JP2023092454A - Titanium alloy, titanium alloy bar, titanium alloy plate, and engine valve - Google Patents

Abstract

A titanium alloy, a titanium alloy rod, a titanium alloy plate, and an engine valve that maintain ductility and have excellent fatigue properties are provided.
Kind Code: A1 A Ti-5Al-2Fe-3Mo or Ti-6Al-4V system α+β type titanium alloy has a two-phase structure consisting of an α phase and a β phase, and Al, Fe, Mo, and V in a cross section. The maximum area of the segregation abnormality part of the element concentration is 2000 μm ² or less. The segregation abnormal part of the element concentration of Al, Fe, Mo, and V in the cross section is an EPMA surface analysis in which the average value of Al and Fe is ± 1% by mass, the average value of Mo is ± 1.5% by mass, and the average value of V is A set of adjacent measurement points outside the value ±2% by weight is meant. As a result, it is possible to provide a titanium alloy, a titanium alloy rod, a titanium alloy plate, and an engine valve that are excellent in fatigue properties and ductility.
[Selection figure] None

Description

The present invention relates to titanium alloys, titanium alloy rods, titanium alloy plates and engine valves.

Titanium alloys are used in various fields because they are lightweight, have high strength, and have good corrosion resistance. Among them, α + β type titanium alloy represented by Ti-6Al-4V has an excellent balance of mechanical properties such as strength, ductility and toughness, and has been widely used in the space and aviation fields for a long time, and in recent years it has also been applied to automobile parts. progressing.

On the other hand, there has been a demand for a low-cost α+β type titanium alloy with better properties.

In Patent Document 1, in mass%, 4.4% or more and less than 5.5% of Al, 1.4% or more and less than 2.1% of Fe, and 1.5% or more and less than 5.5% of Mo , a high-strength α+β-type titanium alloy characterized by containing less than 0.1% of Si and less than 0.01% of C as impurities, with the remainder consisting of Ti and unavoidable impurities. By adding an appropriate amount of Mo to an α+β type titanium alloy containing Al and Fe, high strength, high ductility, and excellent hot workability and cold workability are obtained. We have found an α+β type titanium alloy.

In Patent Document 2, by mass%, it contains 4.4% or more and less than 5.5% of Al, 1.4% or more and less than 2.1% of Fe, and 2.5% or more and less than 5% of Mo, An α+β-type titanium alloy member containing less than 0.1% Si and less than 0.01% C as impurities, the balance being Ti and unavoidable impurities, wherein the area ratio "A" of proeutectoid α-phase grains is An α+β type titanium alloy member having a tensile strength of 1000 MPa or more, characterized by a Young's modulus of 5% or more and less than 49% and a Young's modulus of 75 GPa or more and less than 100 GPa is disclosed. As a result, by using an α + β type titanium alloy having a relatively inexpensive alloy composition, it is possible to obtain a Young's modulus of 75 GPa or more and less than 100 GPa, which is comparable to a β type titanium alloy or lower than a normal α + β type titanium alloy, and a tensile strength of 1000 MPa It is possible to provide an α+β type titanium alloy member of grade or higher and a method for manufacturing the same, and to adjust the Young's modulus in a wider range so that the Young's modulus is 75 to 125 GPa without changing the alloy composition. A method for manufacturing an α+β type titanium alloy member can be provided.

In Patent Document 3, an α + β type titanium alloy composed of 1.4% or more and less than 2.1% Fe, 4.4% or more and less than 5.5% Al, and the balance titanium and impurities, or part of Fe α + β type titanium alloy substituted with less than 0.15% Ni, less than 0.25% Cr, less than 0.25% Mn, or α + β further containing 0.05% or more and less than 0.25% Si Type titanium alloys are disclosed. It is possible to provide an Al--Fe system α+β type titanium alloy having fatigue strength equivalent to that of the conventional one and superior hot or cold workability to the conventional one.

In Patent Document 4, an α + β type titanium alloy composed of 0.5% or more and less than 1.4% Fe, 4.4% or more and less than 5.5% Al, the balance titanium and impurities, or part of Fe , less than 0.15% Ni, less than 0.25% Cr, less than 0.25% Mn α + β type titanium alloy substituted with one or more, or 0.05% or more and less than 0.25% Si An α+β type titanium alloy is disclosed which further contains The present invention can provide a titanium alloy having stable and uniform fatigue strength and high hot workability. Alternatively, it is possible to provide a titanium alloy with even higher creep resistance.

Patent Document 5 discloses an α+β type titanium alloy formed by an α phase and a β phase, in which the area ratio of the α phase having an average equivalent circle diameter of 5 μm or less and an average aspect ratio of 3 or more in the structure is 40%. The following α-β type titanium alloys with excellent machinability are disclosed. As a manufacturing method, according to a conventional method, a slab is hot forged, heat-mechanically processed into a desired shape, and then annealed.

Patent document 6 contains 0.1 to 2.5% by mass of Fe, has a needle-like or lath-like microstructure (structure a), and has a concentration distribution of Fe An Fe-containing titanium material having excellent corrosion resistance is disclosed, characterized in that the area of the portion where the concentration is 1.5 times or more as high as the average concentration is 5% or more. By having the structure a and having the Fe-enriched portion, the corrosion resistance of the titanium material can be enhanced.

Patent Document 7 has a predetermined component element, the difference ΔC between the maximum value C _MAX and the minimum value C _MIN of the measured value of each element is less than 0.2 C _MIN or less than 0.04%, and the metal structure However, it discloses a titanium ingot that can be easily processed into a thin plate or a wire rod, and has a circle-equivalent average crystal grain size of 10 mm or less at the central portion in the thickness direction of the titanium ingot and less than half the thickness. . However, the elemental concentration analysis uses a normal chemical analysis method, does not evaluate microscopic concentration variations in units of μm, and does not refer to the effects on various characteristics including fatigue characteristics.

JP-A-2005-320618 JP 2007-314834 A JP-A-7-62474 JP-A-7-70676 JP-A-2005-320570 JP-A-2005-336551 International publication WO2019/026251

There is a demand for further improvement in fatigue properties of α+β type titanium alloys. In particular, it is desired to suppress early fracture in the fatigue fracture of engine valves, which are manufactured by cutting a bar as a raw material. In addition, if the strength is increased to improve fatigue properties, ductility will decrease, and if the material is deformed at room temperature, there is a concern that it will break, or even if it does not break, voids will form inside, making it difficult to handle. Therefore, it is also required to have sufficient ductility at room temperature.

An object of the present invention is to provide a titanium alloy, a titanium alloy rod, a titanium alloy plate, and an engine valve that maintain ductility and have excellent fatigue properties.

That is, the gist of the present invention is as follows.
[1] It has a two-phase structure consisting of an α phase and a β phase, contains 2.5% by mass or more of Al and at least 0.8% by mass of one or more main elements other than Al, and has a cross section A titanium alloy in which the maximum area of the segregation anomaly in the main element concentration is 2000 μm ² or less.
Here, the main elements are Al, Fe, Mo, and V, and the segregation abnormal portion of the main element concentration in the cross section is an EPMA surface analysis in which the main elements other than Mo and V have an average value of ± 1% by mass, Means a collection of adjacent measurement points where Mo is outside the average ±1.5% by mass and V is outside the average ±2%.
[2] In mass%, Al: 4.5 to 6.5%, Fe: 1.4 to 2.3%, Mo: 1.5 to 5.5%, O and N are total The titanium alloy according to [1], wherein the content is 0.25% or less, and the balance is Ti and impurities.
[3] In mass%, Al: 5.0 to 7.0%, V: 3.5 to 5.0%, the total of O and N is 0.25% or less, and the balance consists of Ti and impurities , the titanium alloy according to [1].
[4] In mass%, Al: 4.5 to 6.5%, Fe: 0.8 to 2.3%, Si: 0.0 to 0.50%, O and N total 0.25 % or less, the balance being Ti and impurities, the titanium alloy according to [1].

[5] A titanium alloy rod made of the titanium alloy according to any one of [1] to [4].
[6] The titanium alloy rod according to [5], wherein the ratio of the needle-like structure is 80 area % or less.
Here, the ratio of the needle-like structure is obtained by performing EBSD evaluation on the cross section perpendicular to the axis of the titanium alloy rod, and obtaining the crystal grain size with the misorientation of 5° or more as the grain boundary in the measurement result. If the ratio of the needle-like structure is 100% by area, and if the average crystal grain size is less than 100 μm, extract data with an image quality (IQ) of 20% or more of the average, and extract the aspect ratio (length An α-grain having a ratio of axis/minor axis) of 3 or less is defined as an equiaxed α-grain, and the ratio of the area occupied by other than the equiaxed α-grain to the total measured area is defined as the acicular structure ratio.

[7] A titanium alloy plate made of the titanium alloy according to any one of [1] to [4].
[8] The titanium alloy plate according to [7], wherein the ratio of the needle-like structure is 80 area % or less.
Here, the ratio of the needle-like structure is obtained by performing EBSD evaluation on a cross section perpendicular to the rolling direction of the titanium alloy plate, obtaining the crystal grain size with a misorientation of 5° or more as the grain boundary in the measurement result, and obtaining an average crystal grain size of 100 μm or more. In the case of , the ratio of the acicular structure is set to 100% by area, and when the average grain size is less than 100 μm, data with an image quality (IQ) of 20% or more of the average is extracted, and the aspect ratio ( α-grains having a long axis/minor axis) of 3 or less are defined as equiaxed α-grains, and the ratio of the area occupied by other than the equiaxed α-grains to the total measured area is defined as the acicular texture ratio.

[9] An engine valve made of the titanium alloy according to any one of [1] to [4].
[10] The engine valve according to [9], wherein the acicular structure has a ratio of 80 area % or less.
Here, the ratio of the acicular structure is obtained by performing an EBSD evaluation on a cross section perpendicular to the axis of the engine valve, obtaining the crystal grain size with a misorientation of 5° or more as the grain boundary in the measurement results, and when the average crystal grain size is 100 μm or more. is assumed to be 100% by area of the acicular structure, and when the average crystal grain size is less than 100 μm, data with an image quality (IQ) of 20% or more of the average is extracted, and the aspect ratio (major axis /minor axis) is 3 or less, and the area ratio occupied by other than the equiaxed α grains to the total measured area is defined as the ratio of the acicular structure.

The present invention provides an α+β type titanium alloy having a two-phase structure consisting of an α phase and a β phase, and having a maximum size of segregation anomaly in the main element concentration of Al, Fe, Mo, V, etc. in the cross section of 2000 μm ² or less. Therefore, it is possible to provide a titanium alloy, a titanium alloy rod, a titanium alloy plate, and an engine valve having excellent fatigue properties.

<<Composition of Titanium Alloy>>
The alloy composition specified in the present invention is an average analytical value for the entire product. 1 mm of the surface layer is removed from the sample for analysis, and the sample is evenly sampled from the entire remaining portion for analysis. The analysis methods are inductively coupled plasma (ICP) emission spectrometry for metal elements, inert gas fusion infrared absorption method for O, inert gas fusion thermal conductivity method for N and H, and high frequency combustion infrared absorption method for C. Hereinafter, % in the alloy composition means % by mass.

《α＋β type titanium alloy》
The titanium alloys targeted by the present invention are primarily α+β type titanium alloys. The α+β type titanium alloy has a two-phase structure consisting of an α phase and a β phase, and 2.5% by mass or more of Al is added to the alloy. It is a titanium alloy containing more than mass %. Specifically, Ti-6Al-4V (ELI), Ti-3Al-2.5V, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1V-1Mo, Ti-6Al described in the AMS standard -2Sn-4Zr-6Mo, Ti-6Al-6V-2Sn, Ti-4.5Al-3V-2Fe-2Mo and other titanium alloys, as well as Ti-5Al-1Fe and Ti-5Al-1.5Fe-0.25Si , Ti-5Al-2Fe-3Mo and Ti-8Al-1Fe-1Nb-0.15Si. The present invention includes any of these α+β type titanium alloys.

<<Ti-5Al-2Fe-3Mo alloy>>
The second titanium alloy composition targeted by the present invention is α+β containing Al: 4.5-6.5%, Fe: 1.4-2.3%, Mo: 1.5-5.5% type titanium alloy, hereinafter referred to as "Ti-5Al-2Fe-3Mo alloy". A Ti-5Al-2Fe-3Mo alloy is a component system that can obtain a strength higher than that of general-purpose Ti-6Al-4V in a solution state, and that can utilize relatively inexpensive elements. It is a titanium alloy having a composition similar to that of the titanium alloys described in Patent Documents 1 and 2.

(Al: 4.5-6.5%)
The higher the Al content, the higher the strength. It also has effects such as controlling the ratio of α phase and β phase and suppressing precipitation of ω phase that causes embrittlement. Therefore, the Al content is preferably 4.5% or more, more preferably 4.8% or more. The upper limit is made 6.5% because if too much is added, the hot workability deteriorates. More preferably, it is 6.0% or less.

(Fe: 1.4-2.3%)
The higher the Fe content, the higher the strength. In addition, 1.4% or more of Fe is added because the β phase ratio at high temperatures is increased and the hot workability is improved. More preferably, it is 1.8% or more. On the other hand, if too much is added, segregation will become noticeable in large ingots, and the segregation will not be eliminated even if measures are taken to alleviate the segregation in subsequent steps. Therefore, the upper limit of the amount of Fe added is set to 2.3%. More preferably, it is 2.1% or less.

(Mo: 1.5-5.5%)
The higher the Mo content, the higher the strength. In addition, 1.5% or more of Mo is added because the β phase ratio at high temperatures is increased and the hot workability is improved. More preferably, it is 2.0% or more. On the other hand, if too much is added, segregation will become noticeable in large ingots, and the segregation will not be eliminated even if measures are taken to alleviate the segregation in subsequent steps. Therefore, the upper limit of the amount of Mo added is set to 5.5%. More preferably, it is 4.5% or less.

(O, N, C, H)
O, N, C, and H are contained in the titanium alloy as impurity elements.
O and N increase strength but decrease ductility. Therefore, the total content of O and N is set to 0.25% or less. Practically, the amount of N contained is small, and the amount of O is large. Although the lower limit is not particularly limited, it is preferable that the sum is 0.05% or more in order to secure the strength.
As the C content increases, the strength increases, the ductility decreases, and the hot workability decreases. Therefore, it is preferably 0.05% or less. Although the lower limit is not particularly limited, it is substantially 0.001% or more.
H is an element that causes embrittlement when contained in a large amount, and is preferably 0.013% or less to prevent embrittlement. Although the lower limit is not particularly limited, it is substantially 0.0001% or more.

<<Ti-6Al-4V alloy>>
The third titanium alloy composition targeted by the present invention is an α+β type titanium alloy containing 5.0 to 7.0% Al and 3.5 to 5.0% V, hereinafter referred to as “Ti-6Al "-4V alloy". It is a component system that includes a representative α+β titanium alloy commonly referred to as Ti-6Al-4V.

(Al: 5.0-7.0%)
The more Al is added, the higher the strength. It also has effects such as controlling the ratio of α phase and β phase and suppressing precipitation of ω phase that causes embrittlement. Therefore, Al is preferably added in an amount of 5.0% or more, more preferably 5.5% or more. If too much is added, the hot workability deteriorates, so the upper limit of the Al content is made 7.0%. More preferably, it is 6.5% or less.

(V: 3.5-5.0%)
The more V is added, the higher the strength. Also, 3.0% or more of V is added because the β phase ratio at high temperatures increases and the hot workability is improved. More preferably, it is 3.5% or more. On the other hand, if the added amount increases, the cost increases, so the upper limit of the added amount of V is set to 5.0%. More preferably, it is 4.5% or less.

(O, N, C, H)
O and N increase strength but decrease ductility. Therefore, the sum is set to 0.25% or less. Practically, the amount of N contained is small, and the amount of O is large. In order to secure the strength, the sum total must be 0.05% or more.
When the amount of C added increases, the strength increases, the ductility decreases, and the hot workability decreases. Therefore, it is made 0.05% or less. Although the lower limit is not particularly limited, it is substantially 0.001% or more.
H is an element that causes embrittlement when added in a large amount. Although the lower limit is not particularly limited, it is substantially 0.0001% or more.

<<Ti-5Al-1.5Fe-Si alloy>>
Fourth, the titanium alloy composition targeted by the present invention is α+β containing Al: 4.5 to 6.5%, Fe: 0.8 to 2.3%, Si: 0.0 to 0.50%. type titanium alloy, hereinafter referred to as "Ti-5Al-1.5Fe-Si alloy". It is a component system including the α+β type titanium alloy described in Patent Documents 3 and 4.

(Al: 4.5-6.5%)
The higher the Al content, the higher the strength. It also has effects such as controlling the ratio of α phase and β phase and suppressing precipitation of ω phase that causes embrittlement. Therefore, the Al content is preferably 4.5% or more, more preferably 4.8% or more. The upper limit is made 6.5% because if too much is added, the hot workability deteriorates. More preferably, it is 6.0% or less.

(Fe: 0.8-2.3%)
The higher the Fe content, the higher the strength. In addition, 0.8% or more of Fe is added because the β phase ratio at high temperatures is increased and the hot workability is improved. More preferably, it is 1.0% or more. On the other hand, if too much is added, segregation will become noticeable in large ingots, and the segregation will not be eliminated even if measures are taken to alleviate the segregation in subsequent steps. Therefore, the upper limit of the amount of Fe added is set to 2.3%. More preferably, it is 2.1% or less.

(Si: 0.0 to 0.50%)
The higher the Si content, the higher the strength. In addition, it is added as necessary because it suppresses oxidation at high temperatures. On the other hand, if too much is added, coarse precipitates are generated in large ingots, which become starting points of fatigue fracture. Therefore, the upper limit of the amount of Si added is set to 0.5%. More preferably, it is 0.4% or less. Si does not have to be contained.

(O, N, C, H)
O, N, C, and H are contained in the titanium alloy as impurity elements.
O and N increase strength but decrease ductility. Therefore, the total content of O and N is set to 0.25% or less. Practically, the amount of N contained is small, and the amount of O is large. Although the lower limit is not particularly limited, it is preferable that the sum is 0.05% or more in order to secure the strength.
As the C content increases, the strength increases, the ductility decreases, and the hot workability decreases. Therefore, it is preferably 0.05% or less. Although the lower limit is not particularly limited, it is substantially 0.001% or more.
H is an element that causes embrittlement when contained in a large amount, and is preferably 0.013% or less to prevent embrittlement. Although the lower limit is not particularly limited, it is substantially 0.0001% or more.

<<Other element content: less than 0.5%>>
In any of the second to fourth titanium alloys, elements that are likely to be mixed as impurities other than the elements described above are Sn, Ni, Cr, Mn, Zr, Nb, Cu, (Si, V, Mo) There is There is a possibility of contamination from raw materials, especially when raw materials such as scrap or low-grade sponge titanium are used. These are elements that should be controlled because there is a concern that their contents will increase. The total content of these elements is preferably less than 0.5% (preferably less than 0.3%), and the individual content of each element is preferably 0.1% or less. In addition, there are elements such as other platinum group elements contained in corrosion resistant alloys, but since these have little effect on mechanical properties, there is no problem even if they are mixed, so there is no need to control them. Since other elements are hardly contained in general-purpose alloys, the possibility of contamination from scrap is low, and the possibility of contamination from sponge titanium or the like is also low.

<<Crystal structure of titanium alloy (1)>>
The titanium alloy having the chemical composition of the present invention is an α+β type titanium alloy, and has an α phase and a β phase in the titanium alloy.
In the composition range of α+β type titanium alloys, at high temperatures, there are two phases, the α and β phases, and the higher the temperature, the more the β phase. β phase. Therefore, when kept at a low temperature of 500° C. or less, or when rapidly cooled, ω phase, α′ phase, or α″ phase may be formed. As they are formed, they become harder and their ductility and toughness at room temperature decrease. Therefore, when handling at room temperature, care must be taken such as avoiding excessive deformation. In particular, the influence of the ω phase is large. Since the α' phase and α'' phase are formed by rapid cooling from a high temperature at which the β phase is abundant, or by holding at a low temperature, they are hardly formed by the usual production methods. Therefore, even if the α' phase and α'' phase are formed, they do not reach about 0.1% and have no effect. Therefore, only the ω phase should be confirmed, and no peak should be confirmed by X-ray diffraction. X-ray diffraction is performed using Cu-Kα or Co-Kα radiation. 2θ is 30 to 90°, and the scanning step is 0.01°.
As described above, the titanium alloy of the present invention, in which no ω phase is confirmed by X-ray diffraction, has a two-phase structure consisting of α phase and β phase.

《Homogeneity of composition of titanium alloy》
There are various causes of fatigue fracture in titanium alloys, but as an example, if there is a difference in strength inside the material, the same portion is likely to become the starting point of fracture. Also, the larger the starting point, the more likely it is to break. In many cases, the starting point of fracture is the grain unit, but the starting point larger than that is generated. There are several causes for this, one of which is segregation that occurs during solidification of the alloy. In particular, alloys intended for high strength tend to have segregation because they contain a large amount of additive elements. In order to prevent fatigue failure of the product, it is desirable that the segregation in the product is small. The same is true for maintaining ductility. Furthermore, large ingots are generally used to improve productivity and provide low cost, but large ingots tend to cause segregation. Therefore, it was conceived that fatigue characteristics can be improved and ductility can be maintained by reducing the segregation of alloying elements even in products manufactured using large ingots. In addition, since it is a phenomenon caused by the difference in strength, it is desirable to confirm it by the difference in strength, but since titanium has anisotropy, even if it is evaluated by hardness etc., the value obtained by evaluation is the fatigue load. Not necessarily a useful number for the direction. Therefore, it is important to evaluate the amount of the element that is the source of the difference in strength regardless of the anisotropy. Elements that initiate fatigue fractures and cause differences in strength are substitution-type strengthening elements such as Al, Fe, Mo, and V, and the effect of improving fatigue characteristics due to the uniformity of the composition was confirmed. In the present invention, these are called main elements in the titanium alloy. Interstitial strengthening elements that diffuse easily and do not cause segregation are considered to be out of scope, but other alloying elements for substitution strengthening are not excluded.

Solidification segregation is particularly likely to occur in large ingots (here, at least 1 ton or more). Unevenness in the blending of raw materials is also considered to be one of the causes of segregation. Since these are relatively large, several millimeters or more, and are formed over a long distance (long distance), it is difficult to homogenize them only by the diffusion phenomenon of the alloying elements due to heat treatment. It is necessary to utilize both the metal flow due to processing and the diffusion due to heat treatment.

In particular, the reduction in the cross-sectional area due to processing significantly reduces and shortens the diffusion distance in heat treatment, making it easier to obtain a uniform element distribution by heat treatment for a short period of time.

On the other hand, since the bar has a smaller cross-sectional reduction rate than the plate, the segregation of the alloying elements is remarkable, and the resulting breakage becomes more conspicuous. More specifically, high fatigue strength is required for engine valves, which are manufactured from bar material, and premature failure due to segregation can be a problem, and it is expected that this will be suppressed. be done.

Therefore, we clarified the effect of variations in the composition (segregation abnormalities) in titanium alloys (products after hot working) on fatigue characteristics, and attempted to realize a manufacturing method that can obtain sufficient ductility. . In particular, the fatigue strength of titanium alloy bars, specifically round bars, has been improved.

(The maximum size of the segregation anomaly in the concentration of the main element in the cross section is 2000 μm ² or less)
Hereafter, the major elements mean Al, Fe, Mo and V. The α+β type titanium alloy of the present invention contains Al and, in addition, at least one major element in an amount of 1% by mass or more.
As will be clarified in Examples described later, it was found that the segregation of the main elements in the alloy affects the 1×10 ⁷ cycle fatigue limit (hereinafter simply referred to as the fatigue limit).

First, a Ti-5Al-2Fe-3Mo alloy will be exemplified. In the specimen with a fatigue limit of about 650 MPa, there were some specimens that fractured even at 600 MPa. The periphery of the fatigue starting point of this test piece was mirror-polished and subjected to surface analysis by EPMA. In the EPMA analysis, an acceleration voltage of 10 kV and a 3 mm square area were measured in steps of 10 μm. Regardless of the fatigue life, the alloying elements are distributed in a roughly normal distribution, but in the case of a short life, a large region in which Al, Fe, and Mo are concentrated or reduced is formed near the fatigue starting point. With respect to Al, Fe, and Mo, this region is a segregation region where Al and Fe are outside the average value ± 1% by mass and Mo is outside the average value ± 1.5% by mass. A test piece fractured at 600 MPa In , the size (area) of the segregation region of any element was equivalent to 3000 μm ² , and the results corresponded to the fatigue test.

Next, a Ti-6Al-4V alloy will be exemplified. For this alloy, some specimens with a fatigue limit of about 630 MPa failed even at 600 MPa. When the same investigation as the above Ti-5Al-2Fe-3Mo titanium alloy is performed, when the region where Al is outside the average value ± 1% by mass and V is outside the average value ± 2% is shown as a segregation region, fracture at 600 MPa The size (area) of the segregation region of any element in the test piece obtained was equivalent to 3500 μm ² , and the results corresponded to the fatigue test.

A segregation region in which the alloying elements are concentrated or reduced in this way is called a segregation anomaly. The segregation anomaly can be evaluated as a region in which the average value in the EPMA surface analysis of a predetermined element fluctuates by a value different for each element or more. In the case of the Ti-5Al-2Fe-3Mo alloy of the present invention, the strengthening elements are targeted, Al and Fe are ±1% by mass, Mo is ±1.5% by mass, and enrichment indicated by + and - Both decreases indicated by are the same, and both can be regarded as segregation regions that constitute segregation anomalies. The fatigue starting point is the weakest part, and the size (area) of the largest segregation abnormality is important. The maximum size of the abnormal segregation part was 2000 μm ² or less in the test pieces that had a long life. Therefore, it was found that breakage due to segregation can be avoided if the maximum area of the segregation anomaly in the element concentration of Al, Fe, and Mo in the cross section satisfies 2000 μm ² or less. In the case of Ti-6Al-4V alloy, the strengthening element is targeted, Al is the average value ± 1 mass%, V is the average value ± 2%, and both the enrichment indicated by + and the decrease indicated by - They are the same, and both can be regarded as segregation regions constituting segregation anomalies.
Moreover, the maximum area of the abnormal segregation part was 2000 μm ² or less in the test piece with a long life. Furthermore, the results (segregation of each element and the maximum area of segregation anomaly) were the same for the Ti-5Al-1.5Fe-Si alloy.

From the above results, in the EPMA surface analysis, the measurement points where the main elements other than Mo and V are outside the average value ± 1% by mass, the average value for Mo is ± 1.5% by mass, and the average value for V is ± 2% are adjacent. A cluster of points where

The segregation abnormal part is evaluated as a collection of adjacent points in mapping by EPMA surface analysis, and the number of measurement points where the measurement points that satisfy the criteria for the segregation area are adjacent is multiplied by the square of the step size. can be found at Adjacent measurement points refer to the case where each measurement point is a square and one side is in contact with the other.

If the object to be evaluated is a bar, the measurement plane shall be either a cross section perpendicular to the axis of the bar or a cross section including the axis. In the case of plate material, the cross section shall be perpendicular to the rolling direction. A wide measurement area is desirable, and the measurement is performed on an area with a corner of 3 mm or more. Also, the square or rectangular area should be as large as possible with respect to the measurement cross-sectional size. In the case of a rectangular shape, the ratio of the long side to the short side (long side/short side) should be 1.5 or less. If it is not possible to secure an angle of 3 mm or more in one field of view, it is necessary to perform measurement in a plurality of fields of view with an angle of 4 mm or more. In this method, an area wider than one field of view is evaluated because accurate measurement becomes difficult if the segregation abnormality is located at the edge of the field of view. In the measurement, the 0.1 mm surface layer is not included in the measurement range because the vicinity of the surface is affected by contamination and accurate measurement is difficult at the edge portion such as the surface.

The EPMA acceleration voltage is 10 to 15 kV, the step size is 0.01 to 0.05 mm, and the beam diameter for measurement is 0.5 to 1 times the step size.

For quantification of the measurement results, the measurement result of pure titanium is assumed to be 0% strength for each element, and the chemical composition of the target α + β alloy, such as Ti-5Al-2Fe-3Mo, Ti-6Al-4V alloy or Ti- A calibration curve is created using the measurement results of a model alloy of the chemical composition of the 5Al-1.5Fe-Si alloy. For example, for a model alloy with a chemical composition of Ti-5Al-2Fe-3Mo, a small arc-melted ingot (approximately 100 g) is produced and heated to 1100 ° C. to eliminate solidification defects. The above processing is performed to form a cross-sectional area of 100 mm ² or more. It can be produced by annealing this at 1100° C. for 10 to 60 minutes and air cooling. After removing 1 mm of the surface layer of this, each element is analyzed by ICP emission spectrometry, and this is used as the composition of the model alloy. Therefore, even when creating a calibration curve, the surface layer of 1 mm is not included in the measurement range, and the measurement is performed in the widest possible area. The measurement at this time is desirably surface analysis, and the average value may be used as the measurement result. The same applies to other alloy compositions.

When processed into a product such as an engine valve, evaluation is possible by performing the evaluation on a cut surface obtained by cutting so as to secure a measurement area.

<<Crystal structure of titanium alloy (2)>>
(Percentage of acicular tissue is 80% or less)
An equiaxed structure and an acicular structure appear as crystal structures in the α+β type titanium alloy. Compared to the acicular structure, the equiaxed structure has higher ductility at room temperature and is superior in handleability at room temperature. In addition, since the acicular structure has low ductility at room temperature, the less the acicular structure, the better the handleability at room temperature.
One of the reasons for the low ductility of the acicular structure is the existence of grain boundary α-grains. However, in a high alloy such as that of the present invention, when the needle-like structure ratio is high, the ductility at room temperature is low even if grain boundary α-grains do not exist. Therefore, the ratio of acicular tissue should preferably be low. The ratio of needle-like structures is an area ratio, preferably 80% or less. More preferably 75% or less, still more preferably 70% or less.

The acicular texture here is a region in which finely elongated α-phase precipitates in the β-phase. Since the α-phase in the needle-like structure is fine, it is difficult to recognize individual crystal grains. Therefore, using the results obtained by the EBSD method, analysis is performed by a method described later.
EBSD measurements are made on cross-sections perpendicular to the axis in the case of titanium alloy bars, for example round bars. The measurement conditions are 500 times, a region of 200 μm or more, and a step size of 0.2 μm. Under these conditions, a total of 3 or more fields of view, ie, 1 field of view at the center of the round bar and 2 fields of view at R/2, are measured. Measurement at R/2 is performed at an arbitrary point and a position rotated 90° from that point with respect to the axis of the round bar. In the case of sheet material, the cross section is perpendicular to the rolling direction. Under the same conditions as for the round bar, measurements are made for a total of 3 or more fields of view at the central position of the thickness of the plate material: 1 field of view at the center of the width, 2 fields of 1/4 width, and 2 fields of 3/4 width.

First, in the measurement results, the crystal grain size of each crystal is obtained with a grain boundary having a misorientation of 5° or more, and the average crystal grain size in the field of view is obtained based on the obtained value.

When the average crystal grain size is 100 μm or more, the ratio of the acicular structure is 100%. The acicular structure is formed by transforming the β phase into the α phase by cooling. In addition, the formed α-grains form regions called colonies in which those having substantially the same crystal orientation are alternately present with very thin β-grains. However, since the β-phase is very thin, it is difficult to detect by measurement, and it is identified that α-grains that should be recognized as different α-grains are adjacent to each other. As a result, in the analysis for calculating the average grain size, the α-grains to be judged as each are judged to be the same grain, and the colony is judged to be one crystal grain. In this case, the crystal grain size of the α grains is only about 30% of the β grain size at the smallest. The β grain size is easily 500 μm or more when the β phase rate is 100%. However, it is difficult for the average grain size of the α grains to exceed 100 μm even if the heat is maintained for a long time in the presence of the α grains having an equiaxed structure. Therefore, when the average crystal grain size is 100 μm or more, it can be determined that 100% of the grains have an acicular structure. On the other hand, when the β phase ratio is less than 100%, the β grains do not become coarse due to the pinning effect of the α grains, and it is difficult for the average grain size of the α grains after cooling to be 100 μm or more. Therefore, when the average grain size of α grains is less than 100 μm, a mixture of acicular and equiaxed structures is obtained.

When the average crystal grain size is less than 100 μm, data with an image quality (IQ) of 20% or more of the average is first extracted. Among the extracted data, α-grains (equiaxed α-grains) with an aspect ratio (major axis/minor axis) of 3 or less are determined, the area ratio of equiaxed α-grains in the field of view is determined, and the equiaxed α-grain ratio ( area %). The crystal structure is a structure in which equiaxed α-grains and needle-like structures are mixed. In the needle-like structure, fine needle-like α-grains are precipitated in the β-phase. Assuming that the sum of the equiaxed α grain ratio (area%) and the needle-like structure ratio (area%) is 100%, the needle-like structure ratio (area%) (described as “needle-like structure ratio” in Table 3) is asked. In addition, the ratio of the acicular structure was determined as the ratio of the area to the measured area.

That is, the ratio of the needle-like structure is evaluated by EBSD evaluation in the cross section perpendicular to the axis in the case of alloy rods and in the cross section perpendicular to the rolling direction in the case of plate materials, and the measurement results show that the crystal grains have a misorientation of 5 ° or more as a grain boundary. If the average crystal grain size is 100 μm or more, the ratio of the needle-like structure is set to 100% by area, and if the average crystal grain size is less than 100 μm, the image quality (IQ) is 20% or more of the average. Among the extracted data, α grains with an aspect ratio (major axis/minor axis) of 3 or less are defined as equiaxed α grains, and the area ratio occupied by other than the equiaxed α grains to the total measured area is the ratio of the acicular structure. (area %).

<<Manufacturing method of titanium alloy>>
(β transformation point temperature)
The β-transformation temperature Tβ (°C) used in the manufacturing method below can be expressed as an expression of the main elements and the contents of O, N, and C in the titanium alloy. For Ti-5Al-2Fe-3Mo alloys, it can be obtained by calculation based on the formula (1). For Ti-6Al-4V alloys, it can be obtained by calculation based on the formula (2).
Tβ (°C) = 880 + 23 x Al + 170 x (O + N) + 100 x C-20 x Mo-8 x Fe (1)
Tβ (°C) = 875 + 23 x Al + 170 x (O + N) + 100 x C-12 x V (2)
The element symbol is the mass % value of the alloying element.
For α+β type titanium alloys other than the above two alloy systems, Tβ (° C.) can be obtained from the equation (3).
Tβ (°C) = 905 + 20 x Al + 170 x (O + N) + 100 x C-20 x Mo-15 x Fe-12V (3)

(ingot)
An ingot manufactured by a general method is used as the ingot. For example, VAR, EB melting, and plasma melting. The ingot may be rectangular or cylindrical, and surface processing such as cutting of the ingot is performed as necessary. When using an ingot of 1 ton or more as the ingot, the effect of the present invention can be particularly exhibited.

(billet forging)
The ingot is forged into a rolled bar, for example, a billet that serves as a raw material for a round bar. The shape of the billet may be any shape such as a round cross-section, square, rectangle, octagon, etc., as long as it is possible to manufacture a rolled round bar or plate material. The forging process consists of two processes: rough forging and billet finish forging. Any forging process must be performed at a temperature higher than the β transformation temperature.

Rough forging is performed to eliminate internal defects formed by solidification shrinkage existing in the ingot and to eliminate coarse solidified structures. Furthermore, it also serves to disperse the segregated structure using the metal flow due to processing.

The ingot is heated to a temperature above the β transformation point and below 1200° C. to form a round cross section of φ200 to 250 mm or a square cross section of 200 to 250 mm square. Further, instead of the square cross section, an octagonal cross section corresponding to these cross sections may be used. In the case of a plate, it may have a rectangular cross section. The heating temperature is preferably +50°C, more preferably +100°C or higher than the β transformation point. The upper limit temperature is set to 1200° C. because the yield is greatly reduced due to oxidation even if the temperature is raised above this value.

The present invention achieves a reduction in variation in the alloy composition by performing one or more upsets in the forging. When a round bar-shaped product is manufactured by normal forging and hot rolling, the round bar is rolled down in the radial direction, but not in the length direction, so metal flow is insufficient to alleviate segregation. becomes. On the other hand, by performing upsetting one or more times, the segregation is relaxed and the dispersion of the alloying elements is reduced. It is desirable that the upsetting is performed at a stage where the ratio (L/Ls) of the length Ls of the short side and short axis of the cross section to the length L is 1 to 4 or less. Care must be taken when L/Ls exceeds 4, because the greater the L/Ls, the more likely it is to buckle. The upsetting is performed at a rolling reduction rate of 20% or more and 60% or less. It is more desirably 25% or more, more preferably 30% or more. In addition, one heat may be used only for the upsetting, or the same heat for forging may be used. The same applies to plate materials.

Depending on the ingot size and forging speed, cracking may occur due to the temperature drop before the full length is forged. In this case, a reheat will be performed. The reheat at this time is also treated as one heat.

Rough forging is performed with a plurality of heats. This is because the temperature of the surface of large ingots in mass production drops during the process and cracks occur, and by maintaining a high temperature above the β transformation point for a long time, a more uniform element distribution can be obtained. . The number of heats is preferably 4 or more, more preferably 6 or more. In addition, it is necessary to carry out processing with a cross-sectional reduction rate of 20% or more in one heat. However, if the upsetting is performed in the same heat, the cross-sectional reduction rate may be less than 20%. The reason why the cross-section reduction rate is set to 20% or more is that if a large ingot is repeatedly processed to a reduction of less than 20%, the productivity drops significantly.

Finish forging is then performed. After the rough forging, the steel may be cooled to around room temperature, or may be immediately reheated for finish forging. A step of removing surface scales and wrinkles may be added before finish forging.

Finish forging is the process of shaping the material for hot rolling, and the finished size varies depending on the size of the manufactured product. In the case of a bar material, the cross-sectional area is 2000 mm ² or more considering the reduction in yield due to oxidation. Although the upper limit is not particularly limited, the cross-sectional area is preferably 18000 mm ² or less. Also, the shape may be any shape that can be rolled, such as an octagonal or square cross-sectional area corresponding to these. In the case of a plate shape, it is rectangular and has a thickness of 50 mm or more. It is desirably 70 mm or more, and more desirably 100 mm or more. The upper limit of the plate thickness and the width size are not particularly limited as long as they are sizes that can be rolled.

The heating temperature for finish forging is the same as that for rough forging, and is preferably +50°C higher than the β transformation point, more preferably +100°C or higher. The number of heats should be 4 or more, preferably 6 or more. However, there is no lower limit to the cross-sectional reduction rate for each heat, and heats that only heat and do not reduce may also be used. In the finish forging process, only forging to reduce the cross-sectional area by pressing from the side is performed without upsetting.

(hot rolling)
After finish forging, hot rolling is performed to form titanium alloys, particularly titanium alloy rods and titanium alloy plates. After the finish forging, it may be cooled to around room temperature, or immediately reheated and hot rolled. A step of removing surface scales and wrinkles may be added before hot rolling.

Hot rolling is performed by heating to a temperature higher than the β transformation point and lower than or equal to 1200°C. Desirably, the temperature is +50°C higher than the β transformation point, more preferably +100°C or higher. As with forging, the upper limit temperature is limited in terms of yield. A preferred upper limit is 1150°C.

Hot rolling is preferable if the area reduction rate is 70% or more in one heat, since the maximum area of the segregation abnormal part can be reduced and the ratio of the acicular structure can be reduced. It is preferably 80% or more, more preferably 85% or more. Although the upper limit is not particularly limited, it is approximately 99% in relation to the product diameter. In manufacturing by hot rolling, if thin products are manufactured, the subsequent descaling, etc., will greatly reduce the yield. Therefore, in the case of bars, the lower limit of the product diameter of the hot-rolled material is φ10 mm. In the case of plate material, the lower limit of the product plate thickness of hot rolled material is 3 mm.

During production, if the β phase is kept at around 300°C in an unstable state, the ω phase may be significantly formed. The partitioning of alloying elements in the β phase occurs, and the β phase is stabilized, so it is rarely formed. If the presence of the ω phase is still a concern, it can be resolved by annealing after hot rolling.

(annealing and descaling)
As mentioned above, the annealing of hot-rolled titanium alloy rods and titanium alloy sheets is a process that must be carried out when the precipitation of the ω phase is suspected or confirmed. or strain removal. Further, in applications where the use environment is 200 to 400° C., precipitation of the ω phase during use causes embrittlement, so annealing can suppress the precipitation of the ω phase during use. Annealing is performed at a temperature of 750° C. or more and less than the β transformation point. A higher temperature is desirable in terms of uniformity of elements. It is more preferably 780° C. or higher, still more preferably 800° C. or higher. In order to make the ratio of the acicular structure 80% or less, it is preferable not to perform annealing, or to perform annealing at a temperature lower than the β transformation point when annealing is performed.

Scales formed by annealing or hot rolling may be removed by mechanical and chemical methods such as shot blasting, pickling and polishing.

《Titanium alloy rod and titanium alloy plate》
As described above, when the titanium alloy is a bar material, for example, a round bar, the area reduction rate during hot working is smaller than that of a plate material, so fracture due to segregation of alloying elements becomes apparent. In contrast, the titanium alloy bar and titanium alloy round bar of the present invention have a small maximum area of abnormal segregation and can achieve excellent fatigue properties. A titanium alloy rod having a cross-sectional area of 80 mm ² to 8000 mm ² can be suitably used. On the other hand, the plate material has a thickness of 3 mm to 50 mm. It is preferably 30 mm or less, more preferably 10 mm or less.

《Engine valve》
An engine valve using a titanium alloy is manufactured using a titanium alloy round bar as a material, as described above. Conventional engine valves using titanium alloys have a large maximum area of abnormal segregation, but the engine valve of the present invention uses the titanium alloy of the present invention, so the maximum area of abnormal segregation is small and excellent fatigue properties can be realized.

Variety No. having the component composition shown in Table 1. For A to F, VAR ingots (1.8 tons) of φ720×1000 mm were produced. For one example (No. 31 in Table 3), it was 0.2 tons. Table 2 shows manufacturing method numbers. 1 to 15 are described. As shown in Table 3, variety No. Using an ingot having the components A to F and having the weight shown in "ingot/ton" in Table 3, manufacturing method No. 2 in Table 2 was used. Rough forging, finish forging, and hot rolling were performed under the conditions described in 1 to 15 to obtain round bars having diameters shown in Table 2, “hot rolling/diameter/after rolling”. In Table 2, numerical values and items outside the preferred range of the present invention are underlined.

Rough forging is carried out at the rate of area reduction described in "rough forging/area reduction rate (%)" for heats 1 to 4, as described in "rough forging/heat No." in Table 2. rice field. In the example described as "yes" in the "rough forging/upsetting" column, in heat 1 of rough forging, the upsetting was performed with a rolling reduction of 20%. No. 10, 13 to 15 are not indented. The rolling reduction described in heat 1 of rough forging is a rolling reduction in consideration of the effect of upsetting.

After the rough forging, it was cooled to room temperature and cut at 5 mm/face to form a 240 mm square cross section. After the rough forging, it was divided in the length direction and subjected to finish forging and subsequent processes under various conditions.

For finish forging, heat to the temperature described in the "finish forging/heating temperature" column in Table 2, and as described in "finish forging/heat No." Finish forging was performed at the area reduction rate described in "/area reduction rate (%)".
No. For 1 to 7 and 11 to 15, half the length was forged and the rest was forged after reheating. Therefore, the cross-sectional shape was uniform over the entire length in two heats. For example, No. In 1, the front half was forged in the first heat with a cross-sectional reduction rate of 65%, and the rear half was not forged. In the second heat, the front half was not forged, and the rear half was forged with a cross-sectional reduction rate of 65%. Furthermore, in the third heat, the front half was forged at a cross-sectional reduction rate of 44%, and the rear half was not forged. In the fourth heat, the front half was not forged, and the rear half was forged with an area reduction rate of 44%.
No. For 8 to 10, the entire length was forged in one heat. No. 8 was forged in the first heat with a cross-sectional reduction rate of 65% over the entire length, and in the second heat with a cross-sectional reduction rate of 44%. No. No. 9 was forged at a cross-sectional reduction rate of 45, 36, 23, and 27% in the first to fourth heats, respectively.

After the finish forging, it was cooled to room temperature, 5 mm of the surface layer was removed by cutting, and hot rolling was performed under the conditions described in Table 2, "Hot Rolling" column. The hot-rolled round bars were heat-treated under the heat treatment conditions described in the "annealing" column of Table 2. Examples with "-" in the "annealing" column were not annealed.

EPMA analysis, X-ray diffraction, EBSD measurement, tensile test, and fatigue test were performed on the round bar manufactured above. Table 3 shows the quality evaluation results. In Table 3, numerical values outside the scope of the present invention are underlined.

The elemental distribution for evaluation of segregation anomalies was measured by EPMA after mirror-polishing a cross section perpendicular to the axis of the round bar. The EPMA measurement was performed at an accelerating voltage of 10 kV, a step size of 0.02 mm, and a beam diameter of 0.02 mm at the center of a 5 mm square region.
In the Ti-5Al-2Fe-3Mo titanium alloy, in the EPMA surface analysis, a set of adjacent measurement points where Al and Fe are outside the average value of ±1% by mass and Mo is outside the average value of ±1.5% by mass. The body was used as the segregation anomaly of the element concentrations of Al, Fe, and Mo in the cross section. Table 3 shows the maximum areas of segregation anomalies in the element concentrations of Al, Fe, and Mo.
In the Ti-6Al-4V titanium alloy, in the EPMA surface analysis, a set of points where the measurement points where the Al is outside the average value ± 1% by mass and the V is outside the average value ± 2% by mass are adjacent to each other in the cross section. was defined as the segregation anomaly of the element concentrations of Al and V. Table 3 shows the maximum areas of segregation anomalies in the element concentrations of Al and V.
For α+β-type titanium alloys other than the above two-component system, aggregates of adjacent measurement points that deviate from the average value ± 1% by mass for Al and Fe, which are the main elements contained, are measured in the cross section for Al and Fe, respectively. was regarded as the segregation anomaly of element concentration.

In the X-ray diffraction, the range of 2θ from 30 to 90° was measured with a Cu—Kα ray at a scan step of 0.01° to confirm the presence or absence of the ω phase.

In the EBSD measurement, the cross section perpendicular to the axis of the round bar is colloidal silica polished, the acceleration voltage is 15 kV, the center of the cross section, the position corresponding to the center of the line segment connecting the surface and the center in the radial direction (R / 2 position), R The measurement was performed in three fields of view at positions where the /2 position was rotated clockwise by 90° in the circumferential direction with respect to the center of the axis of the round bar. The measurement conditions were a magnification of 500 times, a 200 μm square area, and a step size of 0.2 μm. After the measurement, the crystal grain size was determined with a misorientation of 5° or more as the grain boundary, and when the average crystal grain size was 100 μm or more, the ratio of the needle-like structure was set to 100 area %. If the average grain size is less than 100 μm, extract data with an image quality (IQ) of 20% or more of the average, and select α grains with an aspect ratio (long axis/short axis) of 3 or less from the extracted data. The ratio of acicular texture to the total measured area occupied by other than the equiaxed α grains is shown in Table 3 as "ratio of acicular texture".

In the tensile test, using a test piece with a parallel part diameter of φ6.25 mm and a parallel part length of 28 mm from the axis center of the round bar, the distance between the gauge points is 25 mm, and the strain is 0.4% / min up to 2%, and thereafter. was performed at 25%/min until breakage, and the breaking elongation was measured by the butt method.

In the fatigue test, a rotating bending fatigue test piece with a parallel portion of φ6 mm is prepared, and the surface of the parallel portion is finished with #1000 emery paper in the circumferential direction of the parallel portion. Using this test piece, a rotating bending fatigue test is performed with a stress amplitude of 560 to 640 MPa. The stress amplitude adopts a value within the range of 0.54 to 0.56 times the tensile strength. Moreover, the stress amplitude was changed by 20 MPa and set to a predetermined stress amplitude. At this time, when multiple stresses are 0.54 to 0.56 times the tensile strength, the test is performed at the higher stress amplitude. Within this range, the stress level is slightly lower than the fatigue limit, and it is possible to judge the quality. The repetition rate is 50-60 Hz. If it reaches 1 × 10 ⁷ times without breaking under these test conditions, it is marked as a pass, and if it is not reached, it is marked as a failure, and it is indicated in the "fatigue characteristics" column of Table 3. Described.

Inventive Example No. 1 to 14 have the chemical composition of the present invention shown in Table 1, and as a result of manufacturing under the suitable manufacturing conditions of the present invention as shown in Table 2, as shown in Table 3, any alloy element The maximum area of the segregation abnormality was within the range of the present invention, and the fatigue properties were good. Inventive Example No. In No. 4 (manufacturing method No. 4), the annealing temperature after hot rolling was outside the upper limit of the preferred range (β transformation point temperature), so the needle-like structure ratio (acicular structure ratio) was outside the preferred range of the present invention. However, the fatigue property was good.

Comparative example no. 15 to 31 are comparative examples.
Comparative example no. In 15 and 23 (manufacturing method No. 8), the number of heats for finish forging was two, and the number of times of processing and the number of times of holding at high temperature were reduced, so the diffusion of elements was insufficient, and the maximum area of segregation abnormality was obtained. is outside the scope of the present invention.
Comparative example no. In No. 16 and 24 (manufacturing method No. 9), the heating temperature of the finish forging is outside the lower limit of the preferred range, and comparative example No. 17, 25 (manufacturing method No. 10) and No. Comparative Example Nos. 20 to 22 and 28 to 31 (manufacturing methods 13 to 15) were not subjected to upsetting in rough rolling. In No. 18 and 26 (manufacturing method No. 11), the heating temperature in hot rolling was outside the lower limit of the preferred range (β transformation point + 50°C), and comparative example No. In Nos. 19 and 27 (manufacturing method No. 12), the rolling reduction in hot rolling is off the lower limit (70%) of the preferred range. Therefore, No. In all of Nos. 16 to 22 and 24 to 31 (manufacturing method Nos. 9 to 15), the maximum area of the abnormal segregation part of any alloy element was outside the scope of the present invention.
As a result, Comparative Example No. All of Nos. 15 to 31 (manufacturing method Nos. 8 to 15) had poor fatigue properties.

Comparative example no. No. 19 (manufacturing method No. 12) had a large diameter after hot rolling and a low area reduction rate after hot rolling. was off.

Comparative example no. In Nos. 15 and 23 (manufacturing method No. 8), the unstable β phase formed by annealing transformed into the α phase during cooling, resulting in an increase in the ratio of needle-like structures, which was out of the preferred range of the present invention.

Comparative example no. In No. 31, the weight of the ingot was 0.2 tons and less than 1 ton, but the manufacturing method was the manufacturing method No. 31. 14, which is out of the preferred range of the present invention, the maximum area of the abnormal segregation portion is out of the range of the present invention, and the fatigue properties are also poor.

Variety No. having the component composition shown in Table 4. For G and H, VAR ingots (1.8 tons) of φ720×1000 mm were produced. Table 5 shows manufacturing method No. 16 and 17 manufacturing conditions are described. The ingot was produced by the manufacturing method No. in Table 5. Rough forging, finish forging, and hot rolling were performed under the conditions described in 16 and 17, and the plate material shown in Table 5, "hot rolling/plate thickness/after rolling" was obtained. In Table 5, numerical values and items outside the preferred range of the present invention are underlined.

Rough forging is carried out at the rate of area reduction described in "rough forging/area reduction rate (%)" for heats 1 to 4, as described in "rough forging/heat No." in Table 5. rice field. In the example described as "yes" in the "rough forging/upsetting" column, in heat 1 of rough forging, the upsetting was performed with a rolling reduction of 20%. No. 17 has not been set. The rolling reduction described in heat 1 of rough forging is a rolling reduction in consideration of the effect of upsetting.
After the rough forging, it was cooled to room temperature and cut at 5 mm/face to obtain a cross section of 290 mm×490 mm. After rough forging, it was split lengthwise.

For finish forging, heat to the temperature described in the "finish forging/heating temperature" column of Table 5, and as described in "finish forging/heat No." /Reduction of area (%)” was performed to achieve a width of 500 mm.
Production method No. of Example 1; As with 1-7, half the length was forged and the rest was forged after reheating. Therefore, the cross-sectional shape was uniform over the entire length in two heats.

After finish forging, it was cooled to room temperature, 5 mm of the surface layer was removed by cutting, and hot rolling was performed under the conditions described in Table 5, "Hot Rolling" column. The plate material after hot rolling was not annealed.

Various evaluations were performed in the same manner as in Example 1. In the X-ray diffraction, the cross section perpendicular to the rolling direction was measured at the center of the plate thickness at the center of the width. In the tensile test and the fatigue test, a rectangular bar was cut from the center of the plate thickness at the center of the width so that the longitudinal direction was the rolling direction, and the same test piece was processed and tested. The EPMA surface analysis sample for segregation abnormality evaluation was a cross section perpendicular to the rolling direction. In the EBSD measurement, in a cross section perpendicular to the rolling direction, a total of 3 fields of view, 1 field of width center, 2 fields of 1/4 width, and 3/4 width, were measured at the thickness center position of the plate material.

EPMA analysis, X-ray diffraction, EBSD measurement, tensile test, and fatigue test were performed on the round bar manufactured above in the same manner as in Example 1. Table 6 shows the quality evaluation results. In Table 6, numerical values outside the scope of the present invention are underlined.
Inventive Example No. 32 is the variety No. in Table 4; It has the component composition of the present invention shown in Table 5, and the manufacturing method No. in Table 5. No. 16 in Table 6 was produced under the preferred production conditions of the present invention as shown in Table 6. As shown in No. 32, the maximum area of the segregation abnormality was within the range of the present invention for all alloying elements, and the fatigue properties were good.
Comparative example no. No. 33 (manufacturing method No. 17) was rough forged without upsetting, and the maximum area of abnormal segregation of Fe and Mo was outside the range of the present invention, resulting in poor fatigue properties.

Claims (10)

It has a two-phase structure consisting of an α phase and a β phase, contains 2.5% by mass or more of Al and at least 0.8% by mass or more of one or more major elements other than Al, and has a concentration of the major elements in the cross section. A titanium alloy with a maximum area of segregation anomalies of 2000 μm ² or less.
Here, the main elements are Al, Fe, Mo, and V, and the segregation abnormal portion of the main element concentration in the cross section is an EPMA surface analysis in which the main elements other than Mo and V have an average value of ± 1% by mass, Means a collection of adjacent measurement points where Mo is outside the average ±1.5% by mass and V is outside the average ±2%. In % by mass,
Al: 4.5-6.5%,
Fe: 1.4-2.3%,
Mo: contains 1.5 to 5.5%,
2. The titanium alloy according to claim 1, wherein the total content of O and N is 0.25% or less, the balance being Ti and impurities. In % by mass,
Al: 5.0 to 7.0%,
V: 3.5-5.0%,
2. The titanium alloy according to claim 1, wherein the total content of O and N is 0.25% or less, the balance being Ti and impurities. In % by mass,
Al: 4.5-6.5%,
Fe: 0.8-2.3%,
Si: 0.0 to 0.50%,
2. The titanium alloy according to claim 1, wherein the total content of O and N is 0.25% or less, the balance being Ti and impurities. A titanium alloy rod made of the titanium alloy according to any one of claims 1 to 4. 6. The titanium alloy rod according to claim 5, wherein the ratio of needle-like structures is 80 area % or less.
Here, the ratio of the needle-like structure is obtained by performing EBSD evaluation on the cross section perpendicular to the axis of the titanium alloy rod, and obtaining the crystal grain size with the misorientation of 5° or more as the grain boundary in the measurement result. If the ratio of the needle-like structure is 100% by area, and if the average crystal grain size is less than 100 μm, extract data with an image quality (IQ) of 20% or more of the average, and extract the aspect ratio (length An α-grain having a ratio of axis/minor axis) of 3 or less is defined as an equiaxed α-grain, and the ratio of the area occupied by other than the equiaxed α-grain to the total measured area is defined as the acicular structure ratio. A titanium alloy plate made of the titanium alloy according to any one of claims 1 to 4. 8. The titanium alloy plate according to claim 7, wherein the acicular structure has a ratio of 80 area % or less.
Here, the ratio of the needle-like structure is obtained by performing EBSD evaluation on a cross section perpendicular to the rolling direction of the titanium alloy plate, obtaining the crystal grain size with a misorientation of 5° or more as the grain boundary in the measurement result, and obtaining an average crystal grain size of 100 μm or more. In the case of , the ratio of the acicular structure is set to 100% by area, and when the average grain size is less than 100 μm, data with an image quality (IQ) of 20% or more of the average is extracted, and the aspect ratio ( α-grains having a long axis/minor axis) of 3 or less are defined as equiaxed α-grains, and the ratio of the area occupied by other than the equiaxed α-grains to the total measured area is defined as the acicular texture ratio. An engine valve made of the titanium alloy according to any one of claims 1 to 4. 10. The engine valve according to claim 9, wherein the acicular structure has a ratio of 80 area % or less.
Here, the ratio of the acicular structure is obtained by performing an EBSD evaluation on a cross section perpendicular to the axis of the engine valve, obtaining the crystal grain size with a misorientation of 5° or more as the grain boundary in the measurement results, and when the average crystal grain size is 100 μm or more. is assumed to be 100% by area of the acicular structure, and when the average crystal grain size is less than 100 μm, data with an image quality (IQ) of 20% or more of the average is extracted, and the aspect ratio (major axis /minor axis) is 3 or less, and the area ratio occupied by other than the equiaxed α grains to the total measured area is defined as the ratio of the acicular structure.