A hot working method for improving the high-temperature tensile strength of titanium alloy rolled bars

Abstract

The present invention provides a hot working method for improving the high-temperature tensile strength of titanium alloy rolled bars. The bars obtained by blooming and intermediate billet forging are heated and deformed in the β single-phase region to obtain the final rolled bar billets. The heating temperature is set at 30 - 50 <supgt;o< / supgt;C above the phase transformation point, and the holding time is calculated as 1 min / mm. Then, rotary swaging or rolling is carried out for forming, and the deformation amount is 40% - 70%. After deformation, air cooling is carried out. Then, the obtained bar billets are ground to remove surface defects and then heated and rolled in the low-temperature section of the α + β two-phase region. The heating temperature of the final rolled bar billets is set at 60 - 100 <supgt;o< / supgt;C below the phase transformation point, and the holding time is calculated as 1 min / mm. Then, rolling is carried out for forming, and the rolling deformation amount is 60% - 90%. After deformation, air cooling is carried out. By the method of the present invention, a rolled state structure with uniform and fine α phase can be obtained. After the rolled bars containing this structure are subjected to conventional heat treatment, the tensile strength at room temperature and 600 °C is significantly improved and the values are stable.

Description

Technical field:

[0001] The invention belongs to the field of metallurgy and relates to the technical field of hot processing of titanium alloy bars, in particular to a hot processing method for improving the high-temperature tensile strength of titanium alloy rolled bars. Background technology:

[0002] Titanium alloys are widely used in aerospace and other fields due to their high specific strength and excellent corrosion resistance. Among them, near-α alloys have good high-temperature strength and structural stability, and are used as high-temperature titanium alloys above 500°C.

[0003] The traditional hot working process of small-size high-temperature titanium alloy bars is to prepare the intermediate billet by forging or rolling deformation in the α+β two-phase region, and then to form the material in the α+β two-phase region (30-50℃ below the phase transition point). The process route is shown in Figure 1 . However, a large number of practical results show that the size distribution of the primary α phase of the rod obtained by the traditional process is uneven, and large pieces of α exist in local locations. During the tensile process of the material, cracks are easy to initiate at the interface between the large pieces of α and the matrix, which reduces the tensile strength of the material and increases the dispersion of tensile properties, especially high-temperature tensile strength. Therefore, reducing the size of the α phase after rolling in the two-phase region and improving its dimensional uniformity are technical bottlenecks that need to be solved in the rolling of near-α high-temperature titanium alloy rods. Summary of the invention:

[0004] In view of the above-mentioned technical problems in the prior art, the present invention provides a hot processing method for improving the high-temperature tensile strength of titanium alloy rolled rods. The hot processing method for improving the high-temperature tensile strength of titanium alloy rolled rods is intended to solve the technical problem that the process in the prior art to obtain rods reduces the tensile strength of the material and increases the dispersion of tensile properties, especially the high-temperature tensile strength.

[0005] The present invention provides a hot working method for improving the high temperature tensile strength of a titanium alloy rolled bar, comprising the following steps:

[0006] 1) The bars obtained by forging the blank and the intermediate blank are heated and deformed in the β single phase region to obtain the final rolled bar blank. In the process of heating and deforming in the β single phase region to obtain the final rolled bar blank, the heating temperature is set to 30-50°C above the phase transformation point, the holding time is calculated as 1min / mm, and then radial forging or rolling is performed, the deformation amount is 40%-70%, and air cooling is performed after deformation;

[0007] 2) The bar blank obtained in step 1) is ground to remove surface defects and then heated and rolled in the low temperature section of the α+β two-phase region. During the heating and rolling process in the low temperature section of the α+β two-phase region, the heating temperature of the final rolled bar blank is set to 60-100° C. below the phase transformation point, and the holding time is calculated as 1 min / mm. The bar blank is then rolled into shape, and the rolling deformation is 60%-90%, and air-cooled after deformation.

[0008] The preparation process of titanium alloy bars generally includes: ingot melting, blooming, intermediate billet forging, rotary forging / rolling, finish rolling, and heat treatment. This invention mainly focuses on the preparation process and finish rolling process of titanium alloy finish-rolled billets.

[0009] After being prepared by the method of this invention, the α phase in the as-rolled state of the titanium alloy rolled bar exists in a uniform, fine equiaxed or vermicular shape, with a size of 1 - 3 μm, and there is no large α phase. After double annealing heat treatment, the microstructure is a duplex structure, with the primary α phase evenly distributed and a size of 5 - 8 μm, and there is no large α phase. The room-temperature tensile strength and 600 °C high-temperature tensile strength of the material with this microstructure prepared by the method of this invention are significantly improved and the values are stable.

[0010] The process design principle for optimizing the α grain size and uniformity of high-temperature titanium alloys in this invention is as follows:

[0011] (1) In the traditional process of "billet making in the α + β two-phase region" + "rolling in the α + β two-phase region", the billet has a typical α + β duplex structure, containing primary α phase and β transformation structure; during the subsequent heating in the α + β phase region (30 - 50 °C below the phase transformation point), the primary α grains grow, and the α lamellae in the β transformation structure also coarsen. During the rolling process, both the primary α phase and the α lamellae in the β transformation structure start to break, but the degree of breakage is different. The thinner α lamellae are prone to breakage, but the breakage effect of the coarse primary α phase is limited, especially for the large α phase existing in the billet. The uneven deformation leads to non-uniform microstructure of the finish-rolled bar, which in turn affects the tensile strength.

[0012] (2) In the process of "blank making in the β phase region" + "rolling in the α+β two-phase region" proposed by the present invention, the coupling effect and microstructure evolution of the front and back hot working processes are fully utilized. The blank is heated and deformed in the β region to prepare the intermediate blank. By controlling the heating temperature and deformation amount, Widmanstätten structure with uniform lamellar thickness can be formed. The β heating temperature should not be too high and the time should not be too long, and there should be a certain deformation amount. Otherwise, the β grains grow and continuous and flat grain boundary α phase is easily formed during the air cooling process after deformation, which is not conducive to subsequent fragmentation. Research and practice show that the β region heating temperature is selected to be 30-50 °C above the phase transformation point, the holding time is calculated according to 1 min / mm, and the deformation amount is 40%-70%. The obtained Widmanstätten structure has uniform lamellae and there is no continuous and flat grain boundary α. During the subsequent heating in the α+β phase region, the lamellae grow to a certain extent. By controlling the heating temperature and time, excessive growth can be avoided. During the deformation process of this uniform lamella, the lamellar α phase at different positions is uniformly fragmented, and uniform and fine equiaxed or vermicular α can be formed. There is no large α in the entire observation range. Research and practice show that the α+β two-phase region rolling heating temperature is selected to be 60-100 °C below the phase transformation point, the holding time is calculated according to 1 min / mm, and the rolling deformation amount is 60%-90%. The α lamellae in the obtained microstructure are fully fragmented. If the two-phase region heating temperature is too low, rolling cracking will occur; if it is too high, the α phase will dissolve back and the amount of α phase participating in the deformation and fragmentation of the lamellae is limited. If the deformation amount is too small, the α lamellae cannot be fragmented, and if it is too large, there is a risk of excessive temperature rise.

[0013] During the subsequent double annealing heat treatment, part of the α coarsens to form primary α, and the other part forms β transformation structure. Since there is no large α and the size and distribution of the primary α are uniform, 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.

[0014] Compared with the existing technology, the technological progress of the present invention is significant. The hot-rolled titanium alloy bar prepared by the process technology of "blank making in the β phase region" + "rolling in the α+β two-phase region" proposed by the present invention has α phase existing in the form of uniform and fine equiaxed or vermicular shape with a size of 1-3 μm in the as-rolled state, and there is no large α. After double annealing heat treatment, the microstructure is a duplex structure, and the size of the primary α is 5-8 μm, and there is no large α. The material with this microstructure has high room temperature tensile and 600 °C high temperature tensile strength, and the values are stable. The method of the present invention optimizes the α phase size and uniformity, thereby improving its strength and reducing the dispersion of strength, especially the high temperature tensile strength. Description of the drawings:

[0015] Figure 1 It is a schematic diagram of the process route of the traditional "α+β two-phase region blank making" + "α+β two-phase region rolling".

[0016] Figure 2Schematic diagram of the process route of "blanking in the β phase region" + "rolling in the α+β two-phase region" proposed by the present invention.

[0017] Figure 3 Microstructure morphology obtained by using the traditional process route in the comparative example.

[0018] Figure 4 Microstructure morphology obtained by using the preparation process proposed by the present invention in Example 1.

[0019] Figure 5 Microstructure morphology obtained by using the preparation process proposed by the present invention in Example 2. Specific implementation method:

[0020] The high-temperature titanium alloy in the comparative example and the examples is of the Ti-Al-Sn-Zr-Mo-Nb system, 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.

[0021] Comparative example:

[0022] A φ760mm ingot was obtained by three times of vacuum consumable melting, and the phase transformation point was 1020°C. The ingot was bloomed into φ220mm by a quick forging machine, and the forging temperature was 1050 - 1150°C; then it was forged into a φ80mm bar, and the forging temperature was 950 - 970°C. After the bar was ground, it was loaded into an electric furnace at a temperature of 990°C and held for 80 minutes, and then it was radially forged into a blank with a deformation amount of 60%. After the blank was ground to remove surface defects, it was loaded into an electric furnace at a temperature of 980°C and held for 40 minutes, and then rolled into a bar with a deformation amount of 75%. The rolled bar was subjected to double annealing, specifically 960°C / 1h, air cooling + 570°C / 2h, air cooling.

[0023] Figure 3 Microstructure morphology obtained by using the traditional two-phase region rolling process route in the comparative example. It can be seen that the primary α phase in the blank before final rolling is coarse, with a size of 10 - 20μm, and the primary α phase is connected locally to form large α. After final rolling, the degree of fragmentation of the primary α phase is different. Some become fine equiaxed shapes with a size of 2 - 10μm; some form shapes with a slightly reduced size of 8 - 15μm; and there are still some original large α that are not fully fragmented and are inherited. During the subsequent double annealing heat treatment, the fine equiaxed α dissolves and disappears, while the large α still exists. Table 1 shows the room temperature and 600°C high-temperature tensile properties of the rolled bar. Due to the uneven size distribution of the α phase and the existence of large α after rolling, the high-temperature properties of the rolled bar have obvious fluctuations, and most of the values do not meet the relevant material specifications (yield strength requirement ≥550MPa, tensile strength requirement ≥650MPa).

[0024] Example 1:

[0025] The φ80mm bars after blooming + forging in the comparative example were used. After grinding, the bars were loaded into an electric furnace at a temperature of 1070°C and held for 80 minutes, and then upset forged into billets with a deformation amount of 60%. After grinding to remove surface defects, the rolled billets were loaded into an electric furnace at a temperature of 920°C and held for 40 minutes, and then rolled into bars with a deformation amount of 70%.

[0026] Figure 4 It is the microstructure morphology of the bars obtained by using the preparation process proposed in the present invention in Example 1. It can be seen that before the final rolling, the bar billet is a basket weave structure, the α lamellar thickness is uniform, and the size is 1-3 μm. After the final rolling, the lamellar α phase is fully broken, forming uniform and fine equiaxed or worm-like α, with the short side size of 1-3 μm, and there is no large α in the entire observation range. After double annealing heat treatment, part of the α coarsens to form primary α with a size of 5-8 μm, and the other part redissolves and then precipitates to form β transformation structure. Since there is no large α and the size of the primary α is uniform, the tensile strength of the material at room temperature and 600°C has been significantly improved, as shown in Table 1 for details.

[0027] Example 2:

[0028] The φ80mm bars after blooming + forging in the comparative example were used. After grinding, the bars were loaded into an electric furnace at a temperature of 1070°C and held for 80 minutes, and then rolled into billets with a deformation amount of 60%. After grinding to remove surface defects, the rolled billets were loaded into an electric furnace at a temperature of 920°C and held for 40 minutes, and then rolled into bars with a deformation amount of 70%. Compared with Example 1, in Example 2, the rolling process was used to prepare the rolled billets, while in Example 1, the upset forging process was used to prepare the rolled billets.

[0029] Figure 5 It is the microstructure morphology of the bars obtained by using the preparation process proposed in the present invention in Example 2. It can be seen that since the deformation process of the rolled bar billets is also above the phase transformation point, and the deformation amount, final rolling temperature, and final rolling deformation amount are the same as those in Example 1, the structures of the rolled bar billets, the as-rolled structures of the final rolled bars, and the heat-treated structures of the final rolled bars obtained are similar to those in Example 1. Compared with the comparative example, the tensile strength of the material at room temperature and 600°C has been significantly improved, as shown in Table 1 for details.

[0030] Example 3:

[0031] The φ80mm bars after blooming + forging in the comparative example were used. After grinding, the bars were loaded into an electric furnace at a temperature of 1050°C and held for 80 minutes, and then upset forged into billets with a deformation amount of 50%. After grinding to remove surface defects, the rolled billets were loaded into an electric furnace at a temperature of 960°C and held for 40 minutes, and then rolled into bars with a deformation amount of 85%. Compared with the comparative example, the tensile strength of the material at room temperature and 600°C has been significantly improved, as shown in Table 1 for details.

[0032] Example 4:

[0033] The φ80mm bars after blooming + forging in the comparative example are used. After grinding, the bars are loaded into an electric furnace at a temperature of 1070°C and held for 80 minutes, and then rolled into billets with a deformation amount of 65%. After grinding the rolled billets to remove surface defects, they are loaded into an electric furnace at a temperature of 940°C and held for 40 minutes, and then rolled into bars with a deformation amount of 65%. Compared with the comparative example, the tensile strength at room temperature and 600°C of the material has been significantly improved. See Table 1 for details.

[0034] Table 1 Mechanical properties of the examples and comparative examples

[0035]

[0036] Those of ordinary skill in the art in this technical field should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. As long as it is within the scope of the essential spirit of the present invention, changes and modifications to the above-described embodiments will fall within the scope of the claims of the present invention.

Claims

1. A hot working method for improving the high-temperature tensile strength of titanium alloy bars, the titanium alloy being Ti-Al-Sn-Zr-Mo-Nb series titanium alloy, characterized in that comprising the following steps: 1) Heating and deforming the bars obtained by blooming and intermediate billet forging in the β single-phase region to obtain the final rolling bar billet. During the process of heating and deforming the bars in the β single-phase region to obtain the final rolling bar billet, the heating temperature is set at 30 - 50°C above the phase transformation point, the holding time is calculated as 1 min / mm, and then rotary forging or rolling is carried out for forming, the deformation amount is 40% - 70%, and after deformation, air cooling is carried out; 2) After grinding the surface defects of the bar billet obtained in step 1), heating and rolling are carried out 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 heating temperature of the final rolling bar billet is set at 60 - 100°C below the phase transformation point, the holding time is calculated as 1 min / mm, and then rolling is carried out for forming, the rolling deformation amount is 60% - 90%, and after deformation, air cooling is carried out.

Images