CN115178598A - Hot processing method for improving high-temperature tensile strength of titanium alloy rolled bar - Google Patents

Abstract

The invention provides a hot working method for improving the high temperature tensile strength of a titanium alloy rolled bar. The final rolled bar is obtained by heating and deforming a bar obtained by forging a billet and an intermediate billet in a β single-phase region, and the heating temperature is set as The temperature is 30~50 ^o C above the transformation point, the holding time is calculated as 1min/mm, and then the diameter is forged or rolled, the deformation amount is 40%~70%, and the deformation is air-cooled; then the obtained bar is ground to remove the surface After the defect, it is heated and rolled in the low temperature section of the α+β two-phase region. The heating temperature of the final rolled billet is set to 60~100 ^o C below the transformation point, and the holding time is calculated as 1min/mm. The deformation amount is 60%~90%, and it is air-cooled after deformation. Through the method of the present invention, a rolled structure with uniform and fine α phase can be obtained. After conventional heat treatment, the tensile strength at room temperature and high temperature of 600°C of the rolled bar containing the structure is obviously improved and the value is stable.

Description

A kind of hot working method for improving high temperature tensile strength of titanium alloy rolled bar

Technical field:

The invention belongs to the field of metallurgy and relates to the technical field of hot processing of titanium alloy rods, in particular to a hot processing method for improving the high temperature tensile strength of titanium alloy rolled rods.

Background technique:

Titanium alloys are widely used in aerospace and other fields because of their high specific strength and excellent corrosion resistance. Among them, the near-α type alloy has good high temperature strength and microstructure stability, and is used as a high temperature titanium alloy above 500 ℃.

The traditional hot working process of high-temperature titanium alloy small-sized bars is to forge or roll deformation in the α+β two-phase region to prepare the intermediate billet, and then form the bar in the α+β two-phase region (30-50 °C below the transformation point). See the process route. figure 1. However, a large number of practical results show that the size distribution of the primary α phase in the bar obtained by the traditional process is not uniform, and there are large α in the local position. During the tensile process of the material, cracks are easily initiated at the interface between the bulk α and the matrix, which reduces the tensile strength of the material and increases the dispersion of tensile properties, especially the high-temperature tensile strength. Therefore, reducing the size of the α phase after rolling in the two-phase region and improving its size uniformity are the technical bottlenecks that need to be solved in the rolling of near-α type high-temperature titanium alloy bars.

Invention content:

In view of the above-mentioned technical problems in the prior art, the present invention provides a hot working method for improving the high temperature tensile strength of a titanium alloy rolled bar, and the described hot working method for increasing the high temperature tensile strength of a titanium alloy rolled bar To solve the technical problem of reducing the tensile strength of the material and increasing the dispersion of the tensile properties, especially the high temperature tensile strength, obtained by the process in the prior art.

The invention provides a hot working method for improving the high temperature tensile strength of a titanium alloy rolled bar, comprising the following steps:

1) The final rolled billet is obtained by heating and deforming the bar obtained by the billet and intermediate billet forging in the β single-phase region. During the process of heating and deforming the final rolled billet in the β single-phase region, the heating temperature is set to phase transformation. The temperature is 30～50℃ above the point, the holding time is calculated as 1min/mm, and then forging or rolling is formed, the deformation is 40%～70%, and the air cooling after deformation;

2) After grinding the bar obtained in step 1) to remove surface defects, it is heated and rolled in the low temperature section of the α+β two-phase region. During the process of heating and rolling in the low temperature section of the α+β two-phase region, the final rolled bar is The heating temperature of the billet is set to be 60-100°C below the transformation point, the holding time is calculated as 1min/mm, and then rolled into a shape with a rolling deformation of 60% to 90%, and air-cooled after deformation.

The preparation process of titanium alloy bar is generally: ingot smelting, billet opening, intermediate billet forging, radial forging/rolling, final rolling, heat treatment.

After being prepared by the method of the present invention, the α phase in the rolling state of the titanium alloy rolled bar exists in a uniform and fine equiaxed or worm-like shape, the size is 1-3um, and there is no large α phase. After double annealing and heat treatment, the microstructure is a two-state structure, the primary α distribution is uniform, the size is 5-8um, and there is no large α. After being prepared by the method of the present invention, the room temperature tensile strength and the high temperature tensile strength of 600° C. of the material having the structure are obviously improved, and the numerical value is stable.

The process design principle for optimizing the grain size and uniformity of high temperature titanium alloy α in the present invention is as follows:

(1) In the traditional "α+β two-phase region billet making" + "α+β two-phase region rolling" process, the billet is a typical α+β two-state structure, including primary α phase and β transformation structure; During the subsequent heating in the α+β phase region (30-50°C below the transformation point), the primary α grains grow, and the α sheet in the β transformation structure also coarsens. During the rolling process, the primary α phase and β transformation The α-sheets in the tissues all began to break up, but the degree of break-up was different. The α lamellae with smaller thickness are prone to crushing, but the crushing effect of the coarse primary α phase is limited, especially the large pieces of α existing in the billet. The non-uniform deformation leads to the non-uniform structure of the final rolled bar, which in turn affects the tensile strength.

(2) In the "β-phase region billet making" + "α+β two-phase region rolling" process proposed by the present invention, the coupling effect and microstructure evolution of the front and rear hot working links are fully utilized. The billet is heated and deformed in the beta zone to prepare the intermediate billet. By controlling the heating temperature and the amount of deformation, a Widmandelstein microstructure with uniform lamella thickness can be formed. The β heating temperature is not easy to be too high, the time is not too long, and there must be a certain amount of deformation, otherwise the β grains grow and the air cooling process after deformation is easy to form a straight and continuous grain boundary α phase, which is not conducive to subsequent crushing. Research and practice show that the heating temperature in the beta zone is 30-50°C above the phase transition point, the holding time is calculated as 1min/mm, and the deformation is 40%-70%. Continuous grain boundary α. During the subsequent heating in the α+β phase region, the lamellae grow to a certain extent, and the excessive growth can be avoided by controlling the heating temperature and time. During the deformation process of this uniform lamella, the α phase of the lamella is uniformly broken at different positions, and can form a uniform and fine equiaxed or worm-like α, and there is no large α in the entire observation range. Research and practice show that the rolling heating temperature in the α+β two-phase region is 60-100°C below the transformation point, the holding time is calculated as 1min/mm, and the rolling deformation is 60%-90%. The layers are sufficiently broken. If the heating temperature in the two-phase region is too low, the rolling cracks; if the α phase is too high, the α phase will dissolve back, and the α phase participating in the deformation and crushing of the lamellae is limited. If the deformation is too small, the α sheet cannot be broken, and if it is too large, there is a risk of excessive temperature rise.

During the subsequent double annealing heat treatment, part of α is coarsened to form primary α, and other parts form β-transformed structure. Due to the absence of bulk α and the uniform size and distribution of primary α, the room temperature tensile strength and high temperature tensile strength of the material have been significantly improved, and the results of multiple measurements have little fluctuation.

Compared with the prior art, the present invention has significant technical progress. For the high-temperature titanium alloy rolled bar prepared by the process technology of "β-phase region billeting" + "α+β two-phase region rolling" proposed by the present invention, the α-phase exists in a uniform and fine equiaxed or vermicular shape in the rolling state , the size is 1-3um, and there is no large block α. After double annealing and heat treatment, the microstructure is a two-state microstructure, the primary α size is 5-8um, and there is no bulk α. The material with this structure has high tensile strength at room temperature and high temperature at 600°C, and is numerically stable. The method of the present invention improves the strength and reduces the dispersion of the strength, especially the high temperature tensile strength, by optimizing the size and uniformity of the alpha phase.

Description of drawings:

Figure 1 is a schematic diagram of the traditional "α+β two-phase region billet making" + "α+β two-phase region rolling" process route.

Fig. 2 is a schematic diagram of the process route of "β-phase region billet making" + "α+β two-phase region rolling" proposed by the present invention.

Figure 3 shows the microstructure morphology obtained under the traditional process route in the comparative example.

FIG. 4 is the microstructure morphology obtained by using the preparation process proposed by the present invention in Example 1. FIG.

FIG. 5 is the microstructure morphology obtained by using the preparation process proposed by the present invention in Example 2. FIG.

Detailed ways:

The high-temperature titanium alloys in the comparative examples and examples are Ti-Al-Sn-Zr-Mo-Nb systems, containing a small amount of Si and Ce. For other near-α high temperature titanium alloys, the preparation process proposed by the present invention can also achieve the same effect.

Comparative ratio:

The φ760mm ingot was obtained by three vacuum consumable smelting, and the phase transition point was 1020°C. Use a quick forging machine to open the ingot to φ220mm, the forging temperature is 1050～1150℃; then it is forged into a φ80mm bar, the forging temperature is 950～970℃. After grinding, the bar is loaded into an electric furnace, the temperature is 990 ° C, and the temperature is kept for 80 minutes, and then it is forged into a billet with a deformation amount of 60%. After the billet is ground to remove surface defects, it is loaded into an electric furnace, the temperature is 980 ° C, the heat preservation is 40 minutes, and it is rolled into a bar with a deformation amount of 75%. The rolled bar is double annealed, specifically 960°C/1h, air-cooled +570°C/2h, air-cooled.

Figure 3 shows the microstructure obtained under the traditional two-phase rolling process route in the comparative example. It can be seen that the primary α phase in the billet before the final rolling is coarse, the size is 10-20um, and the primary α phase is connected at local positions to form a large block of α. After the final rolling, the primary α phase is broken to different degrees, some become small equiaxed, with a size of 2-10um; some are slightly reduced in size, with a size of 8-15um; and some original large pieces of α are not sufficiently broken. , inherited. During the subsequent double annealing heat treatment, the small equiaxed α re-dissolved and disappeared, and the large α remained. Table 1 shows the tensile properties of the rolled bars at room temperature and 600 °C. Due to the uneven size distribution of the α phase and the existence of large α blocks after rolling, the high temperature properties of the rolled bars have obvious fluctuations, and most of the values do not meet the relevant requirements. The material specification (yield strength requirement ≥ 550MPa, tensile strength requirement ≥ 650MPa).

Example 1:

The φ80mm bar after billeting + forging in the comparative example is used. After grinding, the bar is put into an electric furnace, the temperature is 1070 ° C, and the temperature is kept for 80 minutes, and then it is forged into a billet with a deformation amount of 60%. The rolled billet was ground to remove surface defects and then loaded into an electric furnace at a temperature of 920° C., kept for 40 minutes, and rolled into a bar with a deformation of 70%.

FIG. 4 is the microstructure morphology of the bar obtained by using the preparation process proposed by the present invention in Example 1. FIG. It can be seen that the billet before the final rolling is a basket structure, the thickness of the α lamella is uniform, and the size is 1-3um. After the final rolling, the α-phase of the lamella is sufficiently broken to form a uniform and fine equiaxed or worm-like α with a short side size of 1-3um, and there is no large α in the entire observation range. After double annealing and heat treatment, part of α is coarsened to form primary α, with a size of 5-8um, and other parts are redissolved and then precipitated to form a β-transformed structure. Due to the absence of bulk α and the uniform size of the primary α, the tensile strength of the material at room temperature and high temperature at 600 °C is significantly improved, as shown in Table 1.

Example 2:

The φ80mm bar after billeting + forging in the comparative example is used. After grinding, the bar is loaded into an electric furnace, the temperature is 1070 ° C, and the temperature is kept for 80 minutes, and then it is rolled into a billet with a deformation amount of 60%. The rolled billet was ground to remove surface defects and then loaded into an electric furnace at a temperature of 920° C., kept for 40 minutes, and rolled into a bar with a deformation of 70%. Compared with Example 1, Example 2 adopts a rolling process to prepare rolled billets, while Example 1 adopts a radial forging process to prepare rolled billets.

FIG. 5 is the microstructure morphology of the bar obtained by the preparation process proposed by the present invention in Example 2. FIG. It can be seen that since the deformation process of the rolled bar is also above the phase transition point, and the deformation amount, the final rolling temperature, and the final rolling deformation are the same as those in Example 1, the obtained rolled bar structure and the final rolled bar structure as-rolled And the heat-treated structure of the final-rolled bar is similar to that of Example 1. Compared with the comparative example, the tensile strength of the material at room temperature and high temperature at 600 °C has been significantly improved, as shown in Table 1.

Example 3:

The φ80mm bar after billeting + forging in the comparative example is used. After grinding, the bar is loaded into an electric furnace, the temperature is 1050 ℃, and the temperature is kept for 80 minutes, and then it is forged into a billet with a deformation amount of 50%. The rolled billet was ground to remove surface defects and then loaded into an electric furnace at a temperature of 960° C., kept for 40 minutes, and rolled into a bar with a deformation amount of 85%. Compared with the comparative example, the tensile strength of the material at room temperature and high temperature at 600 °C has been significantly improved, as shown in Table 1.

Example 4:

The φ80mm bar after billeting + forging in the comparative example is used. After grinding, the bar is loaded into an electric furnace, the temperature is 1070 ° C, the temperature is kept for 80 minutes, and then it is rolled into a billet with a deformation amount of 65%. The rolled billet was ground to remove surface defects and then loaded into an electric furnace at a temperature of 940° C., kept for 40 minutes, and rolled into a bar with a deformation of 65%. Compared with the comparative example, the tensile strength of the material at room temperature and high temperature at 600 °C has been significantly improved, as shown in Table 1.

Table 1 Example, Comparative Example Mechanical Properties

Those of ordinary skill in the art should realize that the above embodiments are only used to illustrate the present invention, but not to limit the present invention, as long as the changes to the above embodiments are within the spirit and scope of the present invention , modifications will fall within the scope of the claims of the present invention.

Claims (1)

1. A hot working method for improving the high-temperature tensile strength of a titanium alloy rolled bar is characterized by comprising the following steps:

1) Heating and deforming a bar material obtained by cogging and intermediate billet forging in a beta single-phase region to obtain a final rolled bar blank, and setting the heating temperature to be 30-50 ℃ above the phase transformation point in the process of heating and deforming the bar material in the beta single-phase region to obtain the final rolled bar blank ^o C, keeping the temperature for 1min/mm, then performing radial forging or roll forming, wherein the deformation amount is 40-70%, and performing air cooling after deformation;

2) Grinding the bar stock obtained in the step 1) to remove surface defects, then heating and rolling the bar stock in an alpha + beta two-phase region low-temperature section, and setting the heating temperature of the finally rolled bar stock to be 60-100 ℃ below the transformation point in the process of heating and rolling the bar stock in the alpha + beta two-phase region low-temperature section ^o And C, keeping the temperature for 1min/mm, then rolling and forming, wherein the rolling deformation is 60-90%, and air cooling is carried out after deformation.

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