JP5382518B2 - Titanium material - Google Patents

Description

  The present invention relates to a titanium material, and more particularly, to a titanium material excellent in strength and workability.

Conventionally, titanium materials such as titanium alloys and pure titanium have been widely used in various forms such as plates and rods.
For example, in order to manufacture titanium products, plate-like titanium materials (hereinafter also referred to as “titanium plates”) are widely used, and various processes involving plastic deformation such as bending, overhanging and drawing are performed. It has been used for various purposes.
Titanium plates subjected to such processing are required to have excellent workability.

Recently, as a result of the demand for thinner titanium plates from the standpoints of reducing material costs and reducing the weight of products, improvement in strength has been demanded.
However, conventionally, there is a trade-off relationship between workability and strength in a titanium plate, and it is difficult to satisfy these simultaneously.
That is, the conventional titanium plate has a problem that when the proof stress is increased, the forming process is difficult (the workability is inferior).

On the other hand, Non-Patent Document 1 below shows the results of evaluating the formability of titanium thin plates having different components and crystal grain sizes by a cylindrical deep drawing test, and the finer the crystal grains, the better the workability. (Page 103, 5th line).
And the following patent document 1 is disclosing the manufacturing method of a pure titanium thin plate, and final annealing is performed by the continuous annealing for (600-800) degreeC x (2-5) in an atmospheric condition, It describes that a pure titanium thin plate having a surface gloss reduced by adjusting the average crystal grain size (hereinafter referred to as particle size) of the product to 3 to 60 μm by performing a pickling treatment.

Patent Document 2 discloses pure titanium for building materials, a pure titanium plate, and a method for producing the same. The oxygen content is 900 ppm or less, Fe is 100 ppm or more and 600 ppm or less, and Ni and Cr are contained. It describes a titanium material for building materials having an average crystal grain size of 70 μm or less, which is subjected to pickling treatment with a nitric hydrofluoric acid aqueous solution after cold rolling annealing.
However, in Patent Documents 1 and 2, there is hardly any data evaluated for a crystal grain size of 5 μm or less, and in Patent Document 2, an example in which the crystal grain size is 3 μm is shown. However, at the same time, paragraph [0026] states that “the lower limit is about 5 μm in actual production”, and there is a negative statement that the crystal grain size should be 5 μm or less. Yes.
This is because these documents aim to obtain an excellent titanium material for building materials with suppressed gloss, and the workability in overhanging, deep drawing, etc. has not been sufficiently studied. it is conceivable that.

Patent Documents 3 and 4 listed below disclose titanium plates with excellent workability, but these are excellent in workability but have low strength (yield strength) and can achieve both workability and strength. is not.

JP 63-103056 A JP-A-9-3573 JP 2006-316323 A JP 63-60247 A

“Titanium”, Vol. 57, no. 2 (Japan Titanium Association, issued in April 2009)

  An object of the present invention is to provide a titanium plate having high strength and excellent workability.

Titanium materials can increase strength (proof strength) by adding mainly oxygen (O) or iron (Fe), but at the same time, ductility is lowered and workability is also lowered.
For example, the titanium material specified in JIS class 1 has a low content of oxygen and iron, so a titanium plate using this JIS class 1 material generally has low strength (yield strength) but large ductility and excellent workability. ing.
Use of JIS type 2 titanium material, which contains more oxygen and iron than JIS type 1, increases strength (yield strength) while lowering ductility and processability than titanium material using JIS type 1. Tend to.
Furthermore, JIS type 3 and type 4 having a high oxygen and iron content further increase in strength (yield strength), but further reduce ductility and greatly reduce workability.
That is, there is a certain relationship between strength (yield strength) and formability (hereinafter, this relationship is also referred to as “strength (yield strength) −workability” balance).

By the way, the board | plate material and wire which consist of titanium materials are formed by performing the process accompanying plastic deformation, such as rolling and wire drawing.
Usually, plate materials and wires that have undergone processing with plastic deformation have a processed structure formed inside as they are, so a process called finish annealing is performed to recrystallize the structure. It is used as a member for secondary processing.
For example, in the case of a titanium plate, after adjusting the thickness to a predetermined thickness by performing processing such as cold rolling, etc., batch annealing or continuous annealing is performed to recrystallize the internal processing structure, and equiaxed crystal grains (Hereinafter referred to as “recrystallized grains”) is performed.
These recrystallized grains grow greatly with the lapse of time of annealing, etc., especially immediately after the start of recrystallization with a small recrystallized grain size, the recrystallized grains have a high growth rate and are relatively short in time. The particle size becomes larger than 5 μm.
When the recrystallized grains grow to such a size, normally, no unrecrystallized portion (worked structure) remains and an equiaxed structure formed by the recrystallized grains is formed inside the titanium material. It becomes.

As a result of earnest studies on the problems as described above, the present inventor has adjusted the structure that has not been noticed as a means for improving the strength (proof strength) in the past (by leaving an unrecrystallized portion). It has been found that the strength (proof strength) of a titanium material can be improved by refining crystal grains.
Specifically, an industrial pure titanium plate cold-rolled to a predetermined thickness is subjected to finish annealing in a vacuum using an electric furnace, and various titanium plates having different structures are manufactured by changing the temperature and time. The present invention was found by evaluating the strength (yield strength) and workability (ductility) by a tensile test and an Erichsen test.

  As a result of the evaluation, the finer the crystal grains, the greater the strength (yield strength), and the workability (Ericsen value) tended to decrease, but the average grain size of the recrystallized grains was less than a predetermined size. It has been found that the Erichsen value does not decrease so much and the “strength (yield strength) -workability balance” can be improved as compared with conventional titanium materials.

In addition, even if the average crystal grain size of the recrystallized grains is less than or equal to a predetermined value, the workability (Erichsen value) is lowered, and the “strength (yield strength) -workability balance” can be improved over the conventional titanium material. There was a case where it was not possible.
As a result of detailed investigation of the microstructure of the titanium plate, many non-recrystallized portions were recognized in addition to the grains recrystallized by finish annealing.
Based on the amount of the non-recrystallized portion, the “strength (yield strength) -workability balance” was examined. Particularly, when the area ratio of the non-recrystallized portion in the cross-sectional area exceeds 30%, the workability is extremely reduced. I found out.
Here, the non-recrystallized portion means a portion where a worked structure by plastic working remains.

  That is, the present invention according to the titanium material for solving the above-mentioned problems is that the iron content is 0.60% by mass or less, the oxygen content is 0.15% by mass or less, and the balance is from titanium and inevitable impurities. A titanium material formed of a titanium material, and having a processed structure formed by processing accompanied by plastic deformation, and a recrystallized structure formed by annealing after the processing. The average grain size of the crystal grains of the crystal structure is 1 μm or more and 5 μm or less, and the area of the non-recrystallized portion in the cross-sectional area is more than 0% and 30% or less.

  According to the present invention, it is possible to provide a titanium material having high strength and excellent workability.

The microstructure photograph of the titanium plate of the Example observed with the transmission electron microscope (an unrecrystallized part is recognized in a part between recrystallized grains). The graph which shows the relationship between yield strength and Eriksen value.

Hereinafter, a preferred embodiment of the titanium material according to the present invention will be described using a titanium plate as an example.
The titanium plate in this embodiment has an iron (Fe) content of 0.60 mass% or less, an oxygen (O) content of 0.15 mass% or less, and the balance being titanium (Ti) and inevitable impurities. It is formed in a plate shape by the material.
The titanium plate is formed by being annealed after being processed with plastic deformation, and has a processed structure accompanying the processing inside and a recrystallized structure accompanying the annealing, The average grain size of the crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of the non-recrystallized portion in the cross-sectional area is more than 0% and 30% or less.

As described above, the iron (Fe) is contained at a ratio of 0.60% by mass or less.
Note that the upper limit of the Fe content is 0.60% by mass in the titanium material, Fe is a β-phase stabilizing element, and if the Fe content exceeds 0.60% by mass, the titanium plate is formed. This is because there is a possibility that many β phases other than the α phase may be generated in the tissue.
That is, depending on the size of the β phase to be formed, the ductility is greatly reduced or the corrosion resistance is lowered. Therefore, the content of Fe contained in the titanium material should be 0.60% by mass or less. In addition, it is important in that a titanium plate having excellent workability is formed.

In addition, in terms of forming a titanium plate having high strength and excellent workability, the lower limit of the Fe content is not particularly required, but a titanium plate having Fe of less than 0.01% by mass will be manufactured. Then, expensive high purity sponge titanium must be used as a raw material, which may increase the material cost of the titanium plate.
Therefore, from the viewpoint of the cost of the titanium plate, the Fe content is preferably set to 0.01% by mass or more and 0.60% by mass or less.

For example, in the crawl method, a titanium material having an Fe content of 0.60% by mass or more is usually formed only in a small region near the container.
Therefore, the titanium plate in the present embodiment has a content of iron as its component of 0.01 to 0.60% by mass, so that most materials of sponge titanium by the crawl method can be used and used. It can be said that it is suitable as a consumable material used for forming a molded product in that the material is hardly restricted.

The oxygen (O) is contained in the titanium material at a content of 0.15% by mass or less.
The content of O in the titanium material forming the titanium plate of the present embodiment is 0.15% by mass or less because when the O content is 0.15% by mass or more, the crystal grains are made finer. Even if it is intended to improve the “strength-workability balance”, there is a possibility that the workability may not be sufficiently imparted because the strength is excessively improved, and a titanium plate suitable for processing such as overhanging and deep drawing, Because it becomes difficult to do.

The lower limit of the O content is not particularly defined, but if the O content in the titanium material constituting the titanium plate is set to 0.015% by mass or less, an expensive high-purity sponge titanium is produced as a raw material. There is a risk of having to.
Therefore, the O content is preferably 0.015% by mass or more and 0.15% by mass or less.

In addition, for inevitable impurities such as carbon (C), nitrogen (N), and hydrogen (H), for the purpose of ensuring good processability in the molding process, the content should be less than or equal to JIS type 2. is important.
More specifically, it is important that the contents of C, N, and H are each less than 0.02% by mass.
More preferably, the C content is 0.01% by mass or less, the N content is 0.01% by mass or less, and the H content is 0.01% by mass or less.
From the viewpoint of the workability of the titanium plate, there is no lower limit to the contents of C, N, and H. However, if these contents are extremely reduced, the production cost of the titanium plate is greatly increased. There is a fear.
From the viewpoint of suppressing the cost increase, it is preferable that the C content is 0.0005 mass% or more, the N content is 0.0005 mass% or more, and the H content is 0.0005 mass% or more.

  As described above, the titanium plate of the present invention has a processed structure and a recrystallized structure therein, and the average grain size of the crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less. The area of the non-recrystallized portion in the cross-sectional area is formed so as to be more than 0% and not more than 30%.

The upper limit of the average grain size of the recrystallized structure is set to 5 μm because the average crystal grain size of the equiaxed α grains produced by recrystallization is 5 μm or more. This is because the effect becomes small and it becomes difficult to realize an excellent “strength-formability balance”.
In addition, the lower limit value is set to 1 μm, when processing (rolling, forging, etc.) is performed in actual production (industrially practicable method), and then annealing is performed, when the average crystal grain becomes smaller than 1 μm, the latter stage This is because the area ratio of the non-recrystallized portion (processed structure) described in the above increases, and the strength becomes very large, but the ductility is greatly reduced and it is difficult to realize an excellent “strength-formability balance”. .

The non-recrystallized portion is a processed structure that is plastically deformed by processing (cold rolling, forging, etc.) and the crystal grains are crushed, and the strength can be improved by leaving the processed structure in the titanium plate. .
A titanium plate composed of a processed structure formed by cold rolling or the like exhibits high strength while having very low ductility.
For this reason, conventionally, the work structure is recrystallized into an equiaxed structure by annealing, and a sufficient annealing time is provided so that the work structure does not remain on the titanium plate.
On the other hand, in the titanium plate in the present embodiment, the processed structure is left in the titanium plate by employing the annealing conditions described later, and the grain size of the recrystallized grains is as described above. It has been adjusted.

The non-recrystallized portion (processed structure) is important in that it provides an excellent “strength-formability balance” that is provided so that the area ratio in the cross section of the titanium plate is 30% or less.
When the area ratio of the non-recrystallized portion is larger than 30%, the strength of the titanium plate is increased, but the ductility is lowered and it becomes difficult to exhibit excellent workability on the titanium plate.
As a result, there is a possibility that an excellent “strength-formability balance” cannot be obtained.
From the viewpoint that this excellent “strength-formability balance” can be more reliably imparted to the titanium plate, the area ratio of the non-recrystallized portion is preferably 10% or less.
The lower limit is not particularly limited, but when there is no unrecrystallized portion (the area ratio becomes 0%), the grain size of the recrystallized grains rapidly increases.
Therefore, it is preferable that the area ratio of the non-crystal part is 0.1% or more in that the grain size of the recrystallized grains can be adjusted more surely within the aforementioned range.

  In order to adjust the grain size of the recrystallized grains as described above and to form the non-recrystallized portion, the titanium plate is adjusted to a desired thickness by a general rolling process and then finished under predetermined conditions. The method of implementing annealing is mentioned.

The annealing methods that can be employed in the finish annealing can be broadly classified into a continuous type and a batch type.
Among these, continuous finish annealing is a method of annealing by expanding a cold-rolled coil and passing a titanium plate through the annealing furnace at a constant speed, and the holding time of the heating temperature can be controlled by the passing speed. .
In the conventional finish annealing in the titanium plate, in the case of continuous type, the heating temperature is 700 to 800 ° C., and the heating time is about several tens of seconds to 2 minutes.
On the other hand, batch-type finish annealing is a method in which a coil of titanium plate is heated in an annealing furnace in the state of a coil, and it is slowly performed in order to reduce the difference in how heat is applied between the surface layer of the coil and the inside. It is heated and the cooling rate is very slow.
In the case of the batch annealing method, the heating temperature is 550 to 650 ° C. and the heating time is about 3 to 30 hours in the conventional finish annealing on the titanium plate.

On the other hand, as the finish annealing performed when producing the titanium plate of the present embodiment, for example, in the case of a continuous type, heating is performed at a temperature of 580 ° C. or higher and lower than 600 ° C. for 1 minute or longer and 10 minutes or shorter. It is preferable to carry out the heating under the conditions or at a temperature of 600 ° C. or higher and 650 ° C. or lower for 10 seconds or longer and less than 2 minutes.
As this preferable heating condition, the time of 10 seconds or more is selected because if the holding time of the heating temperature is shorter than 10 seconds, a predetermined annealing speed is applied to the titanium plate, and the plate passing speed, the heating temperature, etc. This is because the appropriate range of the operating conditions becomes very narrow, and high-precision management is required for the apparatus and its operation.
On the other hand, the heating time is preferably 10 minutes or less because if the time of 10 minutes or more is applied, the plate passing speed has to be slowed and productivity is lowered.

Further, as a preferable heating temperature condition, a temperature of 580 ° C. or higher is selected. It is difficult to cause a predetermined recrystallization on the titanium plate with a holding time of 10 minutes or shorter at a heating temperature lower than 580 ° C. This is because the area ratio of the non-recrystallized portion often exceeds 30%.
Furthermore, the heating temperature of 650 ° C. or lower is selected because the recrystallization of the titanium plate is completed even when the heating temperature is higher than 650 ° C. for 10 seconds, and the recrystallized grains have an average grain size of 5 μm or more. This is because there is a possibility of growing to a diameter.

Further, the finish annealing performed when the titanium plate of the present embodiment is manufactured is preferably a heating condition of not less than 3 hours and not more than 50 hours at a temperature of 420 ° C. or more and less than 550 ° C. in the case of a batch type. .
The heating time is preferably 3 hours or longer because if the heating time is shorter than 3 hours, the temperature inside the coil may not reach a predetermined temperature, depending on the size of the coil. It is.
On the other hand, the condition of 50 hours or less as the heating time is preferable because when the time of 50 hours or more is taken, the time required for annealing becomes too long and the productivity of the titanium plate is lowered.

Further, it is preferable that the heating temperature is 420 ° C. or higher. It is difficult to cause predetermined recrystallization on the titanium plate with a holding time of 50 hours or less at a heating temperature lower than 420 ° C. This is because the area ratio often exceeds 30%.
Alternatively, in order to secure a predetermined production amount, it is necessary to have a number of annealing furnaces (heating facilities), which increases the equipment cost and requires a large space for installation.
In the batch method, heating is performed in the coil state, so the temperature rising speed is different between the surface layer portion and the inside of the coil, and the time until reaching the target temperature is also different.
Depending on the size of the coil, the heating temperature, and the heating capability of the annealing furnace, generally, the time to reach the target temperature varies from several tens of minutes to several hours.
For this reason, even if the heating time is slightly different, it is important that the recrystallized grain size does not differ much, that is, it is heated to a temperature range where the growth rate of the recrystallized grains is slow.

  And it is preferable that the heating temperature is less than 550 ° C. Since the growth rate of the recrystallized crystal grains is large at a temperature of 550 ° C. or higher, if the heating time is shortened according to the coil surface layer portion, the target is still within the coil. If the heating time is increased according to the inside of the coil, the coil surface layer portion is recrystallized. This is because the grains grow too much and the average crystal grain size may become 5 μm or more.

In addition, it is desirable that the finish annealing is performed in a vacuum or in an inert gas atmosphere in both continuous and batch processes.
As described above, a titanium plate having an excellent “strength-formability balance” is obtained by adjusting the average grain size of recrystallization and the remaining ratio of the non-recrystallized portion (worked structure) according to the annealing conditions. Can do.

Although not described in detail here, known matters in the conventional titanium plate and titanium plate manufacturing method can be employed in the present invention as long as the effects of the present invention are not significantly impaired.
Moreover, in this embodiment, although the titanium plate is mentioned as an example of the titanium material, in the point where the outstanding "strength-formability balance" is exhibited, not only a titanium plate but a wire, a rod, for example The same applies to various forms of titanium materials such as pipe materials, and these titanium materials are also within the scope of the present invention.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to these.

<Evaluation 1>
(Sample Nos. 1-45)
(Production of test piece)
An ingot (φ140 mm) was produced by small vacuum arc melting, and the ingot was heated to 1050 ° C. and then forged to produce a slab having a thickness of 50 mm.
The slab was hot-rolled to a thickness of 5 mm at 850 ° C., and then annealed at 750 ° C., shot and pickled, and the surface scale was removed to prepare a plate material.
Further, this plate material was cold-rolled to produce a plate-like sample (titanium plate) having a thickness of 0.5 mm.
The 0.5 mm thick titanium plate was subjected to finish annealing for 48 hours or less at a temperature of 400 to 800 ° C. in an argon gas atmosphere to prepare a test piece with adjusted crystal grains.

(Component measurement)
The amount of iron and the amount of oxygen contained in the titanium plate were measured using the plate material after hot rolling from which the surface scale was removed.
The iron content was measured according to JIS H1614, and the oxygen content was measured according to JIS H1620.

(Tensile strength measurement)
Moreover, the tensile strength of the test piece (titanium plate) whose crystal grain size was adjusted as described above was measured according to JIS Z 2241.

(Processability evaluation)
Moreover, the workability of the test piece (titanium plate) whose crystal grain size was adjusted as described above was evaluated.
Evaluation was carried out according to JIS Z2247 by measuring the Erichsen value using graphite grease as a lubricant.

(Organizational survey)
The microstructure of the titanium plate was observed to obtain a structure photograph of crystal grains (recrystallized α grains) and non-recrystallized parts (processed structure).
The observation was performed using a light microscope or a transmission electron microscope.
An example of a structure photograph observed with a transmission electron microscope is shown in FIG. 1 (microstructure of sample No. 28).
In this structural photograph, recrystallized α grains and unrecrystallized parts are shown.
Using this image analysis software, obtain the area other than the unrecrystallized part, obtain the average area of the recrystallized α grains, and calculate the diameter of a circle having the same area as the average area. The average grain size of recrystallized grains was used.
Moreover, the area ratio of the non-recrystallized part was calculated | required from the area of the non-recrystallized capital.
The results are shown in Table 1.

Sample No. above. In Nos. 1 to 30, the average grain size of the recrystallized grains is 5 μm or less, and the non-recrystallized portion is observed in the cross section of the titanium plate with an area ratio of less than 30%. Nos. 31 to 42 are in a state in which no non-recrystallized portion remains as in a conventional titanium plate.
Sample No. Nos. 43 to 45 are prepared by adjusting the annealing conditions so as to leave unrecrystallized portions, but leave the non-recrystallized portions in a state where the area ratio exceeds 30%.

Sample No. above. 1-30 and no. Nos. 31 to 42 are the sizes of crystal grains (α-phase equivalent circle average grain diameter) and the amount of non-recrystallized parts due to differences in annealing conditions while using titanium materials having substantially the same oxygen content and iron content. Is adjusted.
As can be seen from Table 1, the inclusion of the non-recrystallized portion makes it possible to keep the average grain size small and to exert a large proof stress.
In the above evaluation, generally, as the yield strength increases, the workability (Ericsen value) tends to decrease. However, when compared with the same degree of workability (Erichsen value), the yield strength is obtained by the presence of an unrecrystallized part. Is large and it is understood that the intensity is high (for example, refer to comparison between sample Nos. 1 and 31, 9 and 34, and 15 and 39).
That is, it can be seen that if the crystal grain size is 5 μm or less and the non-recrystallized portion is 30% or less, the “balance of yield strength-workability” is good.
On the other hand, sample no. As shown in 43 to 45, when the area of the non-recrystallized portion is more than 30% after the finish annealing, the workability (Ericsen value) is greatly reduced.
This No. The relationship between the proof stresses 1 to 45 and the Eriksen value is shown in FIG.
What is indicated by a black circle legend in the figure is No. 1 to 30 are shown, and the ones indicated by the white circle legend are No. 1 Nos. 31 to 42 are shown. The result of 43-45 is represented.
FIG. 2 also shows that according to the present invention, a titanium plate having high strength and excellent workability can be provided.

<Evaluation 2>
(Sample Nos. A to H)
(Real machine test)
(Production of test coil)
An ingot (φ750 mm) is produced by vacuum arc melting, and the ingot is 850 to 1000 ° C.
After heating, forging was performed to produce a slab having a thickness of 170 mm.
The slab is heated to a temperature of 850 ° C., then hot-rolled to a thickness of 3.5 mm, and the hot-rolled material is annealed at a temperature of 750 ° C., then shot, pickled, and scaled on the surface. The hot rolled coil was produced by removing.
The hot rolled coil was cold rolled to form a cold rolled coil having a thickness of 0.4 to 0.8 mm.
This cold-rolled coil was inserted into a vacuum annealing furnace after washing and removing oils and fats such as cold-rolled oil.
After the inside of the vacuum annealing furnace containing the cold-rolled coil is evacuated, it is replaced with argon gas, heated to 450 to 650 ° C. and held for 4 to 36 hours (Table 2 “Annealing conditions”) To adjust the size of recrystallized grains.
From the obtained titanium plate, “sample measurement”, “tensile strength measurement”, “workability evaluation”, and “structural investigation” are collected in the same size as in the above evaluation 1, and a sample of the required size is collected. Carried out. The results are shown in Table 2.

Sample No. above. A to E are those in which the average grain size of the recrystallized grains is 5 μm or less, and the non-recrystallized portion is observed at an area ratio of less than 30% in the cross section of the titanium plate. F to H are in a state in which no non-recrystallized portion remains as in a conventional titanium plate.
Sample No. above. In A, B, and C, a titanium plate excellent in workability having a yield strength of 200 MPa or more and an Erichsen value of about 13 mm is obtained.
Sample No. In D and E, a high-strength titanium plate having a yield strength of about 400 MPa and a good workability titanium plate having an Erichsen value of about 10 mm is obtained.
On the other hand, sample no. In F to H, although the workability is excellent, the proof stress is smaller than 200 MPa and the strength is not sufficient.
This also shows that according to the present invention, a titanium plate having high strength and excellent workability can be provided.

Claims (1)

A titanium material formed of a titanium material having an iron content of 0.60% by mass or less, an oxygen content of 0.15% by mass or less, and the balance of titanium and inevitable impurities,
It has a processed structure formed by processing with plastic deformation and a recrystallized structure formed by annealing after the processing, and the average grain size of the recrystallized structure is 1 μm or more and 5 μm The titanium material is characterized in that it is formed so that the area of the non-recrystallized portion in the cross-sectional area is more than 0% and 30% or less.