JP7372532B2 - Titanium alloy round rod and connecting rod - Google Patents

Description

The present invention relates to a titanium alloy round bar and a connecting rod.

Titanium alloys have good tensile strength and elongation, and are lightweight alloys. Therefore, from the viewpoint of improving fuel efficiency, it is sometimes used in parts called connecting rods that constitute engines of automobiles and the like. Titanium alloy is suitable as a material for connecting rods because of its light weight. However, there are some problems when using titanium alloys for connecting rods. Although titanium alloy has high tensile strength, it is flexible and has a relatively low Young's modulus.

If the Young's modulus is low, it is easy to deform, which may cause problems in terms of engine safety. Therefore, in order to ensure the Young's modulus, it is necessary to increase the thickness of the part, and the advantage of light weight of titanium alloys has not been fully utilized.

Japanese Patent Application Publication No. 2010-7166

Patent Document 1 discloses a titanium alloy for casting containing 5.5 to 7.0% Al. It is thought that by including Al as in Patent Document 1, the strength is improved and the Young's modulus is also improved.

On the other hand, when Al is added, hot workability decreases and it becomes necessary to mold the part into the shape of the part by casting. The connecting rod is repeatedly loaded by the driving force generated by the piston. For this reason, it is preferable to perform forging from a shape such as a round bar, for example, rather than casting, which tends to leave casting defects and reduce fatigue strength. However, when Al is contained, there is a problem that sufficient hot workability to withstand the forging process required for forming parts cannot be obtained.

An object of the present invention is to solve the above problems and provide a titanium alloy round bar and connecting rod that have good tensile strength and ductility, as well as high Young's modulus and hot workability.

The present invention has been made to solve the above problems, and its gist is the following titanium alloy round bar connecting rod.

(1) A titanium alloy round bar extending in one direction,
The chemical composition is in mass%,
Al: 7.0-9.0%,
O: 0.20% or less,
N: 0.010% or less,
C: 0.010% or less,
H: 0.013% or less,
Nb: 0 to 2.00%,
Si: 0 to 0.30%,
Fe: 0-2.00%,
Cr: 0-2.00%,
The remainder: Ti and impurities,
The following formulas (i) to (iii) are satisfied,
In the metal structure, the area ratio has an α phase of 85.0% or more,
The equiaxed ratio of α-phase crystal grains is 70% or more,
When θ is the angle between the longitudinal direction of the titanium alloy round rod and the c-axis direction of the α-phase crystal lattice, θ is 0° or more and 45° or less for all α-phase crystal grains to be measured. A titanium alloy round bar in which the proportion of α-phase crystal grains is 15.0% or more.
Al+10×O≦10.5...(i)
0.50≦Nb+10×Si...(ii)
1.00≦2×Fe+5×Cr...(iii)
However, each element symbol in the above formula represents the content (mass %) of each element contained in the titanium alloy, and is zero if it is not contained.

(2) A connecting rod using the titanium alloy round bar described in (1) above.

According to the present invention, it is possible to obtain a titanium alloy round bar and connecting rod that have good tensile strength and ductility, as well as high Young's modulus and hot workability.

FIG. 1 is a diagram schematically showing the c-axis direction in the hcp structure. FIG. 2 is a diagram schematically showing the angle θ between the longitudinal direction of the alloy round bar and the c-axis direction.

The present inventors conducted various studies in order to obtain a titanium alloy having good tensile strength and ductility, as well as high Young's modulus and hot workability. As a result, the following findings (a) to (d) were obtained.

(a) Young's modulus can be improved by including Al in the titanium alloy. In addition, Al is a lightweight metal and reduces density, so it is desirable from the viewpoint of weight reduction. On the other hand, since Al increases deformation resistance during hot working, when Al is included, it is inevitably necessary to increase the hot working temperature.

In this case, significant oxidation occurs, and hot workability may deteriorate, such as cracks or oxide scales occurring on the surface of the material. Further, titanium alloy has an hcp structure and has strong anisotropy, so the yield during hot working is poor. Therefore, it is also required from the viewpoint of hot workability to reduce anisotropy and improve yield.

(b) Therefore, it is necessary to limit the Al content to a predetermined amount. In addition, it is also necessary to limit the content of O, which has similar effects, to a certain amount. Furthermore, it is desirable to improve the oxidation resistance by containing a predetermined amount of Nb and/or Si at high hot working temperatures by containing Al.

(c) In addition, in order to ensure a high Young's modulus, it is necessary to make the amount of α phase formed equal to or greater than a predetermined value. On the other hand, in order to ensure hot workability, the composition is designed such that a certain amount of β phase is included in the metal structure of the titanium alloy. This is because the β phase reduces Young's modulus but improves hot workability. That is, it is necessary to control the chemical composition and manufacturing conditions of the titanium alloy so as to adjust the metal structure of the titanium alloy.

(d) Furthermore, in order to improve hot workability, it is preferable to make the crystal grains of the titanium alloy equiaxed and to reduce anisotropy. For this purpose, it is preferable to control the crystal grains formed during the transformation from the β phase to the α phase by controlling the solution treatment conditions during production.

The titanium alloy round bar according to the present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be explained in detail.

The titanium alloy round bar according to the present invention has a shape extending in one direction.

1. Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

Al: 7.0-9.0%
Al has the effect of improving high temperature strength and stabilizing the α phase up to a high temperature range. It also has the effect of improving Young's modulus. By containing Al, good Young's modulus and low density can be achieved. For this reason, the Al content is set to 7.0% or more, preferably 7.5% or more. However, when the Al content becomes excessive, α ₂ phase (Ti ₃ Al phase), which is a brittle phase, begins to precipitate, and ductility may decrease. For this reason, the Al content is 9.0% or less, preferably 8.5% or less.

O: 0.20% or less O is an impurity element contained in the titanium alloy, and has the effect of improving strength. Therefore, the strength can be adjusted by limiting the O content to a predetermined amount. However, excessive O content promotes precipitation of _α2 phase, which is a brittle phase. Also, the ductility at room temperature decreases. Therefore, the O content is set to 0.20% or less. The O content is preferably 0.18% or less, more preferably 0.15% or less. On the other hand, excessive reduction in O content increases manufacturing costs, so it is preferable that O content is 0.01% or more.

Here, both Al and O are elements that contribute to improving the strength. On the other hand, if both elements are contained in excess, the _α2 phase, which is a brittle phase, tends to precipitate, so it is necessary to satisfy the following formula (i).

Al+10×O≦10.5...(i)
When the value on the left side of equation (i) exceeds 10.5, _α2 phase, which is a brittle phase, precipitates, reducing hot workability. Therefore, the value on the left side of equation (i) is set to 10.5 or less. The left-hand side value of equation (i) is preferably 10.0 or less, more preferably 9.5 or less. The lower limit of the value on the left side of equation (i) is not particularly determined, but is generally considered to be 7.1 or more.

N: 0.010% or less C: 0.010% or less N and C are impurities contained in the titanium alloy, and may reduce mechanical properties. The N content shall be 0.010% or less. Further, the C content is 0.010% or less. It is preferable to reduce the N and C contents as much as possible.

H: 0.013% or less H is an impurity element contained in titanium alloys and may cause embrittlement. Therefore, the H content is set to 0.013% or less. The H content is preferably 0.010% or less, more preferably 0.008% or less.

Nb: 0-2.00%
Si: 0-0.30%
Nb and Si are both β-stabilizing elements and have the effect of improving oxidation resistance. In the titanium alloy round bar according to the present invention, the Young's modulus is improved by containing Al, and it becomes necessary to increase the hot working temperature. Therefore, it may be included if necessary. However, when Nb is contained excessively, manufacturing costs increase. Therefore, the Nb content is set to 2.00% or less. Moreover, when Si is contained excessively, intermetallic compounds are formed and the fatigue strength is reduced. Therefore, the Si content is set to 0.30% or less.

In addition, titanium alloys are easily oxidized, and the above-mentioned increase in hot working temperature causes a decrease in yield. Specifically, the decrease in yield is considered to be due to the fact that the oxide scale is pushed in during processing, the hardened layer formed directly under the oxide scale becomes thicker, and it cannot be removed sufficiently. Therefore, in the titanium alloy round bar according to the present invention, the Nb and Si contents satisfy the following formula (ii).

0.50≦Nb+10×Si...(ii)
If the value on the right side of formula (ii) is less than 0.50, oxidation resistance that can withstand increases in hot working temperature cannot be obtained. Therefore, the value on the right side of equation (ii) is preferably 0.50 or more, more preferably 0.80 or more.

Fe: 0-2.00%
Cr: 0-2.00%
Fe is a eutectoid β-stabilizing element and has the effect of stabilizing the β phase. By containing Fe, the β phase can be ensured and hot workability can be improved. Furthermore, since Fe is less distributed in the α phase, it is possible to suppress the increase in strength of the α phase at high temperatures. V and Mo, which are commonly used as optionally added elements, are completely solid solution type elements, but compared to these elements, they are effective in improving elemental hot workability and are also inexpensive. Therefore, it may be included if necessary. However, if Fe is contained excessively, segregation may occur in the ingot. Therefore, the Fe content is set to 2.00% or less.

Like Fe, Cr is also a eutectoid β-stabilizing element and has the effect of significantly stabilizing the β phase. For this reason, Cr may be contained as necessary. However, if Cr is contained excessively, segregation may occur in the ingot, similar to Fe. Therefore, the Cr content is set to 2.00% or less.

Further, by adding Fe and Cr in combination, a certain amount of β phase can be formed. Therefore, in the titanium alloy round bar according to the present invention, at least one of Fe and Cr is contained, and the Fe and Cr contents satisfy the following formula (iii).

1.00≦2×Fe+5×Cr...(iii)
If the right-hand side value of formula (iii) is less than 1.00, sufficient β phase cannot be ensured, and good hot workability and ductility cannot be obtained. Therefore, the value on the right side of equation (iii) is set to 1.00 or more. The value on the right side of formula (iii) is preferably 1.50 or more, more preferably 2.50 or more.

Note that each element symbol in the above formulas (i) to (iii) represents the content (mass %) of each element contained in the titanium alloy, and is zero if it is not contained.

In the chemical composition of the present invention, the remainder is Ti and impurities. In addition to the above-mentioned elements such as O, N, C, and H, "impurities" refer to components that are mixed in when titanium is manufactured industrially due to raw materials such as scrap and various factors in the manufacturing process. It means what is permissible within a range that does not adversely affect the present invention. In particular, when scrap is used as a raw material, elements such as V, Sn, Zr, Mn, Mo, and Cu may be mixed in as impurities. These elements are allowed as impurities if each content is 0.1% or less and the total content is 0.3% or less.

2. Metal structure 2-1. α Phase Area Ratio The metal structure of the titanium alloy round bar according to the present invention has an α phase of 85.0% or more in terms of area ratio. If the area ratio of the α phase is less than 85.0%, a good Young's modulus cannot be obtained. Therefore, the area ratio of the α phase is 85.0% or more, preferably 90.0% or more. On the other hand, if the α phase is excessive, that is, if the β phase is too small, hot workability cannot be ensured. Therefore, it is preferable that the area ratio of the α phase is 99.0% or less.

Note that the area ratio of the α phase can be measured by performing crystal orientation analysis using the EBSD method. Specifically, the central part of the L cross section (a cross section parallel to the longitudinal direction of the round bar) was observed at 1000 times using an SEM (scanning electron microscope), the measurement area was set to 100 μm x 100 μm, and the measurement pitch was set to 0. .2 μm and perform the measurement. Based on the measured results, crystal orientation analysis is performed to identify the α phase and the β phase. The above measurement is performed at three arbitrary locations, the average value is calculated, and this is taken as the area ratio of the α phase. In addition, in the crystal orientation analysis, calculation may be made from Phase-Map using the analysis software OIM-analysis.

2-2. Equiaxed Ratio In order to ensure strength, hot workability, and ductility in the temperature range from room temperature to warm temperature range, the titanium alloy round bar according to the present invention defines the equiaxed ratio of the α phase. When a titanium alloy round bar is deformed by processing or the like, each crystal grain needs to deform continuously with the surrounding crystal grains. For this reason, it is thought that if there are many elongated crystal grains such as those in a rolled structure that is not equiaxed, the length of contact with surrounding crystal grains becomes long, and the deformation resistance increases. This results in reduced ductility.

Therefore, the equiaxed ratio of the α phase is set to 70% or more. The equiaxed ratio refers to the ratio of equiaxed grains to all α phases observed, and is an index indicating the degree of isotropy regarding the shape of crystal grains. Further, equiaxed grains mean that the aspect ratio, that is, the length of the major axis/length of the minor axis, is 3.3 or less.

If the equiaxed ratio of the α phase is less than 70%, good hot workability and ductility in the temperature range from room temperature to warm cannot be ensured. Moreover, the desired strength cannot be obtained. Therefore, the equiaxed ratio of the α phase is set to 70% or more. The equiaxed ratio of the α phase is preferably 80% or more, and preferably 85% or more.

Note that the equiaxedization rate is observed using the following procedure. Specifically, the structure is observed such that the center of the C cross section (a cross section perpendicular to the longitudinal direction) of the titanium alloy round bar serves as the observation surface. The observation magnification is 200 to 300 times, and an optical microscope or SEM is used. Through this microstructure observation, the area of crystal grains with an aspect ratio of 3.3 or less is measured, divided by the area of the entire α phase, expressed as a percentage, and taken as the equiaxed ratio. The aspect ratio may be calculated by assuming that the crystal grain is equivalent to an ellipse and calculating the long axis and short axis.

2-3. Orientation of α-phase crystal grains in the longitudinal direction of the titanium alloy round rod The crystal structure of the α-phase is an hcp structure (hexagonal close-packed structure) as shown in FIG. In the α phase, the direction perpendicular to the bottom surface of the hcp lattice (hereinafter also simply referred to as the "c-axis direction") is the direction in which the Young's modulus is high. Normally, in consideration of processing and other factors, Young's modulus in the longitudinal direction is important for round bars used for connecting rods. For this reason, it is desirable that the longitudinal direction of the alloy round rod and the c-axis direction of the hcp lattice of the α phase be as parallel as possible. That is, as shown in FIG. 2, when θ is the angle between the longitudinal direction of the titanium alloy round bar and the c-axis direction of the α-phase crystal lattice, it is most desirable that θ be 0°.

In the titanium alloy round bar according to the present invention, the proportion of α-phase crystal grains with θ of 0° or more and 45° or less (hereinafter simply referred to as “c-axis oriented α-phase ) shall be 15.0% or more. The c-axis orientation α phase ratio is preferably 17.5% or more, and preferably 20.0% or more. Note that there is no particular upper limit to the c-axis orientation α phase ratio.

The c-axis orientation α phase ratio can be measured by performing crystal orientation analysis using the EBSD method. Specifically, the central part of the L cross section (a cross section parallel to the longitudinal direction of the round bar) was observed at 100 times using an SEM (scanning electron microscope), and the measurement area was set to 1000 μm x 1000 μm, and the measurement pitch was 2 μm. and take measurements. Based on the measured results, crystal orientation analysis is performed to identify the α phase. The above measurement is performed at three arbitrary locations, α-phase crystal grains with θ of 0° or more and 45° or less are extracted, and the ratio (%) to all α-phase crystal grains to be measured is calculated. Note that in the crystal orientation analysis, analysis software OIM-analysis may be used.

3. Targeted Characteristic Values In the titanium alloy round bar according to the present invention, a case where the Young's modulus in the longitudinal direction is 115 GPa or more is evaluated as having a good Young's modulus. Furthermore, when the tensile strength is 950 MPa or more, the tensile strength is evaluated to be good, and when the elongation at break is 15% or more, the ductility is evaluated to be good. Furthermore, regarding hot workability, oxidation weight gain is used as an evaluation index, and when the oxidation weight gain is 10.0 mg/cm ² or less, hot workability is evaluated as good. In addition, in the titanium alloy round bar according to the present invention, in addition to obtaining good specific strength, it is also possible to obtain the effect of low density. A case where the density is 4.40 g/cm ³ or less is evaluated as a good value.

4. Manufacturing Method A preferred method for manufacturing the titanium alloy round bar and connecting rod according to the present invention will be described. Regardless of the manufacturing method, the titanium alloy round bar according to the present invention can obtain the effects as long as it has the above-mentioned configuration. However, for example, it cannot be stably manufactured by the following manufacturing method. can.

4-1. Ingot manufacturing process It is preferable to manufacture an ingot with a predetermined chemical composition by a method such as electron beam melting, vacuum arc melting, plasma arc melting, or the like. The size of the ingot is not particularly limited. However, in vacuum arc melting, as the ingot becomes larger, segregation becomes more pronounced, so in the case of vacuum arc melting, smaller ingots are preferable. Furthermore, when Fe and Cr are contained, it is preferable to use electron beam melting to produce a small ingot because these elements tend to segregate.

4-2. Blossoming process The obtained ingot has a solidified structure including solidification defects. A solidified structure is undesirable because it reduces hot workability. Therefore, the obtained ingot may be subjected to blooming, if necessary. The blooming may be performed by forging or rolling.

In the blooming step, the heating temperature is preferably higher than the β transformation point (°C) and lower than 1150°C. If the heating temperature is below the β-transus point, the solidified structure cannot be sufficiently eliminated. Therefore, the heating temperature is preferably at least the β transformation point, more preferably at least 50° C. above the β transformation point. Note that the β transformation point may be lowered during processing. Moreover, when reheating, it is preferable to heat to the β transformation point or higher. On the other hand, when the heating temperature exceeds 1150°C, the ingot is significantly oxidized. For this reason, the heating temperature is preferably 1150°C or less.

In the blooming process, it is preferable to perform processing with a total cross-sectional reduction rate of 20% or more, that is, rough forging. This is because if the total cross-sectional reduction rate is less than 20%, the cast structure cannot be sufficiently eliminated.

4-3. β solution processing/processing process Next, heating is performed again to above the β transformation point, processing is performed to reduce the area reduction rate from 5 to 30% in the β single phase region, and then rapid cooling is carried out (hereinafter simply referred to as "β solution processing").・It is preferable to do the following. Below the β transformation point, transformation from β phase to α phase occurs. Therefore, the transformation occurs during cooling after heating and processing in the β single phase region.

If the α phase formed by this transformation is formed along the grain boundaries of the β phase before transformation, hot workability will be reduced. Such precipitation of α phase around grain boundaries occurs more significantly as the alloying element increases, but it may inhibit equiaxedization of the titanium alloy round bar and reduce strength, ductility, etc. Therefore, the α phase formed by transformation is preferably formed within the grains of the β phase before transformation.

In order to form the α phase within the grains during transformation, it is preferable to introduce strain that becomes a precipitation nucleus within the grains and to suppress precipitation around the grain boundaries. For this reason, it is preferable to perform processing such that the area reduction rate is 5% or more. On the other hand, if excessive strain is introduced, the strain will not function as precipitation nuclei within the grains, but will accumulate within the structure as a driving force for recrystallization. For this reason, it is preferable to perform processing such that the area reduction rate is 30% or less.

Further, after heating and processing, it is preferable to rapidly cool the β phase alone. The quenching is preferably carried out by water cooling or forced air cooling in such a shape that the cross-sectional area after processing is 100 cm ² (10000 mm ² ) or less. This is because if the shape has a cross-sectional area of more than 100 cm ² and a cooling method with a lower cooling rate than water cooling or forced air cooling is used, rapid cooling cannot be performed at a sufficiently high cooling rate.

Note that processing in the blooming step and processing in the solution treatment/processing step may be performed together. In this case, the rapid cooling described above is performed after a series of decomposition steps, solution treatment, and processing are carried out at a temperature above the β transformation point, and the processing is carried out at a total area reduction rate of 25 to 50%.

4-4. Finish rolling process Next, finish rolling is performed. In finish rolling, it is preferable to set the heating temperature to less than the β-transus point and roll with a reduction in area of 85% or more. If the heating temperature in finish rolling is higher than the β transformation point, transformation occurs and the metal structure cannot be made equiaxed. For this reason, it is preferable that the heating temperature in finish rolling be less than the β transformation point. The heating temperature in finish rolling is more preferably -20°C, β transformation point, and even more preferably -30°C, β transformation point. Similarly, when the area reduction rate in finish rolling is less than 85%, the metal structure cannot be made equiaxed. For this reason, it is preferable that the area reduction rate in finish rolling is 85% or more. Here, the area reduction rate is based on the cross-sectional area at the start of the finish rolling process.

4-5. Heat Treatment Step After finish rolling, heat treatment may be performed as necessary to adjust mechanical properties. Here, the heat treatment temperature is preferably below the β transformation point -50°C. If the heat treatment temperature exceeds the β transformation point -50°C, the obtained equiaxed structure cannot be maintained and the proportion of equiaxed α grains decreases.

4-6. Descaling Step If necessary, descaling may be performed to improve the surface quality. Descaling may be performed by mechanical polishing, shot blasting, pickling, etc., but is not particularly limited.

4-7. Manufacture into Connecting Rod The obtained titanium alloy round bar is preferably subjected to hot die forging, punching, cutting, etc. to form a connecting rod shape. If necessary, descaling, surface treatment, etc. may be performed.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

A titanium alloy round bar having the chemical composition shown in Table 1 was manufactured. Regarding the chemical composition, metal elements were measured by ICP analysis, O by inert gas melt infrared absorption method, N by inert gas melt thermal conductivity method, and C by high frequency combustion infrared absorption method. The analysis sample was taken from a range from the center of the round bar to a position 1/2 of the diameter. Here, a round bar was manufactured using an ingot (diameter 120 to 140 mm) having a predetermined chemical composition under the conditions listed in Table 2. The resulting round bar had a diameter of 25 to 38 mm.

The following values were measured or calculated for the obtained titanium alloy round bar.

(α phase area ratio)
The area ratio of the α phase was determined by crystal orientation analysis using the EBSD method. Specifically, the central part of the L cross section (a cross section parallel to the longitudinal direction of the round bar) was observed at 1000 times using a SEM (JSM-6500F manufactured by JEOL Ltd.), and the measurement area was set as an area of 100 μm x 100 μm. The pitch was set to 0.2 μm and measurements were taken. Based on the measured results, crystal orientation analysis was performed and α phase and β phase were identified. The above measurements were performed at three arbitrary locations, and the average value was calculated, which was taken as the area ratio of the α phase. In the crystal orientation analysis, calculations were made from Phase-Map using analysis software OIM-analysis.

(Equiaxed rate)
The equiaxed ratio was observed using the following procedure. Specifically, the structure was observed so that the center of the C cross section (cross section perpendicular to the longitudinal direction) of the titanium alloy round bar was the observation surface. The observation magnification was 200 to 300 times, and the measuring device used was ECLIPSE LV150N manufactured by Nikon. Through this microstructure observation, the area of crystal grains with an aspect ratio of 3.3 or less was measured, divided by the total area, expressed as a percentage, and defined as the equiaxed ratio.

(c-axis orientation α phase ratio)
The c-axis orientation α phase ratio was measured by performing crystal orientation analysis using the EBSD method. Specifically, the central part of the L cross section (a cross section parallel to the longitudinal direction of the round bar) was observed at 100 times using an SEM (scanning electron microscope), and the measurement area was set to 1000 μm x 1000 μm, and the measurement pitch was 2 μm. Then, measurements were taken. Based on the measured results, crystal orientation analysis was performed and the α phase was identified. The above measurements were performed at three arbitrary locations, α phase crystal grains with θ of 0° or more and 45° or less were extracted, and the proportion (%) to the total α phase measured was calculated. In the crystal orientation analysis, the analysis software OIM-analysis was used, and the measuring instrument was JSM-6500F manufactured by JEOL.

(Tensile strength and elongation at break)
A tensile test was conducted on the obtained titanium alloy round bar, and the tensile strength and elongation at break were calculated. The tensile test was conducted at a tensile test speed of 1 mm/min until the test piece broke. The elongation at break was calculated by the butt method. The tensile test piece used was an ASTM E8 sub-size tensile test piece (parallel part φ6.25 mm, parallel part length 28 mm, distance between gauges 25 mm), and the test piece surface was smooth with a #400 polishing finish or higher.

(Young's modulus and density)
Young's modulus was measured using a resonance method. The shape of the test piece was 60 mm x 10 mm x 3 mm, and the surface of the test piece was finished with #600. It was taken from the center of the diameter. Using test pieces of 15 mm x 15 mm x 20 mm, N≧3 (N is the number of test pieces), density was calculated from the test piece dimensions and mass, and the average value was taken as the density value.

(oxidation increase)
The sample was held at 800° C. for 100 hours, and the value obtained by dividing the change in mass before and after the holding by the surface area was measured as the oxidation weight gain. A test piece with a size of 5 mm x 20 mm x 40 mm was used, and the surface was polished to #600. At this time, the scale that was peeled off during the holding at the above temperature was also included in the mass change. The results are summarized in Table 3 below.

No. 1, which satisfies the provisions of the present invention. Nos. 1, 2, 5, 6, 9, 11, 13, 19, 20, 25, 28-31, 34 and 35 showed good characteristics. On the other hand, examples that did not satisfy the provisions of the present invention had poor results in at least one of each characteristic value.

No. In No. 3, the Al content did not satisfy the content specified by the present invention, the c-axis α phase ratio decreased, and the Young's modulus in the longitudinal direction decreased. In addition, the density value also increased. No. In No. 4, the Al content did not satisfy the content specified by the present invention, the equiaxed ratio decreased, and the ductility (elongation at break) decreased. No. In No. 7, the Fe content did not satisfy the content specified by the present invention, the c-axis orientation α phase ratio decreased, and the Young's modulus in the longitudinal direction decreased.

No. In No. 8, the chemical composition did not satisfy formula (iii), and the elongation at break was decreased. No. In No. 10, the Si content did not satisfy the content specified by the present invention, and the elongation at break was decreased. No. In No. 12, the Cr content did not satisfy the content defined by the present invention, the α phase area ratio decreased, and the Young's modulus in the longitudinal direction decreased.

No. In samples Nos. 14 to 17, the content of any one of the elements O, N, C, and H did not satisfy the content specified by the present invention, and the elongation at break decreased. No. In No. 18, the chemical composition did not satisfy the formula (i), and the elongation at break was decreased. No. In No. 21, the chemical composition did not satisfy the requirements of formula (ii), and the weight gain by oxidation increased. No. No. 32 had a chemical composition that did not satisfy the Al content and the formula (ii). For this reason, the regulation of the c-axis orientation α phase ratio was not satisfied, the tensile strength and Young's modulus in the longitudinal direction were reduced, and the density and oxidation weight gain were also increased. No. No. 33 had a chemical composition that did not satisfy the specifications of Al content, formula (ii), and formula (iii). For this reason, the regulation of the c-axis orientation α phase ratio was not satisfied, the Young's modulus in the longitudinal direction was reduced, and the density and oxidation weight gain were also high.

No. In No. 22, the β-solution treatment/processing step was not performed, so the equiaxedization rate decreased and the elongation at break decreased. No. In No. 23, the β solution temperature was lower than the β transformation point, so the equiaxedization rate decreased and the elongation at break decreased. No. In No. 24, the cooling rate after β-solution treatment and processing was slow, so the equiaxedization rate decreased and the elongation at break decreased. No. In No. 26, the heating temperature during finish rolling was higher than the β transformation point, so the equiaxedization rate decreased and the elongation at break decreased. No. In No. 27, the heat treatment temperature in the heat treatment step was higher than the β transformation point, so the equiaxedization rate decreased and the elongation at break decreased.

Claims (2)

A titanium alloy round bar extending in one direction,
The chemical composition is in mass%,
Al: 7.0-9.0%,
O: 0.20% or less,
N: 0.010% or less,
C: 0.010% or less,
H: 0.013% or less,
Nb: 0 to 2.00%,
Si: 0 to 0.30%,
Fe: 0-2.00%,
Cr: 0-2.00%,
The remainder: Ti and impurities,
The following formulas (i) to (iii) are satisfied,
In the metallographic structure, the area ratio is
having an α phase of 85.0% or more and 99.0% or less ,
If the aspect ratio (major axis length/minor axis length) of a crystal grain is equivalent to an ellipse is 3.3 or less, it is considered an equiaxed grain. When the ratio is taken as the equiaxed rate, the equiaxed rate of α phase crystal grains is 70% or more,
When θ is the angle between the longitudinal direction of the titanium alloy round rod and the c-axis direction of the α-phase crystal lattice, θ is 0° or more and 45° or less for all α-phase crystal grains to be measured. A titanium alloy round bar in which the proportion of α-phase crystal grains is 15.0% or more.
Al+10×O≦10.5...(i)
0.50≦Nb+10×Si...(ii)
1.00≦2×Fe+5×Cr...(iii)
However, each element symbol in the above formula represents the content (mass %) of each element contained in the titanium alloy, and is zero if it is not contained. A connecting rod using the titanium alloy round bar according to claim 1.

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