A heat treatment process for suppressing coarse grains in 7003 welded tubes

Abstract

This invention discloses a heat treatment process for suppressing coarse grain growth in 7003 welded tubes, belonging to the field of metal material heat treatment technology. The method first involves heating a 7003 cast ingot, prepared by semi-continuous casting, to 300℃~450℃ at a heating rate of 20℃ / h, holding it at this temperature for 3~8 hours, and then immediately quenching it in water at 25℃ to room temperature. Next, the short ingot is placed in an extrusion furnace and heated to 450~490℃, using a forward extrusion method to prepare a welded tube. Compared with traditional 7003 welded tube production processes, this invention, by increasing the holding time of the cast ingot and rapidly cooling it, ensures that the Al3Zr precipitate particle size is between 0.1-5μm, achieving the goal of suppressing coarse grain growth in the 7003 welded tube during subsequent heat treatment. Simultaneously, it ensures that the product has high tensile strength and elongation, meeting the needs of high-end users.

Description

Technical Field

[0001] This invention relates to a heat treatment process for suppressing coarse grains in 7003 slotted tubes, belonging to the field of heat treatment processing technology for metallic materials. Background Technology

[0002] 7003 aluminum alloy is a heat-treatable aluminum alloy with high specific strength and specific stiffness, good extrudability and weldability, and is now widely used in aerospace, medical device and other fields. However, after plastic deformation and heat treatment, 7003 aluminum alloy may experience abnormal grain growth, forming coarse grains. Coarse grains can significantly reduce the mechanical properties, corrosion resistance and fatigue resistance of the product, so they must be effectively controlled.

[0003] Currently, there are six main methods to suppress coarse grains in aluminum alloy extruded products: 1. Alloy composition control; 2. Homogenization heat treatment; 3. Extrusion process control; 4. Quenching process control; 5. Die structure adjustment; and 6. Cold deformation treatment. However, existing processes cannot meet customer requirements for 7003 welded tubes. Therefore, there is an urgent need to find an effective method to suppress coarse grains in 7003 welded tubes, so that the product can simultaneously possess high tensile strength and elongation to meet customer requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a heat treatment process for suppressing coarse grains in 7003 slotted tubes. By increasing the heat preservation of the cast rod and the rapid cooling process, the size of Al3Zr precipitates is kept between 0.1-5μm, thereby suppressing coarse grain growth during subsequent heat treatment.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A heat treatment process for suppressing coarse grains in 7003 welded tubes includes the following steps:

[0007] (1) Preparation of 7003 as-cast short bars by semi-continuous casting method;

[0008] (2) The cast short bar is subjected to low-temperature heating and then quenching treatment;

[0009] (3) After the seam tube is extruded, it is immediately cooled by strong air and then left to stand for 72~100 hours before undergoing artificial two-stage aging treatment.

[0010] Preferably, the specific process of step (1) is as follows: the melting temperature is 720℃~780℃, and the casting speed is 1500~2300L / min.

[0011] Preferably, the specific process of step (2) is as follows: the 7003 cast short rod is heated to 350~440℃ at a heating rate of 20℃ / h, held for 3~8h, and then immediately quenched in water at 25℃ to room temperature at a cooling rate of 100℃~300℃. At this time, the size range of Al3Zr phase particles is between 0.1-5μm.

[0012] Preferably, the specific process of step (3) is as follows: the short rod is placed in the extrusion furnace and heated to 450~490℃. After the seam tube is extruded in the forward direction, it is immediately cooled by strong air and then placed for 72~100h. Finally, artificial two-stage aging strengthening is carried out.

[0013] The mass fraction of the 7003 alloy composition mentioned above is: Zn: 5.5-6.0%, Mg: 0.5-0.6%, Cu: 0.01-0.08%, Zr: 0.12-0.18%, with other impurities not exceeding 0.15%, and the balance being Al.

[0014] 7003 aluminum alloy contains Zr. During heat treatment at 350~440℃ with a heating rate of 20℃ / h, Zr precipitates from the aluminum matrix, forming Al3Zr phase particles with the highest density and uniform distribution. To control the Al3Zr particle size, rapid cooling is achieved through water quenching. This rapid cooling rate helps suppress Al3Zr phase growth, keeping the particle size between 0.1-5μm. This prevents recrystallization in the 7003 shock absorber tube during subsequent heat treatment and also improves the tensile strength and elongation of the 7003 welded tube.

[0015] The beneficial effects of this invention are as follows:

[0016] The heat treatment process employed in this invention effectively suppresses coarse grains in 7003 slotted tubes and improves their overall performance. By increasing the holding time of the cast rod and employing rapid cooling, this invention controls the distribution, quantity, and size of Al3Zr particles, ensuring a uniform distribution and a size range of 0.1-5 μm. Al3Zr particles, through heterogeneous nucleation sites, hinder grain growth, refine the grain structure, and contribute to improving the mechanical properties of 7003 slotted tubes. Attached Figure Description

[0017] Figure 1 The particle size of Al3Zr in the cast rod after low-temperature heating and quenching treatment is: a. 100 times the size of Al3Zr particles in the cast rod after low-temperature heating and quenching treatment; b. 200 times the size of Al3Zr particles in the cast rod after low-temperature heating and quenching treatment.

[0018] Figure 2 It contains 100 times more Al3Zr particles than sample 3;

[0019] Figure 3The image shows the microstructure of the sutured tube in Sample 1 of Example 1.

[0020] Figure 4 This is a macroscopic image of Sample 1 from Example 1 after the seamed tube was bent 90° and then alkali-etched.

[0021] Figure 5 The image shows the microstructure of the sutured tube in sample 3 of Comparative Example 1.

[0022] Figure 6 The image shows a macroscopic picture of the seamed tube of Sample 3 in Comparative Example 1 after it was bent 90° and then alkali-corroded.

[0023] Figure 7 Microscopic images of the sutured tube in sample 5 of Comparative Example 2;

[0024] Figure 8 This is a macroscopic image of sample 5 from Comparative Example 2, showing the seamed tube bent at 90° and then subjected to alkali corrosion. Detailed Implementation

[0025] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0026] The present invention tests the tensile strength, yield strength, and elongation of each embodiment and comparative example. The test method for tensile strength is GB / T228.1-2021; the test method for yield strength is GB / T228.1-2021; and the test method for elongation is GB / T228.1-2021.

[0027] The raw material used in the following examples is 7003 aluminum alloy, and the mass percentage of this alloy is:

[0028] Si: 0.08%, Fe: 0.12%, Cu: 0.06%, Mg: 0.65%, Zn: 5.6%, Zr: 0.16%, Ti: 0.07%, balance Al.

[0029] Example 1

[0030] A: Semi-continuous casting method for preparing 7003 as-cast long bars: The melting temperature was 750℃, the casting speed was 1800L / min, the long bars were cut into 270mm short bars, placed in a homogenizing furnace and heated to 430℃ at a heating rate of 20℃ / h, held for 5h, and then rapidly placed in a liquid quenching tank for cooling at a cooling rate of 250℃ / h; the Al3Zr particle size at 100 times and 200 times magnification within the cast bars was measured, see... Figure 1 a represents 100 times the size of Al3Zr particles, with a size of 0.6 μm; b represents 200 times the size of Al3Zr particles, with a size of 0.31 μm.

[0031] B: Place the short bar into the matching heating furnace of the direct extrusion press A, with a heating rate of 100℃ / h, a heating temperature of 470℃, and a heating time of 12h. Then take out the short bar and extrude it into a welded tube with an outer diameter of 19mm and a wall thickness of 1.5mm. The extrusion speed is 1.8mm / s and the extrusion ratio is 98.31.

[0032] C: After extrusion, the seamless tube is naturally cooled by strong wind. After being placed at room temperature for 72 hours, it is first heated to 80°C and kept at that temperature for 5 hours. Then, it is heated to 150°C and kept at that temperature for 6 hours before being taken out and allowed to cool naturally.

[0033] D: After two-stage artificial aging, two 10cm lengths of the seamed tube were cut, designated as Sample 1 and Sample 2. The microstructure was observed and the mechanical properties were tested. Testing revealed that Sample 1 had no coarse grains, and its microstructure was as shown in the diagram. Figure 3 Sample 1 has a tensile strength of 352 MPa, a yield strength of 307 MPa, and an elongation of 16.5%. Sample 2 has no coarse grains; its tensile strength is 353 MPa, its yield strength is 308 MPa, and its elongation is 16.5%.

[0034] E: After double-stage artificial aging, bend the seamless tube 90°, immerse it in a 14% sodium hydroxide solution for 2 minutes, then place it in clean water for 1 minute, then in a neutralization tank for 2 minutes, and finally rinse it in clean water. Observe that there are no coarse grains at the bend. Figure 4 .

[0035] Comparative Example 1

[0036] A: Cut a long rod with the same furnace number as in Example 1 into short rods of 270mm;

[0037] B: The short bar was placed in the same extruder A heating furnace as in Example 1. The heating temperature was 470°C, the heating rate was 20°C / h, and the heating time was 12h. Then the short bar was taken out and extruded into a slotted tube with an outer diameter of 19mm and a wall thickness of 1.5mm. The extrusion speed was 1.8mm / s and the extrusion ratio was 98.31.

[0038] C: After extrusion, the seamless tube is naturally cooled by strong wind. After 72 hours, it is first heated to 80°C and kept at that temperature for 5 hours. Then, it is heated to 150°C and kept at that temperature for 6 hours before being taken out and allowed to cool naturally.

[0039] D: After artificial aging, cut two 10cm lengths of the seamed tube, designated as Sample 3 and Sample 4, and observe the microstructure and test the mechanical properties. Measure 100 times the number of Al3Zr particles in Sample 3, see... Figure 2 Testing revealed that sample 3 had a coarse grain size of 12 μm, and its microstructure was as follows: Figure 5Sample 3 has a tensile strength of 327 MPa, a yield strength of 280 MPa, and an elongation of 17%. Sample 4 has a coarse grain size of 10 μm, a tensile strength of 332 MPa, a yield strength of 279 MPa, and an elongation of 17.5%.

[0040] E: After double-stage artificial aging, bend the seamless tube 90°, immerse it in a 14% sodium hydroxide solution for 2 minutes, then soak it in clean water for 1 minute, then in a neutralization tank for 2 minutes, and finally rinse it in clean water. Directly observe the bend; if coarse crystals are present, such as... Figure 6 .

[0041] Comparative Example 2

[0042] A: Semi-continuous casting method for preparing 7003 as-cast long bars: The melting temperature is 750℃, the casting speed is 1800L / min, the long bars are cut into 270mm short bars, placed in a homogenizing furnace and heated to 430℃ at a heating rate of 40℃ / h, held for 5h and then quickly placed in a liquid quenching tank for cooling at a cooling rate of 150℃ / h.

[0043] B: Place the short bar into the matching heating furnace of the direct extrusion press A, with a heating rate of 100℃ / h, a heating temperature of 470℃, and a heating time of 12h. Then take out the short bar and extrude it into a welded tube with an outer diameter of 19mm and a wall thickness of 1.5mm. The extrusion speed is 1.8mm / s and the extrusion ratio is 98.31.

[0044] C: After extrusion, the seamless tube is naturally cooled by strong wind. After being placed at room temperature for 72 hours, it is first heated to 80°C and kept at that temperature for 5 hours. Then, it is heated to 150°C and kept at that temperature for 6 hours before being taken out and allowed to cool naturally.

[0045] D: Two 10cm lengths of the seamed tube, after double-stage artificial aging, were cut and their microstructure and mechanical properties were observed. Testing revealed that sample 5 contained coarse grains. Figure 7 Sample 5 has a tensile strength of 335 MPa, a yield strength of 296 MPa, and an elongation of 16%. Sample 6 has coarse grains, and its tensile strength is 332 MPa, yield strength is 294 MPa, and elongation is 16%.

[0046] E: After double-stage artificial aging, bend the seamless tube 90°, immerse it in a 14% sodium hydroxide solution for 2 minutes, then soak it in clean water for 1 minute, then in a neutralization tank for 2 minutes, and finally rinse it in clean water. Observe that there are coarse crystals at the bend. Figure 8 .

Claims

1. A heat treatment process for inhibiting coarse grain in 7003 seamless pipe, characterized by, The method comprises the following steps: (1) preparing 7003 as-cast short rods in a semi-continuous casting mode; (2) performing low-temperature heating and quenching treatment on the as-cast short rods: heating the 7003 as-cast short rods to 350-440 DEG C at a heating rate of 20 DEG C / h, and then immediately quenching to room temperature in water at 25 DEG C after holding for 3-8 h, with a cooling rate of 100 DEG C-300 DEG C; (3) placing the short rods into an extrusion furnace and heating to 450-490 DEG C, extruding a slotted pipe in a forward direction, immediately air cooling, and then placing for 72-100 h, and manually performing double-stage aging strengthening; The manual double-stage aging strengthening is specifically: heating to 80 DEG C, holding for 5 h, then heating to 150 DEG C, holding for 6 h, and then taking out and naturally cooling; The mass fraction of the 7003 alloy is: Zn 5.5-6.0%, Mg 0.5-0.6%, Cu 0.01-0.08%, Zr 0.12-0.18%, total amount of other impurities is not more than 0.15%, and the balance is Al.

2. The heat treatment process for inhibiting coarse grain of 7003 seamless tube according to claim 1, characterized in that, The specific process of step (1) semi-continuous casting is as follows: the smelting temperature is 720 DEG C, and the casting speed is 1500-2300 L / min.

3. The heat treatment process for inhibiting coarse grain of 7003 seamless tube according to claim 1, characterized in that, In step (2), the size of the Al3Zr phase particles after quenching ranges from 0.1-5 um.

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