A Post-Hot-Rolling Temperature Control Method for Improving the Product of Strength and Plasticity of Metastable β Titanium Alloy Sheets

Abstract

A method for controlling the temperature after hot rolling to improve the strength-ductility product of metastable β titanium alloy sheets belongs to the technical field of alloy sheet preparation. The alloy sheets of the present invention have the β phase in a metastable state and suitable stability, and the precipitation of secondary α phase can be achieved through the temperature control process after hot rolling. The process of the temperature control method after hot rolling of the present invention is as follows: hot roll the alloy ingot to obtain alloy sheets; cool the alloy sheets, and then slowly cool the alloy sheets; pass the alloy sheets through a heat treatment furnace, cool the alloy sheets again through the heat treatment furnace, heat the alloy sheets, and finally water quench to room temperature. The temperature control method of the present invention obtains dispersed and fine intragranular secondary α phase and discontinuous grain boundary α phase, and while providing the strength of the alloy, reduces the plastic loss as much as possible, thereby improving the strength-ductility product of the alloy. The present invention can be extended to the production of metastable β titanium alloy sheets to optimize the strength-ductility matching of metastable β titanium alloys.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloy sheet preparation, and particularly relates to a post-rolling temperature control method for improving the strength-plasticity product of metastable β titanium alloy sheets. Background Art

[0002] Metastable β titanium alloys have the advantages of high strength and low density, and are ideal materials for manufacturing aerospace structural components. Among them, metastable β titanium alloy sheets are often used to manufacture the skins and outer plates of aerospace aircraft and need to be hot-rolled to form. With the increasing requirements for weight reduction, energy consumption reduction and flight performance of aerospace aircraft, metastable β titanium alloy sheets must continue to develop in the direction of high strength and high plasticity. The strength-plasticity product (the product of tensile strength and elongation at break) is an important index to evaluate whether the strength and plasticity matching of the alloy is good. A higher strength-plasticity product proves that the alloy has both high strength and high plasticity at the same time. The main method to strengthen metastable β titanium alloy sheets is heat treatment, that is, during the heat treatment process, part of the metastable β phase transforms into the α phase, and secondary α phase precipitates on the β matrix to achieve strengthening. However, due to problems such as coarsening of the precipitated secondary α phase and weakening of the grain boundaries by continuous grain boundary precipitates, while strengthening the matrix, there is often a large loss of plasticity. For example, after solution + bipolar aging treatment, the industrial grade TB8 metastable β titanium alloy can reach a tensile strength of more than 1500 MPa, but the elongation after fracture is only less than 4%, and the strength-plasticity product is only 6 GPa·%; after cold rolling + aging treatment, the industrial grade TB3 metastable β titanium alloy sheet can reach a tensile strength of 1500 MPa, but the fracture elongation is only less than 3%, and the strength-plasticity product is only 4.5 GPa·%.

[0003] The Ti-Mo-V-Al-Fe alloy is a metastable β titanium alloy, whose composition is based on Ti element, and α stabilizing element Al and β stabilizing elements Mo, V and Fe are added. The specific components and mass percentages are: Mo 6.3% - 6.8%, V 5.9% - 6.4%, Al 3.7% - 4.2%, Fe 0.9% - 1.3%, C ≤ 0.03%, H ≤ 0.005%, O ≤ 0.08%, N ≤ 0.015%, and the balance is Ti. This alloy sheet also has the problem of low strength-plasticity product after conventional heat treatment, which limits its practical application. Summary of the Invention

[0004] Aiming at the deficiencies in the prior art, the present invention proposes a post-rolling temperature control method for improving the strength-plasticity product of metastable β titanium alloy sheets. This temperature control method replaces the conventional solution aging heat treatment and improves the strength-plasticity product of the alloy sheet.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A titanium alloy sheet, which is a Ti-Mo-V-Al-Fe alloy sheet, and the components and their mass percentages are as follows: Mo 6.3% - 6.8%, V 5.9% - 6.4%, Al 3.7% - 4.2%, Fe 0.9% - 1.3%, C ≤ 0.03%, H ≤ 0.005%, O ≤ 0.08%, N ≤ 0.015%, and the balance is Ti;

[0007] The Mo is added in the form of Mo-Al alloy, the V is added in the form of V-Al alloy, the insufficient part of Al in the Mo-Al alloy and V-Al alloy is added in the form of high-purity aluminum, and the Fe is added in the form of pure iron;

[0008] According to the Ti-Mo-V-Al-Fe alloy composition for batching, and through three times of vacuum consumable melting, an alloy ingot is obtained.

[0009] A post-rolling temperature control method for improving the strength-ductility product of metastable β titanium alloy sheets, comprising the following steps:

[0010] (1) Hot-rolling the alloy ingot to obtain an alloy sheet;

[0011] (2) Using the laminar flow cooling device behind the hot rolling mill to cool the alloy sheet obtained in step (1);

[0012] (3) Using the heat preservation cover behind the hot rolling mill to slowly cool the alloy sheet in step (2);

[0013] (4) Using a heat treatment furnace to cool the alloy sheet;

[0014] (5) Using a heat treatment furnace to heat the alloy sheet;

[0015] (6) Immediately water-quenching the alloy sheet to room temperature.

[0016] The hot rolling temperature in step (1) is 810 - 840 °C.

[0017] The cooling temperature in step (2) is 590 - 610 °C, and the cooling rate is 28 - 32 °C / s;

[0018] The slow cooling in step (3) is to cool to 410 °C - 430 °C, and the cooling rate is 2.5 - 3.5 °C / min;

[0019] The heat treatment furnace used in step (4) should be preheated to 400 - 420 °C in advance. After the alloy sheet enters the furnace, immediately turn off the heating device to ensure that the alloy sheet cools with the furnace; the cooling with the furnace is to cool to 190 °C - 210 °C, and the cooling rate is 70 - 90 °C / h;

[0020] In step (4), the transfer time from the heat preservation cover after the hot rolling mill to the heat treatment furnace should be less than 3 min;

[0021] In step (5), the heating temperature is 590 - 610 °C and the heating rate is 10 - 15 °C / min;

[0022] Advantages of the present invention:

[0023] For the Ti-Mo-V-Al-Fe alloy sheet of the present invention, appropriate mass fractions of β-stabilizing elements Mo, V, Fe and α-stabilizing element Al are added, so that the β-phase is in a metastable state and has appropriate stability, and the precipitation of secondary α-phase can be achieved through the temperature control process after hot rolling. In the post-hot rolling temperature control method of the present invention, in step (2), the metastable β-phase is retained to a temperature suitable for the precipitation of secondary α-phase by rapid cooling; in step (3), through slow cooling, the secondary α-phase is fully nucleated in the temperature range suitable for nucleation, thus providing a basis for the formation of a dispersion-distributed precipitation phase; in step (4), the furnace cooling method is used, which not only provides a driving force for the growth of the nucleated secondary α-phase, but also restricts its excessive coarsening, avoiding the disadvantage that the secondary α-phase is prone to coarsening during the conventional heat treatment (isothermal aging treatment) process; in step (5), by reheating, the secondary α-phase precipitated at the grain boundaries is partially redissolved, making the grain boundary α-phase have a discontinuous morphology, thus avoiding the disadvantage that a continuous grain boundary α-phase is easily formed during the conventional heat treatment (isothermal aging treatment) process and weakening the grain boundaries, and then the regulated structure is retained to room temperature by water quenching. The present invention obtains a dispersion of fine intragranular secondary α-phase and discontinuous grain boundary α-phase, while providing the alloy strength, minimizing the loss of plasticity, thus improving the strength-plasticity product of the alloy. The present invention can be extended to the production of metastable β titanium alloy sheets to optimize the strength-plasticity matching of metastable β titanium alloys. Description of the drawings

[0024] Figure 1 It is a transmission electron microscope image of the intragranular secondary α-phase of the Ti-Mo-V-Al-Fe alloy sheet prepared in Example 1 of the present invention;

[0025] Figure 2 It is a transmission electron microscope image of the grain boundary α-phase of the Ti-Mo-V-Al-Fe alloy sheet prepared in Example 1 of the present invention;

[0026] Figure 3 It is the room temperature tensile curve of the Ti-Mo-V-Al-Fe alloy sheet prepared in Example 1 of the present invention;

[0027] Figure 4 It is a transmission electron microscope image of the grain boundary α-phase of the Ti-Mo-V-Al-Fe alloy sheet prepared in Comparative Example 1-1 of the present invention;

[0028] Figure 5Room temperature tensile curve of the Ti-Mo-V-Al-Fe alloy sheet prepared in Comparative Example 1-1 of the present invention;

[0029] Figure 6 Transmission electron microscope image of the intragranular secondary α phase of the Ti-Mo-V-Al-Fe alloy sheet prepared in Comparative Example 1-2 of the present invention;

[0030] Figure 7 Room temperature tensile curve of the Ti-Mo-V-Al-Fe alloy sheet prepared in Comparative Example 1-2 of the present invention. Detailed description of the specific implementation mode

[0031] The present invention will be further described in detail below in conjunction with the embodiments.

[0032] In the embodiment of the present invention, the Ti-Mo-V-Al-Fe alloy includes components and mass percentages: Mo 6.5%, V 6.1%, Al 4.0%, Fe 1.1%, C ≤ 0.03%, H ≤ 0.005%, O ≤ 0.08%, N ≤ 0.015%, and the balance is Ti.

[0033] Mo is added in the form of Mo-Al alloy, V is added in the form of V-Al alloy, the insufficient part of Al in the Mo-Al alloy and V-Al alloy is added in the form of high-purity aluminum, and Fe is added in the form of pure iron.

[0034] Example 1

[0035] A method for controlling the temperature after hot rolling of the Ti-Mo-V-Al-Fe alloy sheet, comprising the following steps:

[0036] (1) According to the composition of the Ti-Mo-V-Al-Fe alloy, a circular ingot with a diameter of 100 mm and a mass of 20 kg was prepared by three times of vacuum consumable melting. After hot rolling, the ingot became an alloy sheet with a thickness of 30 mm and a width of 150 mm. At the end of hot rolling, the temperature of the alloy sheet was 835 °C;

[0037] (2) Using the laminar flow cooling device after the hot rolling mill, the alloy sheet was quickly cooled to 608 °C, and the cooling rate was 32 °C / s;

[0038] (3) The alloy sheet was sent into the heat preservation cover after hot rolling through the roller table and cooled to 426 °C in the heat preservation cover, and the cooling rate was 3.3 °C / min;

[0039] (4) The alloy sheet was quickly transferred to a heat treatment furnace preheated to 420 °C, and then the heating device of the heat treatment furnace was immediately turned off and the furnace door was closed, so that the alloy sheet was cooled with the furnace to 206 °C, and the cooling rate was 85 °C / h;

[0040] (5) Turn on the heating device of the heat treatment furnace, set the heating rate to 15 °C / min, and heat the alloy sheet to 610 °C;

[0041] (6) Quickly take out the alloy sheet from the heat treatment furnace and put it into the quenching tank, and water quench it to room temperature.

[0042] As Figures 1-3 shown, secondary α-phase has precipitated in the grains of the Ti-Mo-V-Al-Fe alloy sheet of this embodiment. The secondary α-phase is needle-shaped, showing a dispersed distribution, and has a small width, 70 nm to 90 nm, greatly improving the strength of the alloy. The secondary α-phase precipitated at the grain boundaries has a discontinuous morphology, improving the plasticity of the alloy; the tensile strength of the alloy sheet is 1411 MPa, the elongation after fracture is 12%, and the strength-ductility product is 16.9 GPa·%.

[0043] Example 2

[0044] A post-rolling temperature control method for improving the strength-ductility product of Ti-Mo-V-Al-Fe alloy sheets, comprising the following steps:

[0045] (1) Prepared a round ingot with a diameter of 100 mm and a mass of 20 kg according to the Ti-Mo-V-Al-Fe alloy composition by three times of vacuum consumable melting. After hot rolling, the ingot became an alloy sheet with a thickness of 30 mm and a width of 150 mm. At the end of hot rolling, the temperature of the alloy sheet was 821 °C;

[0046] (2) Use the laminar flow cooling device behind the hot rolling mill to quickly cool the alloy sheet to 592 °C, where the cooling rate is 29 °C / s;

[0047] (3) Send the alloy sheet into the post-rolling insulation cover through the roller table, and cool it to 415 °C in the insulation cover, where the cooling rate is 2.2 °C / min;

[0048] (4) Quickly transfer the alloy sheet to a heat treatment furnace preheated to 420 °C, then immediately turn off the heating device of the heat treatment furnace and close the furnace door, and let the alloy sheet cool with the furnace to 191 °C, where the cooling rate is 72 °C / h;

[0049] (5) Turn on the heating device of the heat treatment furnace, set the heating rate to 15 °C / min, and heat the alloy sheet to 590 °C;

[0050] (6) Quickly take out the alloy sheet from the heat treatment furnace and put it into the quenching tank, and water quench it to room temperature.

[0051] Secondary α-phase precipitates within the grains of the Ti-Mo-V-Al-Fe alloy sheet. The secondary α-phase is needle-shaped, shows a dispersed distribution and has a small width. The secondary α-phase precipitated at the grain boundaries shows a discontinuous morphology. The tensile strength of the alloy sheet is 1423 MPa, the elongation after fracture is 11.2%, and the product of strength and plasticity is 15.9 GPa·%.

[0052] Example 3

[0053] A post-rolling temperature control method for improving the product of strength and plasticity of a Ti-Mo-V-Al-Fe alloy sheet, comprising the following steps:

[0054] (1) Ingredients were prepared according to the Ti-Mo-V-Al-Fe alloy composition and a round ingot with a diameter of 100 mm and a mass of 20 kg was prepared by three times of vacuum consumable melting. After hot rolling, the ingot became an alloy sheet with a thickness of 30 mm and a width of 150 mm. At the end of hot rolling, the temperature of the alloy sheet was 829 °C;

[0055] (2) Using the laminar flow cooling device behind the hot rolling mill, the alloy sheet was rapidly cooled to 602 °C, and the cooling rate was 31 °C / s;

[0056] (3) The alloy sheet was sent into the heat preservation cover after hot rolling through the roller table and cooled to 422 °C in the heat preservation cover, and the cooling rate was 2.9 °C / min;

[0057] (4) The alloy sheet was quickly transferred to a heat treatment furnace preheated to 420 °C, then the heating device of the heat treatment furnace was immediately turned off and the furnace door was closed, and the alloy sheet was cooled in the furnace to 198 °C, and the cooling rate was 81 °C / h;

[0058] (5) The heating device of the heat treatment furnace was turned on, the heating rate was set to 15 °C / min, and the alloy sheet was heated to 600 °C;

[0059] (6) The alloy sheet was quickly taken out of the heat treatment furnace and put into a quenching tank, and quenched in water to room temperature.

[0060] Secondary α-phase precipitates within the grains of the Ti-Mo-V-Al-Fe alloy sheet. The secondary α-phase is needle-shaped, shows a dispersed distribution and has a small width. The secondary α-phase precipitated at the grain boundaries shows a discontinuous morphology. The tensile strength of the alloy sheet is 1389 MPa, the elongation after fracture is 12.4%, and the product of strength and plasticity is 17.2 GPa·%.

[0061] Comparative Example 1-1

[0062] The Ti-Mo-V-Al-Fe alloy components and their mass percentages, melting, and hot rolling are the same as in Example 1, except that after hot rolling, the alloy sheet is air-cooled to room temperature and then subjected to isothermal aging treatment at a temperature of 400 °C for 6 h using a heat treatment furnace. As Figures 4-5 shown, after testing, compared with Example 1, the grain boundary α phase of the prepared Ti-Mo-V-Al-Fe alloy sheet is relatively wide and continuously distributed at the grain boundaries, seriously damaging the plasticity of the alloy. The tensile strength of the prepared Ti-Mo-V-Al-Fe alloy sheet is 1233 MPa, the elongation after fracture is 4.2%, and the product of strength and plasticity is 5.2 GPa·%.

[0063] Comparative Example 1-2

[0064] The Ti-Mo-V-Al-Fe alloy components and their mass percentages, melting, and hot rolling are the same as in Example 1, except that after hot rolling, the alloy sheet is air-cooled to room temperature and then subjected to isothermal aging treatment at a temperature of 600 °C for 6 h using a heat treatment furnace. As Figures 6-7 shown, after testing, compared with Example 1, the intragranular secondary α phase of the prepared Ti-Mo-V-Al-Fe alloy sheet is relatively coarse, with a width of 140 nm to 160 nm, restricting the improvement of the alloy strength. The tensile strength of the prepared Ti-Mo-V-Al-Fe alloy sheet is 1359 MPa, the elongation after fracture is 7.1%, and the product of strength and plasticity is 9.6 GPa·%.

Claims

1. A post-rolling temperature control method for improving the strength-ductility product of metastable β titanium alloy plates, characterized in that, it includes the following steps: (1) Hot-roll the alloy ingot to obtain alloy plates. The alloy components and their mass percentages in the alloy ingot are as follows: Mo 6.3% - 6.8%, V 5.9% - 6.4%, Al 3.7% - 4.2%, Fe 0.9% - 1.3%, C ≤ 0.03%, H ≤ 0.005%, O ≤ 0.08%, N ≤ 0.015%, and the balance is Ti; the Mo is added in the form of Mo-Al alloy, the V is added in the form of V-Al alloy, the insufficient part of the total addition amount of Al in the Mo-Al alloy and Al in the V-Al alloy is added in the form of high-purity aluminum, and the Fe is added in the form of pure iron; (2) Use the laminar flow cooling device after the hot rolling mill to cool the alloy plates obtained in step (1). The cooling temperature is 590 - 610 °C, and the cooling rate is 28 - 32 °C / s; (3) Use the heat preservation cover after the hot rolling mill to slowly cool the alloy plates in step (2) to 410 °C - 430 °C, and the cooling rate is 2.5 - 3.5 °C / min; (4) Use the heat treatment furnace to cool the alloy plates. The heat treatment furnace used should be preheated to 400 - 420 °C in advance. After the alloy plates are put into the furnace, immediately turn off the heating device to ensure that the alloy plates cool with the furnace; the cooling with the furnace is to cool to 190 °C - 210 °C, and the cooling rate is 70 - 90 °C / h; (5) Use the heat treatment furnace to heat the alloy plates. The heating temperature is 590 - 610 °C, and the heating rate is 10 - 15 °C / min; (6) Immediately water-quench the alloy plates to room temperature.

2. The post-rolling temperature control method for improving the strength-ductility product of metastable β titanium alloy plates according to claim 1, characterized in that, the alloy ingot in step (1) is an alloy ingot obtained by vacuum consumable melting.

3. The post-rolling temperature control method for improving the strength-ductility product of metastable β titanium alloy plates according to claim 1, characterized in that, the hot rolling temperature for hot rolling in step (1) is 810 - 840 °C.

4. The post-rolling temperature control method for improving the strength-ductility product of metastable β titanium alloy plates according to claim 1, characterized in that, in step (4), the transfer time from the heat preservation cover after the hot rolling mill to the heat treatment furnace should be less than 3 min.

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