JPS646268B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a rolled titanium alloy material with a good structure, and particularly to a method for manufacturing a rolled titanium alloy material with a good structure by continuous rolling on a multistage stand. Titanium alloy has specific strength (ratio of strength to weight)
Because of its large strength, it is used in applications that require high reliability, including fields such as aircraft and space development equipment that require light weight and high strength. However, for these applications, mere high strength is not sufficient, especially when supplied in the form of wire or rod, which must be formed during manufacturing into the final product as a bolt or structural part. Appropriate ductility is essential as it undergoes a processing process. In order to improve this ductility, it is essential to have a uniform and fine structure. By the way, titanium alloy material is one of the difficult-to-process materials.
There are few reports regarding its manufacturing method. For example, forged materials are disclosed in JP-A-51-77385, but as far as the present inventors know, there are no reports regarding manufacturing conditions for rolled materials. By the way, in the production of the forged material mentioned above, after β forging,
Continuously perform processing of 10% or more in the α+β region, then heat to the β region, and then process α at a cooling rate of 20℃/min or more.
This is done by cooling to the +β region or α region, thereby making the structure finer. However, when a titanium alloy round or small bar is produced by forging, there are fundamental problems inherent to the forging method, such as variations in the structure in the longitudinal direction of the forge and variations in the structure depending on the location within the cross section. In addition, in terms of production efficiency, it is incomparable to the rolling method because heat forging is repeated several times.
Less efficient. In addition, even if rolling is carried out, titanium alloy is a difficult-to-process material, so surface flaws are likely to occur, and even if heated at a high temperature within the α + β range, surface flaws will not occur in rough or intermediate rolling rows. I can't. Therefore, it is necessary to perform whole-surface peeling in a subsequent process, which results in a decrease in yield and an increase in the number of man-hours, which inevitably leads to a decrease in efficiency. Thus, an object of the present invention is to provide a method for producing a rolled titanium alloy material with a good structure and few surface flaws. By the way, in order to improve the structure, it is necessary to lower the rolling temperature, and on the other hand, the higher the processing temperature, the lower the frequency of surface flaws. Therefore,
The requirement to improve the structure and the requirement to reduce surface defects are mutually contradictory. As a result of intensive research on continuous rolling of titanium alloys, the present inventors have found that by controlling the heating temperature during rolling and controlling the material temperature during rolling, it is possible to simultaneously satisfy the contradictory demands described above and eliminate surface defects. We succeeded in obtaining a Ti alloy rolled material with a good structure and no oxidation. In particular, the present invention is based on the discovery of the criticality of the combination of heating in the β range, sufficient working degree in the rough row, and sufficient working degree in the α+β region in the intermediate and finishing rows to eliminate surface defects. It is something. Here, in the present invention, after blooming an α+β type titanium alloy, it is heated to a β region of 1150°C or less, processed by a continuous rolling mill to a forging ratio of 3 or more in the β region, and then continuously processed in the α+β region. This is a method for producing a rolled titanium alloy material with a good structure, which is characterized by adding processing at a forging ratio of 10 or more and, if necessary, subjecting the obtained hot rolled material to equiaxed crystal formation treatment. As described above, one of the features of the present invention is heat rolling in the β region, and by performing rough processing in the β region, which has good workability, it not only prevents surface flaws during rough processing, but also improves the subsequent rough processing. It has the effect of suppressing the occurrence of surface flaws during processing in the α+β region. The reason why the heating temperature in the β region was set to 1150°C or less is that if heated to too high a temperature, the gas absorption layer (so-called α-
case) is generated and the workability is deteriorated.
Temperature control during continuous rolling becomes difficult, especially for α+β.
This is because there is a risk that the degree of processing in the area may not be sufficient. A larger training ratio is advantageous for preventing surface flaws in the α+β region, but it cannot be increased in practice due to restrictions on material dimensions. By setting the forging ratio to 3 or more, a practically sufficient surface quality can be obtained. Furthermore, according to the present invention, the processing degree in the α+β region is regulated, but as described above,
Another feature of the present invention is that processing is performed with a forging ratio of 10 or more in the area. In other words, by performing β region forging in the first half and α+β region forging in the second half of one heat, a Ti alloy rolled material with a good structure can be produced extremely efficiently and without surface flaws. Also,
The forging ratio in the α+β region plays a very important role in improving the structure, and from a structural standpoint, the larger the forging ratio in the α+β region, the finer the equiaxed grains are formed. There are also constraints, and if we look at the permissible limit of the organization empirically set from a practical standpoint, it is sufficient to set the training ratio to 10 or more.
Although the processing temperature in the α+β range is not particularly limited here, if the temperature is just below the β-transazole, the transformed β structure increases within the structure, which is undesirable. In addition, if the processing temperature is lower than about 700℃, the movement of grain boundaries and recrystallization during processing will be extremely slow, so it is necessary to perform the equiaxed crystal formation process described below to achieve equiaxed crystallization. is preferred. Therefore, the processing temperature in the α+β range is preferably 800-900°C, although it is not necessarily limited thereto. Here, equiaxed crystal formation treatment refers to hot rolling followed immediately by slow cooling, preferably furnace cooling to 700℃.
Cool at a rate of 150°C/hr or less, then leave to cool to room temperature, or maintain at a temperature of 800 to 950°C for a predetermined time, generally 30 minutes or more, preferably 1 hour or more after hot rolling, or 700℃ after rolling
First let it cool down to below 700℃, then reheat it in a furnace maintained at 850 to 950℃ and keep it at 800 to 950℃ for a specified period of time, generally at least 30 minutes, preferably at least 1 hour, then let it cool down to 700℃ or below. It is a cooling process,
Such treatment promotes the formation of equiaxed crystals. Slow cooling of hot rolled material immediately after rolling is due to α+β
This is because it is effective in releasing crystal strain accumulated by rolling in the region and achieving equiaxed crystallization. To achieve this, it is necessary to allow sufficient holding time at temperatures above 700°C, where grain boundary movement is likely to occur, and for this purpose slow cooling is performed, but as mentioned above, preferably 150°C/hr up to 700°C. Requires slow cooling at: A similar effect can be obtained by maintaining the temperature at 800 to 950°C after hot rolling. In addition, in order to refine the crystal grains by cooling the hot rolled material and then reheating it, it is necessary to
Heating to 950°C is effective. If the temperature exceeds 950°C, the β phase increases and the acicular structure increases after air cooling, which is not preferable. On the other hand, below 800℃, 700℃
Although it is possible to refine the crystal grains by heating to around .degree. C. for a long time, it is not economical because it takes a long time. The equiaxed crystal thus obtained is a crystal whose crystal grain morphology is isotropic, and ideally consists of crystal grains that do not extend in any particular direction in an arbitrary cross section. In carrying out the present invention, continuous rolling using a multi-stage stand is generally assumed, and since rolling is started after heating to the β region, the rolling temperature is controlled so that the temperature drops to the α + β region during rolling. There is a need to. In addition, heat is generated from the rolled material during processing, and cooling by water cooling, etc. is required at appropriate parts of the continuous rolling mill row.Water cooling between rolls, water cooling immediately after rolling, or a combination of both may be considered. , but are not necessarily limited to these. When water cooling is employed, the temperature may be adjusted by adjusting the flow rate. Typical α+β type titanium alloys to which the present invention is applied are Ti-6Al-4V and Ti-4Al-4Mn, and other examples include Ti-8Mn and Ti-5Al-
2.75Cr―1.25Fe, Ti―4Al―4Mo―4V, Ti―4Al
-3Mo-1V etc. However, it will be understood that the invention is not limited thereto. The invention will now be further explained in connection with examples. Embodiment A 1 ton ingot was produced by vacuum arc melting of Ti-6Al-4V alloy, and after blooming and rolling, peeling was performed to remove surface defects, and then ingot was prepared according to the present invention. As shown in Table 1, billets with a diameter of 130 mm and a square of 53 mm were produced and rolled into round bars with a diameter of 9 mm using a continuous hole rolling material at the finish rolling speed also shown in Table 1. Since the rolling was carried out at a high speed, a temperature increase was observed due to rolling, so in accordance with the present invention, cooling was carried out between the rolls and/or immediately after rolling by water spraying. For each of the obtained rolling mills, the structure determination and surface flaw determination described below were performed. The rolling conditions and results are summarized in Table 1 together with those of comparative examples. It should be noted that various companies have come up with various methods for determining the organization based on their past experience and achievements. As shown in the attached drawing, the α+β equiaxed crystal structure is classified as Class 1 (see Figure a), and the state in which the isotropy of the crystal grains has not progressed, that is, the state in which something close to the processed grain remains is classified as Class 1. (see Figure d), intermediate stages with a high degree of residual processed structure (see Figure c), and lower stages, i.e., those with insufficient equiaxed crystallization (see Figure b). , for a total of 4 classes. Grades 1 and 2 are structures that can withstand practical use, and grades 3 and 4 are structures that require improvement. Surface flaws are determined by visually inspecting the surface of the product, and when surface flaws are detected, the area is ground with a grinder and the depth of the flaw is measured. Depending on the depth of the flaw, it is classified into 1st to 4th grade as shown in Table 2.

[Table] Grades 1 and 2 are structures that can withstand practical use, and grades 3 and 4 are those that require further care.

[Table] In the table, Examples A to I show examples of the present invention, and Examples J to O show comparative examples. Examples H and I show examples in which the hot-rolled materials obtained in Examples C and D (indicated as Material C and Material D, respectively) were subjected to equiaxed crystal formation treatment. In the example of the present invention, the above-mentioned structure evaluation and surface flaw evaluation were both 1 or 2, which was satisfactory. However, as shown in Example J, when the working ratio in the α+β region is less than 10, the structure is not refined sufficiently; on the other hand, as shown in Examples K and L, when rolling is performed only in the α+β region, , the occurrence of surface flaws is inevitable. In the case of continuous rolling, in order to obtain a good structure, α
Hot rolling in the +β range is generally considered, but in that case, the overall material temperature is low and the corner part is lower due to the shape, so that part is subjected to tension during rolling and is originally two-phase. It is thought that surface flaws occur because the deformation is poor in the mixing zone. In addition, if rolling is started in the α+β region and the temperature is raised during rolling as shown in Examples M and N, if the temperature is raised to the β region as in Example M, the occurrence of surface defects will be reduced, but in both examples Even under these conditions, tissue deterioration cannot be prevented. Example O shows the case where the heating temperature is higher than the temperature specified by the present invention. As described above, in the present invention, prior to hot rolling, surface flaws are prevented from occurring during rough rolling or intermediate rolling by heating the material in the β range to raise the material temperature and form a single-phase structure. After increasing the training ratio,
Controlled cooling is performed so that intermediate rolling to finish rolling can be performed in the α+β region. As a result, as shown in Table 1, even when heated and rolled to a fairly high temperature in the β region,
By combining this with controlled cooling, we succeeded in obtaining rolled Ti alloy material with good structure and surface flaws. As shown in the comparative examples, even if these conditions are variously changed as long as they are outside the scope of the present invention,
It is difficult to simultaneously satisfy both the structure and the flaws at a level that is acceptable for practical use.

[Brief explanation of drawings]

The attached drawings are schematic diagrams of crystal structures showing the criteria for determining equiaxed crystal structures, and figures a, b, c, and d correspond to the structures of each grade of 1st, 2nd, 3rd, and 4th grade, respectively. .

Claims (1)

[Claims] 1 After blooming an α+β type titanium alloy, it is heated to the β region of 1150°C or less, processed by a continuous rolling mill to a forging ratio of 3 or more in the β region, and then continuously processed to the α+β region. A method for manufacturing a rolled titanium alloy material with a good structure, which is characterized by applying processing at a forging ratio of 10 or more. 2 After blooming α+β type titanium alloy, it is heated to the β region of 1150℃ or less, processed with a continuous rolling mill to a working ratio of 3 or more in the β region, and then continuously processed to a working ratio of 10 or more in the α+β region. 1. A method for producing a rolled titanium alloy material with a good structure, the method comprising processing and then subjecting the obtained hot rolled material to equiaxed crystal formation treatment.