JP2009299110A - HIGH-STRENGTH alpha-beta TYPE TITANIUM ALLOY SUPERIOR IN INTERMITTENT MACHINABILITY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength α-β type titanium alloy which is superior in hot workability, and besides, shows superior intermittent machinability when being worked by an end mill, a milling cutter, or the like. <P>SOLUTION: The titanium alloy includes, by mass%, 0.06 to 0.13% C, 3.0 to 8.5% Al, further one or more elements of 5.0% or less V, 3.0% or less Cr, less than 2.5% Fe, 5.0% or less Mo, 5.0% or less Ni, 5.0% or less Nb and 5.0% or less Ta in a total amount of 2.0 to 10.0% and the balance Ti with unavoidable impurities. An average area ratio of TiC precipitates in a titanium alloy matrix is 0.5% or less and a mean equivalent circle diameter of the TiC precipitates is 1 μm or less by average. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a high-strength α-β type titanium alloy excellent in intermittent cutting performance by end milling or milling.

  High strength α-β type titanium alloys represented by Ti-6Al-4V are lightweight, high strength, and high corrosion resistance, and can easily change the strength level by heat treatment. Has been used extensively. In order to further utilize these characteristics, in recent years, automobile parts such as automobiles and two-wheel engine members, sports equipment such as golf equipment, civil engineering and building materials, various tools, eyeglass frames and other consumer goods fields, Application to deep seas and energy development applications is also expanding.

  However, the remarkably high production cost of α-β type titanium alloys and poor workability, particularly machinability, hinders the expansion of its application, and the range of use is currently limited. In view of such circumstances, in recent years, various proposals have been made for titanium alloys with improved machinability.

  Patent Document 1 includes a rare earth element (REM) and Ca, S, Se, Te, Pb, Bi as appropriate, and forms a granular compound, thereby suppressing toughness and ductility deterioration, and machinability. A titanium alloy for connecting rods with improved (cutability) is described. Patent Document 2 describes a free-cutting titanium alloy that improves machinability (cutability) by containing rare earth elements (REM) and improves hot workability by containing B. ing. Furthermore, in Patent Document 3, as a free-cutting component, P and S, P and Ni, or P and S and Ni, and further, in addition to these elements, REM is added to reduce the ductility of the matrix and inclusions. Is described, and a free-cutting titanium alloy that suppresses a decrease in hot workability and fatigue strength while improving the free-cutting property is described.

  Further, in Patent Document 4, by positively adding C, the strength in the higher temperature forging temperature range is reduced without reducing the strength in the practical temperature range from room temperature to 500 ° C. Α-β type titanium with improved fatigue characteristics by improving hot workability from Ti-6Al-4V alloy, which is an alloy, and suppressing the area ratio of TiC deposited in the titanium alloy substrate to 3% or less Alloys are described.

  Furthermore, in Patent Document 5, in the α-β type titanium alloy to which C is positively added, the ratio [Cr] / [Fe] of the Cr content and the Fe content to be added together is controlled. On the other hand, an α-β type titanium alloy having improved machinability (cutability) and hot workability by increasing the solid solubility limit of C by increasing the amount of Fe and suppressing the amount of TiC precipitation is described Has been.

  Further, in Patent Document 6, by adding C positively, while ensuring hot workability including forgeability, the amount of TiC precipitate is reduced and refined, and further, by limiting the upper limit of Cr concentration, it is excellent. An α-β type titanium alloy having both machinability (cutability) and hot workability is described.

  Although these patent documents have a description of the effect of improving machinability (cutability), the techniques described in these patent documents, in particular, the techniques described in Patent Documents 5 and 6, are drills. This technology is intended for continuous machinability, and is not a technology that has been studied for intermittent machinability by end milling or milling. In particular, the effect of TiC precipitated by positively adding C in order to improve hot workability such as forgeability is more conspicuous than intermittent machinability, not just continuous machinability. The development of a high-strength α-β type titanium alloy excellent in interrupted machinability has been awaited.

Japanese Patent Publication No. 6-99764 Japanese Patent Publication No. 6-53902 Japanese Patent No. 2626344 JP 2004-91893 A JP 2007-84864 A JP 2007-84865 A

  The present invention has been made as a solution to the above-described conventional problems, and provides a high-strength α-β type titanium alloy having excellent hot workability and excellent intermittent cutting performance by end milling or milling. This is a problem.

The invention according to claim 1 contains, in mass%, C: 0.06 to 0.13%, Al: 3.0 to 8.5%, V: 5.0% or less, Cr: 3. One or more of 0% or less, Fe: less than 2.5%, Mo: 5.0% or less, Ni: 5.0% or less, Nb: 5.0% or less, Ta: 5.0% or less Is a titanium alloy containing 2.0 to 10.0% in total with the balance being Ti and inevitable impurities, and the average area ratio of TiC precipitates in an arbitrary cross section of 0.25 mm ² or more in the titanium alloy substrate Is a high-strength α-β type titanium alloy excellent in intermittent machinability, characterized in that the average equivalent circle diameter of the TiC precipitate is 1% or less.

  Invention of Claim 2 is a high intensity | strength alpha-beta type titanium alloy excellent in the intermittent cutting property of Claim 1 which contains Si: 1.0% or less further by the mass%.

  The invention according to claim 3 further contains, in mass%, one or two of Zr: 5.0% or less and Sn: 5.0% or less in total of 7.0% or less. It is a high-strength α-β type titanium alloy having excellent intermittent cutting performance.

  The α-β type titanium alloy of the present invention has the effect that it has high strength and excellent hot workability, as well as continuous machinability, and is also excellent in intermittent machinability by end milling and milling. ing.

  The inventors have investigated the cause of the decrease in tool life during cutting of α-β type titanium alloys to which C has been added. Even when the amount of TiC precipitates produced is small, coarse TiC precipitates are obtained. It has been found that the tool life is reduced when the is present. In particular, it was found that the effect of tool life reduction due to TiC precipitates was more pronounced during intermittent cutting by end milling or milling than by continuous cutting with a drill.

  As a result of repeated experiments and research based on these assumptions, the present inventors have improved hot workability such as forgeability by positive addition of C, and further reduced the amount of TiC precipitates and further refined. The present inventors have succeeded in obtaining a high-strength α-β type titanium alloy excellent in intermittent cutting performance.

  Hereinafter, the present invention will be described in more detail based on embodiments.

The high-strength α-β-type titanium alloy having excellent intermittent machinability according to the present invention contains, in mass%, C: 0.06 to 0.13%, Al: 3.0 to 8.5%, and V : 5.0% or less, Cr: 3.0% or less, Fe: less than 2.5%, Mo: 5.0% or less, Ni: 5.0% or less, Nb: 5.0% or less, Ta: 5 0.01% or less of a total of 2.0 to 10.0% of a titanium alloy containing Ti and unavoidable impurities, and 0.25 mm ² or more in the titanium alloy substrate The average area ratio of the TiC precipitates in the arbitrary cross section is 0.5% or less, and the average value of the average equivalent circle diameter of the TiC precipitates is 1 μm or less.

  The α-β type titanium alloy refers to a titanium alloy in which an α phase which is a dense hexagonal crystal (HCP) and a β phase which is a body centered cubic crystal (BCC) are mixed in the structure.

  First, the reasons for limiting the components of the high-strength α-β type titanium alloy excellent in intermittent machinability of the present invention will be described. Hereinafter, the ratio of each element is simply described as%, but all indicate mass%.

C: 0.06-0.13%
C has an effect of improving the strength, and since it precipitates finely as TiC in the β temperature range, it also has an effect of refining β phase crystal grains and improving hot workability by the refinement. If the content is less than 0.06%, the action is insufficient. On the other hand, if the content exceeds 0.13%, coarse TiC with an average equivalent circle diameter of more than 1 μm that does not dissolve in the α-phase at room temperature remains, and the mechanical characteristics deteriorate, resulting in intermittent cutting. It will adversely affect sex. Therefore, the lower limit of the C content is 0.06%, preferably 0.07%, more preferably 0.08%, and the upper limit is 0.13%, preferably 0.12%, more preferably. 0.11%.

Al: 3.0 to 8.5%
Al is an α-stabilizing element and is an element added to generate an α-phase. If the content is less than 3.0%, the α phase is excessively generated, sufficient strength is not exhibited, and a tensile strength (TS) of 900 MPa or more cannot be obtained. On the other hand, when the content exceeds 8.5% and becomes excessive, ductility deteriorates and elongation (EL) decreases to less than 10%. Therefore, the lower limit of the Al content is 3.0%, preferably 3.2%, and the upper limit is 8.5%, preferably 8.0%.

V: 5.0% or less, Cr: 3.0% or less, Fe: less than 2.5%, Mo: 5.0% or less, Ni: 5.0% or less, Nb: 5.0% or less, Ta: 5.0 to 10.0% of one or more of 5.0% or less in total
These elements are all β-stabilizing elements and are elements added to generate a β-phase. If the total content of these elements is less than 2.0%, the amount of β-phase produced is too small. Therefore, it may be 2.0% or more, preferably 3.0% or more. Although these elements also have an effect of improving the strength, if they are added exceeding the upper limit of the content of each element, and if the total content exceeds 10.0%, the elongation (EL) is deteriorated. Will be invited. In particular, when the Fe content is excessive, the elongation (EL) decreases remarkably. In addition, if the Cr content is excessive, the machinability is lowered. Therefore, the upper limit of the content of each element is specified as described above, and the upper limit of the total content of these elements is 10.0%.

  The α-β type titanium alloy of the present invention is composed of Ti and unavoidable impurities in addition to the above elements, but may contain the following elements alone or in combination.

Si: 1.0% or less The strength is further improved by adding Si, but if it exceeds 1.0%, the ductility deteriorates and the required elongation (EL) cannot be obtained. Therefore, when adding Si, the upper limit of the content is made 1.0%.

One or two of Zr: 5.0% or less and Sn: 5.0% or less is 7.0% or less in total. The addition of Zr or Sn can further improve the strength, but each of Zr and Sn is independent. If over 5.0%, and over 7.0% in total, the ductility deteriorates and the required elongation (EL) cannot be obtained. Therefore, when adding Zr or Sn, the content of Zr and the content of Sn are 5.0% or less each independently, and the total content is 7.0% or less.

  In the above chemical components, the ratio of Cr content to Fe content [Cr] / [Fe] is preferably 3.0 or less. As described above, both Cr and Fe are β-stabilizing elements, but Fe has an effect of expanding the solid solubility limit of C as compared with Cr. When the ratio [Cr] / [Fe] of Cr content to Fe content exceeds 3.0, the effect of expanding the solid solubility limit of C by Fe is lost, and the effect of suppressing the precipitation of TiC can be exhibited. . Therefore, when Fe is contained, the upper limit of the Cr content / Fe content ratio [Cr] / [Fe] is 3.0, preferably 2.5.

The titanium alloy of the present invention is an α-β type titanium alloy whose structure is composed of an α phase and a β phase at room temperature. By adding C, TiC precipitates in the structure of the α-β type titanium alloy, that is, in the titanium alloy substrate. The average area ratio of TiC precipitates in an arbitrary cross section of 0.25 mm ² or more in the titanium alloy substrate is 0. When the TiC precipitate is present at 5% or less, the average value of the average equivalent circle diameter of the TiC precipitate is 1 μm or less.

TiC is generated by the addition of C, and the structure transforms into α phase with the TiC as a nucleus. As a result, the α-β structure is refined, so that hot workability including forgeability is achieved. Will improve. On the other hand, TiC has a great influence on machinability (cutability), and when the amount of TiC deposited is large, or when the size of the TiC precipitate is large, tool wear is remarkably promoted. In particular, the influence on interrupted cutting performance by end milling or milling becomes large. Therefore, the upper limit of the average area ratio of TiC precipitates in an arbitrary cross section of 0.25 mm ² or more in the titanium alloy substrate is 0.5%, preferably 0.25%, and when TiC precipitates are present, The upper limit of the average value of the average equivalent circle diameter of the object is 1 μm, preferably 0.8 μm.

If the α-β type titanium alloy having the above component composition is processed in a hot working process such as rolling or forging under the production conditions exemplified in the following examples, 0.25 mm ² or more in the titanium alloy substrate The average area ratio of the TiC precipitates in the arbitrary cross-section can be 0.5% or less, and the average value of the average equivalent circle diameter of the TiC precipitates can be 1 μm or less.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention. These are all included in the technical scope of the present invention.

  In this example, first, an ingot of about 20 kg made of a titanium alloy having each component composition shown in Table 1 was cast by CCIM (cold crucible induction heating method) melting. The shape of the ingot is a columnar shape having a diameter of 150 mm and a height of 150 mm.

  Using this ingot, hot forging was performed under the following production conditions. The hot forging is performed by heating the ingot at 1200 ° C, forging at a forging ratio of about 1.5, cooling to water, heating to 900 ° C, and forging at a forging ratio of about 3.2 or higher. did. By this hot forging, a long plate-like titanium alloy plate having a cross-sectional shape of thickness 20 mm × width 190 mm was obtained. Next, after air cooling to room temperature, annealing was performed at 800 ° C. for 2 hours to obtain a final titanium alloy plate used in the test. From the titanium alloy plate, a structure observation test piece, an end mill cutting test piece, and a tensile test piece were sampled and subjected to the following tests.

<Tissue observation test>
In the structure observation, the average area ratio of the TiC precipitates and the average value of the average equivalent circle diameter of the TiC precipitates were obtained by investigating in the following manner.

Tissue observation specimens taken from the steady part of the titanium alloy plate (parts excluding 50 mm from the outer periphery of the four circumferences and parts excluding 2 mm from the front and back surfaces) are embedded in the synthetic resin holder so that the specimens are exposed. Polished and further buffed. Four fields of view of the polished specimen surface were photographed at a magnification of 400 × (total observation area is 0.25 mm ² ), and image analysis software (Nano System NS2K, manufactured by Nanosystem Co., Ltd.) was used from the image data. -Lt), the area ratio of the TiC precipitates in each photograph and the average equivalent circle diameter thereof were determined, and the average area ratio of the TiC precipitates and the average of the average equivalent circle diameter of the TiC precipitates as the average value of the whole The value was calculated. Table 2 shows the average area ratio of the TiC precipitates of each sample and the average value of the average equivalent circle diameter of the TiC precipitates obtained in this test. In each sample in the examples, no precipitates and inclusions other than TiC precipitates were present.

<Intermittent cutting test (end mill cutting test)>
An end mill cutting test piece having a thickness of 15 mm, a width of 180 mm, and a length of 145 mm was collected from the titanium alloy plate. An intermittent cutting test (end mill cutting test) was performed using this test piece, and the amount of tool wear (flank) was confirmed. From the test results, a tool wear amount of 60 μm or less was accepted, and the titanium alloy was judged to be excellent in intermittent cutting performance. The intermittent cutting test conditions are shown below. Table 2 shows the amount of tool wear (flank) of each sample obtained in this test.

・ Tool: H13A chip ・ Cutting speed: 25 m / mm
・ Cutting feed: 0.2 mm / rev
・ Cut amount: 0.5mm in the radial direction, 1.0mm in the axial direction
・ Cutting length: 7.25m (1cut 145mm)
・ Cutting oil: Water-soluble lubricant

<Room temperature tensile test>
A tensile test piece was collected from the titanium alloy plate described above. The tensile test was performed in accordance with ASTM standard E8. The shape and dimensions of the tensile test piece are shown in FIG. The test temperature is room temperature (25 ° C.), and the strain rate is 100 / sec. From the test results, those having a tensile strength (TS) of 900 MPa or more and an elongation (EL) of 10% or more were accepted and judged to be a high-strength titanium alloy. Table 2 shows the tensile strength (TS) and elongation (EL) of each sample obtained in this test together with the other test results described above.

  Sample No. described in Table 1 And sample Nos. Are consistent. Sample No. 1 in which the component composition shown in Table 1 and the average area ratio and average circle equivalent diameter of TiC precipitates shown in Table 2 both satisfy the conditions of the present invention. Examples 4 and 6 to 11 are invention examples, and the others are all comparative examples.

  Sample No. of Invention Example In Nos. 4 and 6 to 11, the tool wear amount obtained in the intermittent cutting test and the tensile strength (TS) and elongation (EL) obtained in the room temperature tensile test all satisfied the acceptance criteria. From the result, sample no. It can be judged that 4 and 6 to 11 are high-strength titanium alloys excellent in interrupted machinability.

  On the other hand, in the case of comparative examples (samples Nos. 1 to 3, 5, 12 to 19) that do not satisfy any one of the conditions of the present invention, the amount of tool wear determined by the intermittent cutting test and the room temperature tensile test The acceptance criteria could not be satisfied with at least one of the tensile strength (TS) and elongation (EL) determined in (1). That is, sample no. 1 to 3, 5 and 12 to 19 cannot be determined to be a titanium alloy and / or a high strength titanium alloy having excellent interruptability.

It is explanatory drawing which shows the shape and dimension of the tensile test piece used by the room temperature tensile test of the Example.

Claims (3)

In addition to containing C: 0.06 to 0.13%, Al: 3.0 to 8.5%, V: 5.0% or less, Cr: 3.0% or less, Fe: 2.% by mass. Less than 5%, Mo: 5.0% or less, Ni: 5.0% or less, Nb: 5.0% or less, Ta: 5.0% or less, or a total of 2.0 to 10 A titanium alloy containing 0.0% and the balance being Ti and inevitable impurities,
The average area ratio of TiC precipitates in an arbitrary cross section of 0.25 mm ² or more in the titanium alloy substrate is 0.5% or less, and the average value of the average equivalent circle diameter of the TiC precipitates is 1 μm or less. High-strength α-β type titanium alloy with excellent interruptability.   Furthermore, the high intensity | strength alpha-beta type titanium alloy excellent in the intermittent machinability of Claim 1 which contains Si: 1.0% or less by mass%.   Furthermore, it is excellent in the intermittent cutting property of Claim 1 or 2 which contains 1 type or 2 types of Zr: 5.0% or less and Sn: 5.0% or less in total 7.0% or less by mass%. High strength α-β type titanium alloy.

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