JP2013001961A - α-TYPE TITANIUM MEMBER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of a spotted pattern that can be visually observed even when cutting work is applied to an α-type titanium member which is produced by forging and has at least one external surface to which cutting work is applied.SOLUTION: In the α-type titanium member which is produced by forging and has at least one external surface to which cutting work is applied, the surface to which cutting work is applied has an inclination of the c-axis of 15° or less and 75° or more, the inclination being the angle of the direction perpendicular to a cutting surface to the c-axis of a crystal lattice, and has a crystal orientation distribution in which the number of crystals having a crystal grain size of 100 μm or more is 10 pieces/mmor less.

Description

  The present invention relates to an α-type titanium member that is manufactured by forging and has at least one outer surface on which cutting is performed.

  For example, a member made of an α-type titanium member (pure titanium member), such as a pure titanium round bar, has its entire surface or a part of its surface cut by a cutting tool (for example, turning with a lathe). There is.

  At this time, a visible mottled pattern composed of countless white spots may be formed on the cutting surface. This mottled pattern greatly reduces the surface quality of the product, and even if it is removed by polishing or the like, a significant increase in manufacturing cost is inevitable.

  Although it has not been reported so far that such a mottled pattern is generated on the cutting surface by cutting the pure titanium member, Patent Document 1 discloses that the surface of the titanium ring for electrodeposition drum is polished. An invention relating to the resulting crepe pattern is disclosed.

  Although this crepe pattern is different from the mottled pattern, the surface hardness is locally different because the C-axis of the hexagonal crystal grains of titanium is close to the vertical direction on the titanium plate surface. A dust pattern is generated by polishing.

  Patent Documents 2 and 3 disclose inventions related to the control of the crystal orientation of the titanium material, but each invention is intended to improve press formability, and does not relate to the mottled pattern. Absent.

Japanese Patent Laid-Open No. 9-20971 JP 2010-150607 A JP 2011-26649 A

  An object of the present invention is to provide an α-type titanium member that has at least one outer surface that is manufactured by forging and that is subjected to cutting, and that does not generate a visible mottled pattern even when the outer surface is cut. Is to provide.

The present invention is an α-type titanium member that is manufactured by forging and has at least one outer surface to be cut, and the surface to be cut is perpendicular to the cutting surface and the c-axis of the crystal lattice A crystal orientation distribution in which the inclination of the c-axis, which is an angle formed by the above, is 15 ° or less and 75 ° or more, and the crystal grain size is 100 μm or more and 10 crystals / mm ² or less. Type titanium member.

  According to the present invention, a visible mottle pattern is not generated even when cutting is performed on this one outer surface of an α-type titanium member which is manufactured by forging and has at least one outer surface to be cut. Can do.

1 (a) and 1 (b) are metallographic photographs showing an example of SEM surface observation results after a cutting test of a mottled pattern. FIGS. 1 (b) and 1 (d) It is a metallographic photograph showing an example of the surface observation result by an optical microscope after electropolishing and etching the mottled portion. FIG. 2 is a graph showing the results of investigating the relationship between the c-axis tilt of a pure titanium crystal lattice (fine hexagonal lattice) and the Schmitt factor (cosφ × cosλ). FIG. 3 is a graph showing the results of determining the relationship between the c-axis inclination of the pure titanium crystal lattice and the slip generation stress σ0. FIG. 4 is a graph showing the results of EBSD results for Sample A. FIG. 5 is a graph showing the results of EBSD results for Sample B.

A mode for carrying out the present invention will be described with reference to the accompanying drawings.
1 (a) and 1 (b) are metallographic photographs showing an example of a surface observation result by an optical microscope after a cutting test of a mottled pattern. FIGS. 1 (b) and 1 (d) ) Is a metallographic photograph showing an example of a surface observation result by an optical microscope after electropolishing of a mottled portion.

  As shown in the encircled portion of FIG. 1A, the surface of the mottled pattern is stripped by cutting. When the same portion is observed after etching after electropolishing, as shown in FIG. 1C, the mottled portion is a collection of crystal grains having several orientations called one crystal grain or colony. It turns out that it corresponds to the body.

  In addition, as shown in FIGS. 1 (b) and 1 (d), even in a titanium material in which a mottled pattern is not visually observed, a phenomenon in which the surface is stripped occurs when observed with an optical microscope. It was found that the crystal grains were fine and could not be recognized visually.

  Furthermore, the size of the crystal observed as a mottled pattern when the surface is peeled off by cutting is of a crystal grain size of approximately 100 μm or more from several visual results and observation results with an optical microscope. Was confirmed.

  On the other hand, it is known that compressive stress is generated in front of the cutting tool tip (uncut surface) on the cutting surface during cutting, and tensile stress is generated behind the cutting tool tip (cut surface by the tool). Yes. For this reason, it is considered that the tensile stress generated behind the cutting bit tip (cutting surface by the cutting tool) greatly affects the surface of the cutting surface, and slip deformation of pure titanium crystal grains (fine hexagonal lattice) that is α-type titanium. The effect of crystal orientation on the was simulated.

First, when a tensile stress is applied to pure titanium grains, which are fine hexagonal lattices, the active slip surface and its direction are investigated.
Using Schmitt's law that the critical resolved shear stress (CRSS) is constant, the Schmid factor of each slip surface of the α phase is calculated, and the slip deformation is calculated from the value of the critical shear stress CRSS described in the literature. The stress (yield stress) σ 0 for starting the process was determined.

  Schmidt's law is expressed by equation (1). Here, τ0 is the critical shear stress (CRSS, MPa), φ is the angle (°) between the cutting direction and the normal of the sliding surface, and λ is the angle (°) between the cutting direction and the sliding direction. , (Cos φ × cos λ) is a Schmitt factor.

σ0 = τ0 / (cosφ × cosλ) (1)
The planes and directions in which the α-phase slip deformation occurs are limited to the four shown in FIG. For these four slip planes and directions, Schmid factor was calculated as an axial ratio c / a = 1.58829). The calculation is performed by changing the inclination of the c-axis (the angle θ between the cutting direction and the c-axis) at which the Schmid factor changes the most, and the Schmitt factor when the c-axis is inclined in the a-axis direction <11-20> is obtained. It was.

FIG. 2 is a graph showing the results of investigating the relationship between the c-axis tilt of a pure titanium crystal lattice (fine hexagonal lattice) and the Schmitt factor (cosφ × cosλ).
As shown in the graph of FIG. 2, the bottom slip and the conical surface (conical surface a + c) slip in the a + c axis direction are large, and the conical surface (conical surface a) slip in the a axis direction is small, less than 0.2. I understand.

  Literature on critical shear stress CRSS of α phase Titanium and Its Alloys, Lesson5. Transformation and Reclamation of Titanium and Its Alloy P. 3 (ASM International), τ0 = 107 MPa, column surface τ0 = 90 MPa, and conical surface aτ0 = 97 MPa were used. Although τ0 (critical shear stress CRSS) of the conical surface a + c is unknown, it is calculated as 720 MPa because it is considered to be about eight times the normal column surface slip here. The slip generation stress σ0 at which slip deformation starts is determined by substituting the Schmitt factor and CRSS into equation (1).

FIG. 3 is a graph showing the results of determining the relationship between the c-axis inclination of the pure titanium crystal lattice and the slip generation stress σ0.
Since the active slip is the slip surface where the slip generation stress σ0 is the lowest, the c-axis inclination θ = 0 to 6 ° is the conical surface a + c, θ = 6 to 69 ° is the bottom surface, and θ = 69 to 90 °. Then, it becomes each slip surface of the column surface. However, the conical surface a + c and the weight surface a have a large slip generation stress, and the column surface is 69 ° or more and lower than the bottom surface slip generation stress. Only slipping needs to be considered.

On the other hand, since the tensile strength of JIS type 2 pure titanium is about 400 MPa, the titanium material itself will be destroyed when more tensile stress is applied.
Therefore, if the inclination of the c-axis of the crystal lattice of pure titanium is 15 ° or less and 75 ° or more, the titanium material is destroyed with almost no crystal slippage.

Therefore, the phenomenon that the surface was stripped was assumed to be related to the crystal orientation of pure titanium, and the crystal orientation analysis of the ground surface was performed.
Two types of samples A and B (made of pure titanium equivalent to JIS type 2) formed into a round bar by forging were cut into circles, and the cut surfaces were cut to investigate the occurrence of mottled patterns.

Forging and heat treatment of each test piece were performed under the conditions described below.
For sample A, a titanium material corresponding to JIS type 2 was manufactured by a VAR melting method, and heating and forging were repeated in a temperature range equal to or higher than the β transformation point, and forging with a total cross-section reduction rate of 60% or more was performed. Thereafter, heating, forging, and rolling were performed in a temperature range below the β transformation point to obtain a round bar having a diameter of 64 mm. This round bar having a diameter of 64 mm was subjected to strain relief annealing at 630 ° C.

  Sample B was formed into a round bar having a diameter of 86 mm by heating, forging, and rolling in an ingot produced by the VAR melting method in a temperature range below the β transformation point. The round bar with a diameter of 86 mm was subjected to strain relief annealing at 705 ° C.

  As a result of cutting the cut surface of each test piece, Sample A was at a pass level although a mottled pattern was slightly generated. On the other hand, Sample B had a lot of mottled patterns and was at a reject level.

In order to measure the crystal orientation of samples A and B, the surface after cutting of samples A and B was smoothed by removing the strain layer on the surface by electropolishing, and then the crystal orientation was measured by EBSD (electron backscatter diffraction). Distribution was measured. The field of view for observation is 1.17 mm × 0.89 mm (= 1.0413 mm ² ).

  FIG. 4 is a graph showing the result of the EBSD result of the sample A, and FIG. 5 is a graph showing the result of the EBSD result of the sample B. This result was arranged in such a manner that the c-axis of the fine hexagonal lattice, which is a crystal lattice of pure titanium, was inclined from the direction perpendicular to the cutting surface, as in the test whose result is shown in FIG.

  From the observation result by the optical microscope described above, the phenomenon that the surface is peeled off and it can be recognized as a mottled pattern by visual observation is the crystal grain size of about 100 μm or more. We focused on crystal grains. However, even if the crystal grain is 100 μm or less, it may be visually confirmed as a mottled pattern when a colony that is an aggregate of crystal grains having the same crystal orientation is formed.

  In the sample A, there are 17 crystals having a crystal grain size of 100 μm or more in a region where the inclination of the c-axis (the angle θ formed by the cutting direction and the c-axis) is 65 ° or more, whereas the sample B has the c-axis It was found that 34 crystals were present in a region where the inclination of 65 was 65 ° or more. Further, when the inclination of the c-axis was 75 ° or more, they were 10 and 28, respectively.

From the above results, the following was found.
(1) The crystal orientation of the cross section of the pure titanium forged material is a mottled pattern from the results of observation with an optical microscope, with crystals having a grain size of 100 μm or more present in crystal grains having a c-axis inclination of 65 ° or more. What is visually observed is a crystal grain of titanium of 100 μm or more.

  (2) As a result of the simulation, there is a correlation between the inclination of the c-axis and the slip generation stress. In the region of 15 ° or less and 75 ° or more, the tensile strength of titanium is exceeded and cutting is performed. The titanium crystal breaks with almost no slip deformation.

(3) As a result, the inclination of the c-axis has an angle greater than or less and 75 ° 15 °, and 100μm or more crystal grains 10 / mm ² to super present, plaques when performing cutting It becomes a pattern and becomes a reject level.

For this reason, in an α-type titanium member that is manufactured by forging and has at least one outer surface that is subjected to cutting, the angle between the surface to be cut and the direction perpendicular to the cutting surface and the c-axis of the crystal lattice If there is a crystal orientation distribution in which the inclination of the c-axis is 15 ° or less and 75 ° or more and the crystal grain size is 100 μm or more is 10 pieces / mm ² or less, Even after cutting, no visible mottled pattern is generated.

Claims (1)

An α-type titanium member manufactured by forging and having at least one outer surface on which cutting is performed, and an angle formed between a direction perpendicular to the cutting surface and a c-axis of the crystal lattice on the surface on which the cutting is performed An α-type titanium member having a crystal orientation distribution in which the inclination of the c-axis is 15 ° or less and 75 ° or more and the crystal grain size is 100 μm or more is 10 pieces / mm ² or less .