Heat treatment process for regulating and controlling uniform precipitation of carbides in carbon microalloying TC4 titanium alloy plate blank

Abstract

The invention belongs to the technical field of metal material heat treatment, and particularly relates to a heat treatment method for regulating and controlling dispersed precipitation of carbon microalloyed TC4 titanium alloy carbide. And through combination of beta-phase region solution treatment and two-stage aging, a high-density uniformly-distributed carbide reinforced phase is separated out in a titanium alloy matrix, and the comprehensive mechanical property is remarkably improved. Compared with two-phase region solid solution, beta-phase region solid solution eliminates the alpha-phase pinning effect, carbon atom diffusion activation energy is increased, and synchronous nucleation of carbides in the crystal / crystal boundary is promoted; and the Ostwald curing process is regulated and controlled through two-stage aging, and a high-density dispersion strengthening phase is formed. The tensile strength of the alloy treated through the technology is larger than or equal to 1200 MPa, the ductility is kept at 15%, and high-toughness matching is achieved. Air cooling is adopted, the unit energy consumption is reduced by 30% compared with quenching, plate microcracks and deformation caused by quenching stress are avoided, and the method is suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of titanium alloy heat treatment, and particularly relates to a heat treatment process for regulating uniform precipitation of carbides in a carbon micro-alloyed TC4 titanium alloy slab, wherein the uniform dispersion precipitation of carbides is realized through multi-stage temperature control heat treatment and phase transformation structure regulation. BACKGROUND

[0002] As a typical alpha + beta dual-phase titanium alloy, TC4 (Ti-6Al-4V) titanium alloy has been widely used in aerospace, weapon equipment and medical implantation fields due to its excellent specific strength characteristics, good corrosion resistance and biocompatibility. However, with the rapid development and iterative upgrading of the advanced manufacturing field, the mechanical properties of the traditional TC4 titanium alloy are difficult to meet the stringent requirements of the service performance of structural materials in strength and toughness. Therefore, improving the mechanical properties of the TC4 titanium alloy has important engineering significance for maintaining the advanced performance of titanium alloys, broadening the application fields of titanium alloys and promoting the upgrading of related industries.

[0003] Carbon micro-alloying is considered to be one of the effective ways to improve the mechanical properties of TC4 titanium alloy. Through the solid solution strengthening effect of carbon atoms and the precipitation strengthening effect of carbides, the simultaneous optimization of the mechanical properties of titanium alloy can be realized. However, the low solubility and high diffusion barrier of carbon atoms in the titanium matrix easily lead to the non-uniform precipitation of carbides at the grain boundaries or local areas, forming coarse particles and causing stress concentration, which causes the deterioration of material performance. However, the existing patents and documents show that only through simple ordinary annealing, solid solution aging treatment and other conventional heat treatment processes, it is difficult to balance the intrinsic contradiction between the diffusion rate of carbon atoms and the nucleation density, which easily causes the uneven distribution of carbide precipitation phase, and cannot solve the technical problem of carbide precipitation in the titanium alloy matrix, thereby affecting the synergistic strengthening of the strength and toughness of the TC4 titanium alloy. Therefore, it is the key to realize the strengthening and toughening of TC4 alloy to precisely regulate the carbon diffusion kinetics and precipitation phase nucleation behavior by using a new type of heat treatment system to regulate the uniform precipitation of carbides in the titanium matrix.

[0004] In addition, the heat treatment system of titanium alloy in laboratory research usually needs to be carefully designed to maximize the performance of the material, and in industrial production, there are often multiple contradictions and challenges in production efficiency, production cost, product precision and stability. In particular, high-cost heat treatment equipment and methods are often used in laboratory conditions to carry out organization regulation and performance optimization, while in industrial production, especially in large-scale structural manufacturing process, simple heat treatment methods are often used to control costs and maximize benefits. For example, the water quenching, furnace cooling or oil cooling methods commonly used in the laboratory are not suitable for large-scale production in enterprises, and the traditional quenching process consumes a lot of energy and easily causes deformation of the plate.

[0005] Based on this, it is urgent to develop a new heat treatment process that takes into account the balance between strengthening and toughening of titanium alloys and industrial feasibility. By precisely controlling the carbon diffusion kinetics and the nucleation barrier of the precipitate phase, the nanoscale uniform dispersion distribution of carbides can be achieved, thus solving the technical bottlenecks of traditional processes in energy consumption, organizational uniformity and deformation control. Summary of the Invention

[0006] The purpose of the present invention is to provide a multi-stage heat treatment process for regulating the uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab. This method maximizes the elimination of the α-phase pinning effect and increases the diffusion activation energy of carbon atoms through solid solution treatment above the phase transformation point. Through the multi-stage heat treatment process design, the simultaneous nucleation of carbides within the grains and at the grain boundaries in the subsequent aging stage is promoted, thereby achieving uniform dispersion distribution of carbides in the titanium alloy matrix, significantly improving its strength, while maintaining good plasticity, and achieving high strength and toughness matching of the TC4 alloy.

[0007] The technical solution provided by the present invention comprises the following steps:

[0008] (1) Step 1: Preheating the carbon microalloyed TC4 titanium alloy slab to 400-600°C for preheating;

[0009] (2) Step 2: high temperature solution treatment, the preheated carbon microalloyed TC4 titanium alloy slab is continuously heated to 20 to 100°C above the β phase transformation point for solution treatment, and then air-cooled to room temperature;

[0010] (3) Step 3: Primary aging treatment, subjecting the solution-treated carbon microalloyed TC4 titanium alloy slab to a primary aging treatment at 200-300°C, followed by air cooling to room temperature;

[0011] (4) Step 4: Secondary aging treatment, subjecting the carbon microalloyed titanium alloy slab that has undergone primary aging treatment to secondary aging treatment at 500-600° C., and then air cooling to room temperature;

[0012] Preferably, the preheating time in step 1 is: t1=20+φ×d (d: thickness, unit: mm; φ=1-2, unit: min / mm; time unit: min.)

[0013] Preferably, the duration of the high-temperature solution treatment in step 2 is: t2=40+λ×d (d: thickness, unit: mm; λ=1.5-3, unit: min / mm; time unit: min).

[0014] Preferably, the duration of the primary aging treatment in step 3 is: t3 = 2 + μ × d (d: thickness, unit: mm; μ = 0.1-0.3, unit: h / mm; time unit: h).

[0015] Preferably, the length of the secondary aging treatment in step four is: t4 = 1 + ω x d (d: thickness, unit: mm; ω = 0.05-0.1, unit: h / mm; the time unit is h.)

[0016] Compared with the prior art, the application has the following beneficial effects:

[0017] (1) The application breaks through the technical bottleneck that carbides are easy to segregate in the titanium matrix by synergistically regulating the phase transition of the titanium alloy and the carbon atom diffusion activation energy through heat treatment, and better solves the technical problem of the increase of the brittleness of the titanium alloy caused by the uneven precipitation of carbides in the matrix. The related technical achievements can be expanded to various carbon micro-alloyed titanium alloys (such as near-alpha type and alpha + beta type), and are compatible with industrial production equipment.

[0018] (2) In the application, the heat treatment process is simple, the cooling method is air cooling, which is more suitable for industrial production compared with other cooling methods, has low energy consumption, and has strong market competitiveness.

[0019] (3) In the application, the alloying elements of the material are simple, the cost is low compared with other dual-phase titanium alloys, only a trace amount of carbon element is added on the basis of TC4 titanium alloy, but the performance is greatly improved, especially the excellent strength and plasticity matching. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is the OM microstructure diagram of Example 1;

[0021] Figure 2 is the OM microstructure diagram of Example 2;

[0022] Figure 3 is the OM microstructure diagram of Comparative Example 1;

[0023] Figure 4 is the OM microstructure diagram of Comparative Example 2;

[0024] Figure 5 is the tensile property curve diagram of the examples and comparative examples;

[0025] Figure 6 is the flow chart of the heat treatment process. DETAILED DESCRIPTION

[0026] The technical solutions in the application will be described clearly and completely below in combination with the embodiments of the application. Obviously, the described embodiments are only a part of the application. All other achievements obtained by a person skilled in the art without creative labor on the basis of the embodiments in the application are within the protection scope of the application.

[0027] Examples and comparative examples:

[0028] Heat treatment step Example 1 High temperature solution + two stage ageing 1 Example 2 High temperature solution + two stage ageing 2 Comparative Example 1 High temperature solution + single stage ageing Comparative Example 2 As-rolled plate

[0029] The TC4 titanium alloy used in the present application mainly comprises 6% Al, 4% V, 0.23% C, and the balance of Ti and other impurity elements, and the content of the impurity elements meets the national standard GB / T 3620.1-2016. The ingredients are prepared according to the mass percentage, and the ingredients are prepared from sponge titanium with a purity of 99.95%, aluminum beans with a purity of 99.5%, aluminum vanadium intermediate alloy particles, and high-purity carbon sources. The alloy is repeatedly flipped and melted for 5 times, each time lasting for 3 min. During the melting process, electromagnetic stirring is started to make the melting uniform. After the melting is completed, the sample is taken after the furnace is cooled to room temperature. The phase change point of the alloy is measured to be 1025±5℃.

[0030] The melted alloy ingot is respectively subjected to blooming and upsetting at 1050℃ and 980℃. After the forging is completed, the forged piece is subjected to rolling after being kept at 960℃ for 30 min, and the total deformation is 75%. The titanium alloy plate blank with a thickness of 8.4 mm is finally rolled, and the rolling speed and the amount of depression are both empirical values. After the rolling is completed, the heat preservation treatment is carried out at 1000℃ in the β phase zone to eliminate carbon segregation and promote solid solution, and the slow cooling is carried out to room temperature at a rate of ≤5℃ / s to avoid composition segregation caused by sudden cooling.

[0031] Example 1

[0032] (1) Step one: preheating treatment

[0033] The carbon micro-alloyed TC4 titanium alloy plate blank is heated to 600℃ at a rate of 6℃ / min, and is kept for 30 min for preheating treatment;

[0034] (2) Step two: high-temperature solid solution treatment

[0035] The preheated carbon micro-alloyed TC4 titanium alloy plate blank is continuously heated to 1100℃ at a rate of 6℃ / min for solid solution treatment, and is kept for 60 min, and then is air-cooled to room temperature;

[0036] (3) Step three: primary aging treatment

[0037] The carbon micro-alloyed TC4 titanium alloy plate blank after the solid solution treatment is subjected to primary aging treatment at 300℃, and is kept for 6 h, and then is air-cooled to room temperature;

[0038] (4) Step four: secondary aging treatment

[0039] The carbon micro-alloyed titanium alloy plate blank after the primary aging treatment is subjected to secondary aging treatment at 550℃, and then is air-cooled to room temperature;

[0040] The heat-treated sample obtained in Example 1 was subjected to quasi-static room temperature mechanical property test and OM microstructure characterization. The experiment showed that the yield strength was 1109.62 MPa, the tensile strength was 1233.47 MPa, and the elongation after fracture was 18.89%, and the sample had excellent strength and plasticity matching. Figure 1 The OM microstructure of the sample is shown, and obvious widmanstatten structure and uniformly dispersed carbide precipitates in the structure can be observed. After two-stage aging treatment, the carbides are in high-density and uniform distribution, and there is no obvious segregation phenomenon. In the first aging process, high diffusion rate is used to promote uniform nucleation, and then in the second aging process, low diffusion rate is used to limit the growth rate and maintain uniformity of size. The difference in carbide density between the intracrystalline and grain boundary regions is small (no continuous chain structure is formed near the grain boundary), which meets the uniform precipitation characteristics of two-stage aging.

[0041] Example 2

[0042] (1) Step one: preheating treatment

[0043] The carbon microalloyed TC4 titanium alloy slab was preheated at 600℃ for 30 min at a temperature rising rate of 6℃ / min;

[0044] (2) Step two: high-temperature solid solution treatment

[0045] The preheated carbon microalloyed TC4 titanium alloy slab was solid solution treated at 1050℃ for 60 min at a temperature rising rate of 6℃ / min, and then air-cooled to room temperature;

[0046] (3) Step three: first aging treatment

[0047] The solid solution treated carbon microalloyed TC4 titanium alloy slab was subjected to first aging treatment at 300℃ for 6h, and then air-cooled to room temperature;

[0048] (4) Step four: second aging treatment

[0049] The first aging treated carbon microalloyed titanium alloy slab was subjected to second aging treatment at 550℃, and then air-cooled to room temperature;

[0050] The heat-treated sample obtained in Example 2 was subjected to quasi-static room temperature mechanical property test and OM microstructure characterization. The experiment showed that the tensile strength-yield strength-elongation reached 1158.75 MPa-1034.96 MPa-18.75%, and had certain strength and plasticity matching. Figure 2The OM microstructure of the sample is shown. The obvious Widmanstatten structure and uniformly dispersed precipitates can be observed. The distribution of the microstructure is similar to that of Example 1. After the two-stage aging treatment, the carbides are in high-density and uniformly dispersed distribution, and there is no obvious segregation phenomenon. The high diffusion rate is used to promote uniform nucleation in the primary aging process, and the low diffusion rate is used to limit the growth rate in the secondary aging process, thereby maintaining the uniformity of the size. The difference in carbide density between the intracrystalline and grain boundary regions is small (no continuous chain structure is formed near the grain boundary), which is consistent with the uniform precipitation characteristics of the two-stage aging.

[0051] Comparative Example 1

[0052] (1) Step one: preheating treatment

[0053] The carbon microalloyed TC4 titanium alloy slab was preheated at 600℃ for 30 min at a temperature rising rate of 6℃ / min;

[0054] (2) Step two: high-temperature solid solution treatment

[0055] The preheated carbon microalloyed TC4 titanium alloy slab was solid solution treated at 1100℃ for 60 min at a temperature rising rate of 6℃ / min, and then air-cooled to room temperature;

[0056] (3) Step three: primary aging treatment

[0057] The solid solution treated carbon microalloyed TC4 titanium alloy slab was subjected to primary aging treatment at 550℃ for 6 h, and then air-cooled to room temperature. The heat-treated sample obtained in Comparative Example 1 was subjected to quasi-static room temperature mechanical property test and OM microstructure characterization. The experiment showed that the yield strength was 1036.12 MPa, the tensile strength was 1187.59 MPa, and the elongation was 15.40%. Figure 3 The OM microstructure of the sample is shown. The Widmanstatten structure and uniformly dispersed precipitates can be observed. Compared with the two-stage aging, the single-stage aging structure is obviously coarser, the grain size is uneven, the structure is not uniform, the TiC precipitation also has a local enrichment phenomenon, and the performance is not as outstanding as that of the two-stage aging.

[0058] Comparative Example 2

[0059] The original slab was not subjected to any post-treatment.

[0060] The sample obtained in Comparative Example 2 was subjected to quasi-static room temperature mechanical property test and OM microstructure characterization. The experiment showed that the yield strength was 913.69 MPa, the tensile strength was 1111.49 MPa, and the elongation was 16.72%. Figure 4 The OM microstructure of the sample is shown. The original rolling slab structure mainly consists of ap and pt structures.

[0061] The heat-treated sample is subjected to heat treatment by a box resistance furnace (KSL-1200X, Hefei Jiesheng Material Technology Co., Ltd.).

[0062] Compared with the original slab and the slab subjected to the conventional solid solution and aging treatment, as shown in the attached Figures 1-4 After the heat treatment according to the present application, the carbides are uniformly and dispersedly precipitated in the structure, the tensile property is significantly improved, and a good match between the strength and the ductility is achieved.

Claims

1. A heat treatment process for regulating the uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab, characterized in that: The following steps are involved: Step 1: Preheat The rolled carbon microalloyed TC4 titanium alloy slab is placed in a heat treatment furnace and heated to 400°C to 600°C for preheating; Step 2: High temperature solution treatment The carbon microalloyed TC4 titanium alloy slab preheated in step 1 is further heated to 20 to 100° C. above the β transformation point for solution treatment, and then air-cooled to room temperature; Step 3: First-level aging treatment The carbon microalloyed TC4 titanium alloy slab after the solution treatment in step 1 is subjected to a primary aging treatment at 200° C. to 300° C., and then air-cooled to room temperature; Step 4: Secondary aging treatment The carbon microalloyed TC4 titanium alloy slab after the primary aging treatment in step 2 is subjected to a secondary aging treatment at 500-600° C., and then air-cooled to room temperature. The thermal treatment temperature deviation is no more than ±5°C.

2. The heat treatment process for regulating uniform precipitation of carbides in a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The carbon microalloyed TC4 titanium alloy slab has the following composition by mass percentage: Al: 5.50% to 6.75%, V: 3.50% to 4.50%, C: 0.1% to 0.5%, and the balance is Ti. The contents of other impurity elements such as O, N, and H except C comply with the national standard GB / T 3620.1-2016.

3. The heat treatment process for regulating uniform precipitation of carbides in a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that The β phase transition temperature of the carbon microalloyed TC4 titanium alloy is obtained by metallographic testing or differential scanning calorimetry.

4. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The heating rate in step 1 and step 2 is 5-15°C / min.

5. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The preheating time in step 1 is: t1 = 20 + φ × d (d: thickness, unit: mm; φ = 1-2, unit: min / mm; time unit: min).

6. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The duration of the solution treatment in step 2 is: t2=40+λ×d (d: thickness, unit: mm; λ=1.5-3, unit: min / mm; time unit: min.).

7. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The duration of the primary aging treatment in step 3 is: t3 = 2 + μ × d (d: thickness, unit: mm; μ = 0.1-0.3, unit: h / mm; time unit: h).

8. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The duration of the secondary aging treatment in step 4 is: t4 = 1 + ω × d (d: thickness, unit: mm; ω = 0.05-0.1, unit: h / mm; time unit: h).

9. The heat treatment process for regulating uniform precipitation of carbides inside a carbon microalloyed TC4 titanium alloy slab according to claim 1, characterized in that: The thickness of the slab is 4 mm to 15 mm.

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