.5 μm, and the welding is good.

[0083] Example 4

[0084] This preferred embodiment provides a method for preparing a large diameter thin-walled Al-Mg-Si aluminum alloy round tube. Different from Example 3, in this exemplary embodiment, the large diameter thin-walled aluminum alloy round tube 10 is in the form of multiple rows of stacking, and the large diameter thin-walled aluminum alloy round tube 10 located on the upper side is stacked between the two large diameter thin-walled aluminum alloy round tubes 10 located below. The large diameter thin-walled aluminum alloy round tubes 10 are uniformly arranged in a unified direction, and the contact method is "surface contact". The multi-row stacking method improves production efficiency, and the gaps between them allow the furnace gas to flow freely during heating, and the heating temperature rises more evenly, avoiding the local soft band performance of the product and avoiding the generation of coarse grains.

[0085] Comparative Example 1

[0086] This comparative example provides a method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, and the specific steps are as follows:

[0087] (1) Ingredients: Ingredients are prepared according to the corresponding chemical compositions and mass percentages in Table 1.

[0088] (2) Melting: Put aluminum ingots, magnesium ingots, zinc ingots and master alloys and other raw and auxiliary materials into a dome hydraulic tilting melting furnace for melting, stir them thoroughly and remove slag. The melting furnace is kept at a temperature of 720-740°C and the melting time is 7 hours.

[0089] (3) Casting: Water-cooled semi-continuous casting is used for casting, with a casting temperature of 680-700°C, a casting speed of 30 mm / min, a casting cooling water temperature of 27-34°C, and a casting cooling water flow rate of 420 m 3 / h, the diameter of the obtained round ingot is 580mm, and the final car body is a round ingot with a diameter of 566mm. The length of the round ingot is 7000mm, and the ingot is cut into ingot blanks with a length of 1800mm after the head and tail waste are cut off.

[0090] (4) Ingot homogenization heat treatment: The metal insulation temperature of the ingot homogenization heat treatment is 570±10℃, and the metal insulation time is 14h.

[0091] (5) Preheating: The ingot is placed in an induction furnace for heating and insulation. The heating temperature is 480°C. A gradient heating method is adopted. The temperature of the front end of the ingot decreases every 300 mm. There are 6 heating zones with a temperature gradient of 10°C, i.e. 480°C-470°C-460°C-450°C-440°C. The insulation time is 15-20 min. The mold heating temperature is 480°C±10°C, and the extrusion barrel heating temperature is 430°C±10°C.

[0092] (6) Extrusion: The preheated ingot is placed in an extrusion barrel of an extruder for extrusion to obtain a round tube profile. The extrusion speed is 7.5 m / min, and the profile inlet temperature for online quenching and cooling is 525°C.

[0093] (7) Online quenching: After extrusion molding, online quenching is carried out, using strong air cooling. The total capacity of the supporting motor is 5.5KW, and the quenching air volume is 80%, which quickly cools the profile to below 50°C.

[0094] (8) Stretching: The quenched round tube profile is stretched and straightened with a stretching rate of 1.6%. During stretching, stretching pads are placed in the inner cavity of the head end and the tail end of the round tube.

[0095] (9) Annealing: Annealing temperature is 390±5℃, and the holding time is 3.9h. After the holding period, the furnace is air-cooled.

[0096] (10) Use an automated grinder to grind the inner wall of the entire round tube.

[0097] The large-diameter thin-walled round tube prepared in the comparative example of the present invention has multiple dent defects on the surface after annealing, all of which appear at the contact position between the round tube and the annealing furnace rack; fine cracks appear on the outer surface of the round tube, and there are multiple mold marks and "pitted" on the inner wall surface. After grinding, the roughness of the inner wall is Ra0.9μm, and the scratch line cannot be completely eliminated. There are multiple poor welds at the weld position.

[0098] In the exemplary embodiments of the present invention, Example 1, Example 2 and Example 3 all adopt the specific ingot casting process, extrusion process, heat treatment process and inner cavity grinding process of the present invention, and adopt specific auxiliary tooling during annealing. With the combination of multiple improvements, the inner cavity of the round tube has no dent defect, and the wall roughness after grinding is less than Ra0.5μm, and all welds of the round tube are well welded. Comparative Example 1 does not place a bottom round tube on the annealing furnace rack, and adopts low-temperature rapid extrusion. There are multiple dent defects on the surface, all of which appear at the contact position between the round tube and the annealing furnace rack; fine cracks appear on the outer surface of the round tube, and there are multiple mold marks and "pitted" on the inner wall surface. After grinding, the inner wall roughness is Ra0.9μm, and the wound line cannot be completely eliminated; there are multiple poor welds at the weld position.

[0099] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube, comprising the following steps: Batching, smelting, refining, casting, ingot homogenization heat treatment, preheating, extrusion, online quenching, stretching and straightening, sawing, inner wall grinding and annealing, characterized in that the heat treatment insulation temperature in the annealing process is 390±5℃, the insulation time is 3-6h, and the furnace is air-cooled after the insulation is completed; a row of bottom-laying round tubes with an outer diameter of 100-200mm and a wall thickness of 10-40mm are placed on the bottom frame of the annealing furnace, and large-diameter thin-walled round tubes are evenly arranged and placed on the bottom-laying round tubes when the furnace is loaded.

2. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: There is a certain gap between the bottom circular tubes.

3. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: The chemical composition and mass percentage of the aluminum alloy material ingredients are: Si: 0.65%-0.75%, Fe: 0.15-0.25%, Cu: ≤0.12%, Mn: 0.20%-0.35%, Mg: 0.50-0.65%, Cr: 0.12%-0.20%, Zn: ≤0.20%, Ti: ≤0.05%, Zr: ≤0.05%, and the rest are Al and unavoidable impurities, each unavoidable element is less than 0.05%, and the total amount of unavoidable elements is less than 0.10%.

4. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: During stretching, stretching pads are placed in the inner cavities of the head end and the tail end of the aluminum alloy round tube respectively, and the stretching rate is 0.5-2.5%.

5. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: During the homogenization heat treatment of the ingot, the metal holding temperature is 570±10℃, and the metal holding time is 7-14h.

6. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 5, characterized in that: The ingot is heated by electromagnetic induction gradient, the heating temperature of the ingot head is 500-520℃, and the temperature gradient decreases by 10℃ at an interval of 300mm; the mold heating temperature is 500℃±10℃, the extrusion barrel heating temperature is 430℃±10℃, the extrusion product speed is 2.5-6.0m / min, the profile online quenching cooling inlet temperature is ≥500℃, and the profile online quenching cooling outlet temperature is ≤50℃.

7. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: The outer diameter of the aluminum alloy round tube is φ300-600mm, the wall thickness of the round tube is 5-10mm, and the finished product length is 6-12m.

8. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1, characterized in that: During smelting, the ingredients are put into the smelting furnace for smelting at a temperature of 720-760°C. After smelting, the materials are refined at a temperature of 710-730°C and a refining chlorine flow rate of 0.1-0.3Nm 3 / h, use electromagnetic stirring device to stir, remove gas and slag, let it stand for 30-45 minutes after refining before casting.

9. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 1 or 8, characterized in that: Casting speed 25-40mm / min, casting temperature 690-710℃, casting cooling water temperature 28-35℃.

10. The method for preparing a large-diameter thin-walled Al-Mg-Si aluminum alloy round tube according to claim 8, characterized in that: The casting is carried out by water-cooled semi-continuous casting, with a casting temperature of 700-730°C, a casting cooling water temperature of 28-35°C, a casting stable material temperature of 690-710°C, a casting speed of 25-40mm / min, and a casting cooling water flow of 380-420m 3 / h.

Images