JP7401760B2 - Manufacturing method of α+β type titanium alloy bar material - Google Patents

Description

The present invention relates to a method for manufacturing an α+β type titanium alloy bar.

For example, for connecting rods used to connect a crank to a piston pin or crosshead pin of an engine, forged products of iron-based materials have conventionally been mainly used. Iron-based materials have a high specific gravity, so there is a limit to how lightweight connecting rods can be made, which has been an obstacle to achieving improvements in fuel economy by reducing the weight of the engine, as well as improving power performance by increasing rotation speeds. .

Generally, connecting rods used in automobiles, industrial machinery, etc. are required to have fatigue strength, toughness and ductility, wear resistance, machinability, etc. Titanium alloys have excellent properties that meet these demands, and connecting rods made of titanium alloys are used even in special applications such as racing cars. A typical example is Ti-6Al-4V.

Patent Document 1 states that the surface layer is a fine equiaxed crystal structure that is difficult to form cracks that become the starting point of fracture, and the interior is a rough plate-like or acicular crystal structure that makes it difficult for cracks to propagate. Certain titanium alloys with improved fatigue strength have been disclosed.

Japanese Patent Application Publication No. 05-311366

Generally, α+β type titanium alloy round bars are manufactured through the following process.

First, an ingot is manufactured using a vacuum arc melting method, an electron beam melting method, a plasma melting method, etc. using sponge titanium, master alloy, titanium scrap, etc. as melting raw materials. Next, after heating the ingot to the β region, a billet is obtained through a blooming process by blooming forging or blooming rolling, and after cooling, it is heated to the β region or α+β region, and a round bar is manufactured by rolling. In general, α+β region rolling provides a microstructure composed of finer, nearly equiaxed crystal grains than β region rolling, resulting in a round bar with higher fatigue strength.

Blossom forging involves repeating heating and forging, but in order to make the microstructure fine and equiaxed, it is common to perform β region heating in the first stage and α+β region heating in the second stage. Therefore, the production efficiency is lower than that of blooming.

In blooming rolling, rolling is performed in a highly efficient rolling line after heating to the β region, so the temperature inside the material remains in the β region even at the end of rolling. Therefore, it is difficult to obtain a good microstructure. Therefore, it is necessary to improve the microstructure by cooling the billet after blooming and cutting it, charging it into a heating furnace again, heating it in the α+β region, and rolling it. Therefore, it cannot be said that the production efficiency is sufficiently high. Note that the reason for cutting is that the material is elongated by β region rolling and therefore often does not enter the heating furnace.

Furthermore, if the initial heating for blooming rolling is performed at a low temperature, the rolling motor torque will increase due to a decrease in the material temperature during rolling, and the torque may exceed an equipment allowable value or the rolling may stop.

In view of the above circumstances, it is an object of the present invention to provide a method for manufacturing an α+β type titanium alloy bar material that has high fatigue strength while ensuring good rollability.

The present inventors have conducted intensive studies on a method for manufacturing an α+β type titanium alloy bar material that has high fatigue strength while ensuring good rollability. As a result, they discovered that by giving the titanium alloy an appropriate thermal history, it is possible to improve the microstructure of the titanium alloy, obtain high fatigue strength, and also suppress the increase in rolling motor torque.

The present invention has been made based on the above findings and further studies, and the gist thereof is as follows.

(1) A first rolling step in which an α+β type titanium alloy ingot is heated to Tβ+30°C or more and less than Tβ+250°C, and the heated titanium alloy ingot is subjected to a first rolling with an area reduction rate of 70% or more, and the rolled A holding step in which the average surface temperature of the titanium alloy is held in the range of 800°C or more and Tβ-50°C or less for 2 minutes or more and 6 minutes or less, and a second process with an area reduction rate of 50% or more is applied to the titanium alloy after holding. 1. A method for producing an α+β type titanium alloy bar, comprising a second rolling process in which two rolling steps are performed. Here, Tβ is the β transformation temperature.

(2) The method according to (1) above, further comprising a leading end heating step of heating the leading and trailing ends of the titanium alloy to 1000° C. or higher between the first rolling step and the second rolling step. A method for manufacturing α+β type titanium alloy rods.

(3) The method for producing an α+β type titanium alloy bar according to (1) or (2) above, wherein the α+β type titanium alloy bar is a bar for a connecting rod for an automobile.

(4) The chemical components of the α+β type titanium alloy ingot are Al: 2.5 to 8.0%, Fe: 0.5 to 3.0%, and O: 0.01 to 0.30 in mass %. %, and the remainder is Ti and impurities.

(5) Production of the α+β type titanium alloy bar according to (4) above, containing at least 3.0% each of one or more of Sn, Zr, Mo, Si, Cu, and Nb in place of a part of the Ti. Method.

According to the present invention, fatigue strength is improved while ensuring good rollability in a titanium alloy bar manufactured by directly processing an α+β type titanium alloy ingot.

In the method for producing an α+β type titanium alloy bar according to the present invention, an ingot is directly hot-rolled without going through a blooming process and subsequent cooling to obtain a bar with high fatigue strength. This mechanism is estimated as follows.

A coarse cast structure such as columnar crystals and equiaxed crystals is formed in the as-cast ingot. In a typical manufacturing method, the cast structure is destroyed by blooming forging to obtain a medium-fine crystal structure. When producing a bar by directly hot rolling an ingot without undergoing blooming forging, the casting structure tends to be insufficiently refined, resulting in a coarse microstructure. As a result, fatigue strength in particular tends to decrease.

Generally, fatigue fracture is caused by the occurrence of cracks on the surface of a bar, so making the structure of the surface layer of the bar finer is effective in improving fatigue strength. As a result of studies by the present inventors, it has been found that in order to refine the cast structure, it is effective to apply strain in both temperature ranges during the process of decreasing the temperature from the β region to the α+β region.

After heating the titanium alloy ingot to the β region, strain is applied to the surface layer in the first rolling process, which makes it easier for the α phase to precipitate, and as the temperature decreases, the α phase forms at the β grain boundaries and within the β grains. Precipitate. Furthermore, by holding the temperature in the temperature range where the α phase precipitates quickly (the nose of the TTT diagram), the α phase precipitation is further promoted and it becomes easier to apply strain to the α phase.

By performing a second rolling after that, a fine structure is formed by dividing the α phase precipitated at β grain boundaries and within β grains, and increasing the number of new precipitation sites. .

Hereinafter, the method for manufacturing an α+β type titanium alloy bar according to the present invention will be described in detail.

First, an α+β type titanium alloy ingot is manufactured. The titanium alloy ingot may be manufactured by a known method. Specifically, an ingot can be manufactured using a vacuum arc melting method, an electron beam melting method, a plasma melting method, etc. using sponge titanium, master alloy, titanium scrap, etc. as melting raw materials.

Next, the titanium alloy ingot is heated to Tβ+30° C. or higher and lower than Tβ+250° C., where the β transformation temperature is Tβ, and subjected to first rolling with an area reduction rate of 70% or higher. When the heating temperature is Tβ+30° C., surface cracks are likely to occur due to a decrease in surface temperature, and surface flaws are likely to increase. When the heating temperature exceeds Tβ+250° C., the surface layer is significantly oxidized and surface flaws are likely to increase. The area reduction rate is set to 70% or more in order to sufficiently refine the surface layer structure.

Next, the surface temperature of the rolled titanium alloy is maintained in the range of 800° C. or more and Tβ-50° C. or less for 2 minutes or more and 6 minutes or less. By providing a cooling period after β region rolling, precipitation of α phase is promoted in the surface layer of the titanium alloy, improving the microstructure. A heat retaining cover may be used to maintain the surface temperature within the above range. Subsequently, the titanium alloy is subjected to a second rolling process with an area reduction rate of 50% or more.

The rolling motor torque due to the temperature drop during rolling increases more significantly in α+β type titanium alloys than in steel. The reason for this is not simply due to the temperature drop, but is thought to be because the α phase (hcp structure) precipitates from the β phase (bcc structure) due to the temperature drop, and the ratio of the α phase, which is difficult to deform, increases. As a countermeasure to this problem, it is possible to cut and remove the temperature-reduced portion. However, since this measure causes a decrease in yield, it is desirable to heat the material in the rolling line.

According to the present invention, by setting the surface temperature in the holding step after the first rolling to 800°C or higher and Tβ-50°C or lower, a large amount of the easily deformable β phase is retained in the center of the material during the second rolling. Therefore, the increase in rolling motor torque at the leading and trailing ends of the material can be suppressed. The temperature in the second rolling is determined because if the temperature is too low, surface flaws are likely to occur due to a decrease in surface temperature, and excessive increase in α phase ratio makes it difficult to obtain the breaking effect due to strain, making it difficult to refine the structure. The temperature is preferably 800°C or higher, and more preferably the higher of Tβ-200°C or 800°C. Further, in order to prevent coarsening of the structure due to processing heat, it is more preferable to set the temperature to Tβ-70°C or lower.

As described above, it is possible to simultaneously improve the microstructure of the surface layer of the material and improve the rollability.

Furthermore, by heating the front and rear ends of the titanium alloy to 1000° C. or higher between the first rolling and the second rolling, an increase in rolling motor torque can be more effectively suppressed. It is preferable to perform this heating on-line since there is no need to cut the titanium alloy. As the heating method, one or a combination of two or more of heating using flammable gas and oxygen, induction heating, arc heating, electron beam heating, and laser heating can be used. In order to compensate for the temperature drop in the lower surface layer due to heat radiation in a short time, it is desirable that the surface temperature be high.

The α+β type titanium alloy bar produced by the method for producing an α+β type titanium alloy bar of the present invention is suitable for use as a bar for connecting rods for automobiles, for example.

The chemical composition of the α+β type titanium alloy rod of the present invention is not particularly limited. As an example, it contains Al: 2.5 to 8.0%, Fe: 0.5 to 3.0%, and O: 0.01 to 0.30% in mass %, with the remainder being Ti and impurities. It can be a titanium alloy bar.

Further, in place of a part of the Ti, one or more of Sn, Zr, Mo, Si, Cu, and Nb can be contained in an amount of 3.0% or less each.

[Example 1]
An α+β type titanium alloy ingot manufactured by electron beam melting and represented by Ti-5%Al-1%Fe-0.18%O (Tβ is 1015°C) with a cross section of 310 x 440 mm or a diameter of 300 mm, The surface temperature of the rolled titanium alloy is heated to the temperature listed in Table 1, the first rolling is performed at the first rolling area reduction rate listed in Table 1, and the surface temperature of the rolled titanium alloy is maintained within the range listed in Table 1 for the holding time. Then, a second rolling was performed at the second rolling area reduction ratio shown in Table 1 to obtain a titanium alloy round bar.

During manufacturing, during the second rolling process, the rolling motor torques at the steady section and the leading and trailing ends are measured when passing through the final rolling stand, and the torque ratio (rolling motor torque value at the leading and trailing ends relative to the rolling motor torque value at the steady section) is calculated. The ratio) was determined and shown in Table 1.

Here, the rolling motor torque at the leading and trailing ends was set to be higher at the leading and trailing ends. The rolling motor torque in the steady portion was the average value excluding the torque increasing portion near the leading and trailing ends. In this example, when the torque ratio was 150% or less, it was determined that the manufacturing conditions were good from the viewpoint of the load on the rolling equipment.

A fatigue test piece with a parallel portion of φ8 mm was taken from near the surface layer of the obtained titanium alloy round bar. For the fatigue test, a rotary bending fatigue test was performed up to 1×10 ⁷ times at room temperature, and the maximum stress that did not cause rupture was taken as the fatigue strength, and 500 MPa or more was judged to be good. The results are shown in Table 1.

No. Tests 1 to 10 were conducted using an ingot measuring 310 x 440 mm.

No. In Samples No. 1, 2, 3, 5, 7, and 8, both the area reduction rate and the holding temperature were within the range of the present invention, the torque ratio was 150% or less, and the fatigue strength was 500 MPa or more.

No. No. 4 has a high holding temperature; No. 6 had a small area reduction rate in the second rolling, which was outside the scope of the present invention, and the fatigue strength was less than 500 MPa.

No. Sample No. 9 had a low holding temperature and many cracks occurred on the surface of the round bar, so it was not possible to collect fatigue test pieces.

No. Tests 11 to 13 were carried out using an ingot with a diameter of 300 mm.

No. No. 11 is an example of the present invention, and a fatigue strength of 500 MPa or more was obtained.

No. No. 12 has a low holding temperature; In No. 13, the area reduction rate in the first rolling was insufficient, and the fatigue strength in all cases was less than 500 MPa.

No. No. 14 is an example in which the front and rear ends were heated after the first rolling. Gas heating was performed using LPG and oxygen, and the temperature was increased to 1000°C or higher. No. 1 with the same heating conditions and rolling conditions. Compared to No. 1, the torque ratio decreased, but the fatigue strength was the same.

[Reference example]
An α+β type titanium alloy ingot with a diameter of 730 mm and represented by Ti-5%Al-1%Fe-0.18%O (Tβ is 1015°C) manufactured by conventional technology using a consumable electrode vacuum arc melting method. was made into a rolled material with a diameter of 200 mm by a process of heating to β region and forging. Next, it was heated to the α+β region and rolled with an area reduction rate of 78% to obtain a titanium alloy round bar. The fatigue strength of the obtained titanium alloy round bar was measured in the same manner as in Example 1. As a result, the maximum stress that did not break even after 1×10 ⁷ cycles was 500 MPa. From this result, even if Example 1 according to the method for producing a titanium alloy bar of the present invention omitted the blooming process, the fatigue was equivalent to or higher than that of the titanium alloy round bar produced by the conventional technology including the blooming forging process. It was confirmed that strength was obtained.

[Example 2]
An α+β type titanium alloy ingot with a diameter of 300 mm and a chemical composition shown in Table 2 was heated to the temperature shown in Table 2, and the first rolling was performed at a first rolling area reduction rate of 74%, resulting in a rolled titanium alloy. The surface temperature of the sample was maintained for the temperature range and holding time shown in Table 2. Subsequently, a second rolling was performed at a second rolling area reduction rate of 78% to obtain a titanium alloy round bar.

As in Example 1, the torque ratio in the second rolling was determined and shown in Table 2.

The fatigue strength of the obtained titanium alloy round bar was measured in the same manner as in Example 1. The results are shown in Table 2.

No. The holding temperatures and holding times of Samples Nos. 1, 3, 5, 7, 8, 9, 12, and 13 were all within the range of the present invention, and the torque ratio was 150% or less.

No. Tests Nos. 2, 4, 6, 10, 11, and 14 had holding temperatures or holding times outside the range of the present invention, and some had torque ratios exceeding 150%.

When comparing α+β type titanium alloys with the same chemical composition, No. Nos. 1, 3, 5, 7, 8, 9, 10, 12, and 13 had higher fatigue strength.

Furthermore, No. When comparing 7 and 8, No. 7 (Tβ-maximum holding temperature = 55°C), No. 8 (Tβ-maximum holding temperature = 75°C) had higher fatigue strength.

Similarly, No. When comparing 12 and 13, No. No. 12 (Tβ-maximum holding temperature = 60°C). No. 13 (Tβ-maximum holding temperature = 80°C) had higher fatigue strength.

According to the method for producing a titanium alloy bar of the present invention, it was confirmed that a titanium alloy base material having high fatigue strength could be produced even if the blooming step was omitted.

Claims (5)

Heating the α+β type titanium alloy ingot to Tβ+30°C or higher and lower than Tβ+250°C,
a first rolling step of subjecting the heated titanium alloy ingot to a first rolling with an area reduction rate of 70% or more;
A holding step of maintaining the average surface temperature of the rolled titanium alloy in a range of 800°C or higher and Tβ-50°C or lower for 2 minutes or more and 6 minutes or less,
A method for producing an α+β type titanium alloy bar, comprising a second rolling step of subjecting the titanium alloy after holding to a second rolling with an area reduction of 50% or more.
Here, Tβ is the β transformation temperature. The α+β type according to claim 1, further comprising a leading and trailing end heating step of heating the leading and trailing ends of the titanium alloy to 1000° C. or higher between the first rolling step and the second rolling step. A method for manufacturing titanium alloy bars. 3. The method for manufacturing an α+β type titanium alloy bar according to claim 1 or 2, wherein the α+β type titanium alloy bar is a bar for a connecting rod for an automobile. The chemical components of the α + β type titanium alloy ingot include Al: 2.5 to 8.0%, Fe: 0.5 to 3.0%, and O: 0.01 to 0.30% in mass %. The method for producing an α+β type titanium alloy bar according to any one of claims 1 to 3, wherein the remainder is Ti and impurities. The method for producing an α+β type titanium alloy bar according to claim 4, which contains at least 3.0% of each of Sn, Zr, Mo, Si, Cu, and Nb in place of a part of the Ti.