JP2007100666A - High strength titanium alloy engine valve for automobile - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength titanium alloy engine valve for automobile excellent in strength, fatigue strength and hot workability, and having sufficient wear resistance. <P>SOLUTION: This engine valve for automobile is made of titanium alloy which contains Al of 4.4 wt.% or more and 5.5 wt.%, Fe of 1.4 wt.% or more and 2.3 wt.% and Mo of 1.5 wt.% or more and 5.3 wt.%, and satisfies conditions of impurities that Si is less than 0.1 wt.% and C is less than 0.01 wt.%, and of which remaining is titanium and unavoidable impurities. Oxidized hardened layer is formed on a part or whole of a surface of the engine valve, is coated by hard coating or is applied to peening process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an automotive engine valve made of a high-strength titanium alloy.

  Conventionally, the engine performance (higher output, lower fuel consumption, lower noise) has been achieved by using a light and strong titanium alloy for an automobile engine valve. However, as titanium alloys are applied to higher performance engines, the strength and fatigue of titanium alloys are reduced due to problems such as an increase in load due to an increase in inertia weight accompanying an increase in engine speed and a decrease in strength due to a rise in temperature. The problem of lack of strength has become apparent. In addition, there is a strong demand for inexpensive titanium alloys in order to expand application to mass-produced vehicles.

  In order to apply a titanium alloy to an engine valve for an automobile, it is necessary to improve wear resistance which is inferior to steel. There is a method of forming a hard film on the surface for improving the wear resistance (see, for example, Patent Document 1). However, since the cost is high, it is applied only to a portion that requires a particularly high level of wear resistance. It is common. As a cheaper method, there is a method of forming an oxide hardened layer (see, for example, Patent Document 2), but there is a problem of further reducing the fatigue strength due to the deterioration of the surface properties. The demands for higher fatigue strength are becoming stricter.

  On the other hand, the Ti-6Al-4V titanium alloy, which has been used mainly from the viewpoint of strength, workability, and cost, has a problem that the strength and fatigue strength are insufficient with respect to the above-described problems. Further, as a method for improving the fatigue strength of a metal material, a peening treatment in which metal or non-metal particles are projected at a high speed is generally known. This is because compressive residual stress is applied to the surface layer of the metal material by the peening process, and fatigue cracks are less likely to occur. However, the Ti-6Al-4V automotive engine valve has a small effect of the peening process and is sufficient. Fatigue strength cannot be obtained.

  When other existing titanium alloys are applied to engine valves for automobiles, a typical Near α-type alloy Ti-6Al-2Sn-4Zr-2Mo titanium alloy or a titanium alloy further containing 0.1% Si is used. Although these are excellent in high-temperature strength, they are poor in hot workability and are more expensive than Ti-6Al-4V titanium alloys. In addition, the α-type titanium alloy has lower strength than the Ti-6Al-4V titanium alloy. In addition, β-type titanium alloys are high in strength and excellent in workability, but are expensive due to the large amount of alloy added. TiAl-based intermetallic compounds have very high strength, but as disclosed in Patent Document 3, not only is cold workability inferior, but also hot workability is inferior, so after precision casting, HIP (Hot Isostatics Press; Manufactured by a high temperature isostatic pressing process. However, such a manufacturing method is expensive because productivity is remarkably deteriorated.

  On the other hand, an inexpensive titanium alloy system has been studied based on a Ti-6Al-4V titanium alloy while maintaining or improving its strength and fatigue strength characteristics. As an example of applying this titanium alloy to an automobile engine valve, for example, as disclosed in Non-Patent Document 1, there is an example in which inexpensive off-grade sponge titanium is used as a raw material, but the strength is not sufficient. Although not limited to automotive engine valve applications, an alloy system using Fe as an alternative to the expensive V of Ti-6Al-4V titanium alloy has been studied.

  Further, in Patent Document 4, as an alloy excellent in hot workability and cold workability, Fe of 1.4% or more and less than 2.3% by mass, Al of 4% or more and less than 5.5%, An α + β type titanium alloy consisting of the balance titanium and unavoidable impurities is disclosed. However, the titanium alloy of the invention described in Patent Document 4 has a tensile strength of less than 1000 MPa, which cannot be said to have sufficient strength, and has a problem that it is insufficient in hot workability and room temperature ductility. Hold it.

  Patent Document 5 discloses that at least one of the solid solution type β stabilizing elements is 2.0 to 4.5% in terms of Mo equivalent, and at least one of the eutectoid type β stabilizing elements is 0 equivalent in terms of Fe equivalent. A high-strength, high-ductility α + β-type alloy characterized by containing 3 to 2.0%, an Al equivalent of 3 to 6.5%, and further containing 0.1 to 1.5% of Si is proposed. ing. However, when the titanium alloy of the invention described in Patent Document 5 contains 0.1% or more of Si, a compound of Ti and Si precipitates at the interface between the α phase and the β phase, and deteriorates fatigue characteristics and room temperature ductility. There is.

Japanese Patent Laid-Open No. 03-249313 Japanese Patent Laid-Open No. 2002-097914 Japanese Patent Laid-Open No. 06-002095 Japanese Patent Laid-Open No. 07-062474 Japanese Unexamined Patent Publication No. 2000-204425 “Titanium” Japan Titanium Association, Vol.50, No.2, p.93-97

  The present invention advantageously solves the above-described problems and provides a high-strength titanium alloy automotive engine valve having excellent strength, fatigue strength, and hot workability, and sufficient wear resistance. .

  As a result of intensive studies to achieve the above object, the present inventors have first found a titanium alloy having a strength superior to that of Ti-6Al-4V, fatigue strength, and excellent in hot workability. It was found that a titanium alloy automotive engine valve with sufficient wear resistance and greatly improved fatigue strength characteristics can be obtained by defining the surface treatment and microstructure using the alloy. .

The gist of the present invention is as follows.
(1) By mass%, containing 4.4% or more and less than 5.5% Al, 1.4% or more and less than 2.3% Fe, 1.5% or more and less than 5.5% Mo, impurities A high-strength titanium alloy automotive engine lube characterized in that Si is less than 0.1%, C is less than 0.01%, and the balance is made of a titanium alloy consisting of titanium and inevitable impurities.
(2) A part of the Fe is replaced by one or more of Ni of less than 0.15% by mass, Cr of less than 0.25%, and Mn of less than 0.25%. A high-strength titanium alloy automobile engine valve according to (1) above.
(3) The oxide hardened layer having a thickness of 5 to 25 μm from the surface and a Vickers hardness Hv of 500 or more is formed on a part of or the entire surface of the engine valve (1) Or the engine valve for motor vehicles made from a high strength titanium alloy as described in (2).
(4) A part or the whole of the surface is covered with a hard film having a thickness of 1 to 10 μm, and is made of the high-strength titanium alloy according to any one of the above (1) to (3) Automotive engine valves.
(5) The high-strength titanium alloy automobile engine valve according to any one of the above (1) to (4), wherein a part or the whole of the surface is subjected to shot peening treatment.
(6) The high-strength titanium according to any one of (1) to (5) above, wherein at least the α phase of the microstructure of the shaft portion is composed of an acicular α phase having a length of 50 μm or less. Alloy automotive engine valve.

  The high-strength titanium alloy automobile engine valve of the present invention has sufficiently higher strength and fatigue strength than conventional titanium alloy automobile engine valves, and is excellent in hot workability and low cost. In addition to contributing to higher output, lower fuel consumption, and lower noise of automobile engines, it is possible to obtain a wide range of effects by expanding application to mass-produced products, so industrial effects are immeasurable .

  The present invention will be described in detail below.

  The mechanical properties of the present invention are that the tensile strength at room temperature is 1050 MPa or more, which exceeds the room temperature tensile strength of Ti-6Al-4V automobile engine valves, and the elongation exceeds 5%, which is not a problem in practice. It was used as an index. In addition, the hot workability of the present invention is based on the presence or absence of cracks during engine valve manufacture. Further, the fatigue strength of the present invention is equal to or higher than the fatigue strength of a Ti-6Al-4V automobile engine valve manufactured by an equivalent manufacturing method, that is, when oxidation treatment is performed at 500 MPa or more when oxidation treatment is not performed. The index was 330 MPa or more. Moreover, it was set as the parameter | index that the abrasion resistance of this invention is more than the abrasion resistance of the engine valve for vehicles made from Ti-6Al-4V manufactured by the equivalent manufacturing method.

  In the present invention described in claim 1, the component ranges of Al, Fe, Mo, Si, and C for achieving the above-described index are defined.

  Al is an element having a high solid solution strengthening ability. Increasing the amount added increases the tensile strength at room temperature and high temperature. In order to obtain a strength of 1050 MPa or more at room temperature, it is necessary to add 4.4% or more. However, if Al is added in an amount of 5.5% or more, the stacking fault energy is increased and twin deformation is suppressed, so that hot and room temperature ductility deteriorates, and hot workability deteriorates. Furthermore, Al strengthens the α phase, but induces a smooth local slip. Therefore, if added in an amount of 5.5% or more, fatigue cracks are likely to occur where the local slip occurs, and the fatigue characteristics deteriorate. Therefore, the Al component range is set to be 4.4% or more and less than 5.5%.

  Fe is a β-stabilized substitutional solid solution element, and its strength increases with the amount of addition. By adding simultaneously with the α stabilizing element Al, an α + β type high strength alloy can be obtained. In order to obtain sufficient strength of 1050 MPa or more at room temperature, addition of 1.4% or more is necessary. However, when the amount of Fe exceeds a certain amount, segregation becomes significant. Segregation of Fe is likely to occur during solidification, and the influence cannot be eliminated in a manufacturing process such as a subsequent heat treatment. In large ingots of several hundred kilograms or more, segregation becomes prominent when 2.3% or more is added, so the amount of Fe added is less than 2.3%.

  Mo has the effects of increasing strength and improving workability. Mo is a β-stabilized substitutional element and, like Fe, functions to improve room temperature strength and high temperature strength, room temperature ductility and fatigue strength, and improve hot workability. In order to improve room temperature ductility and hot workability, addition of 1.5% or more is necessary. On the other hand, if the addition amount exceeds a certain amount, a problem of solidification segregation also occurs. Therefore, the addition amount at which solidification segregation does not become noticeable in a large ingot is set to less than 5.5%.

  Si and C are specified as impurity elements, but when these elements are contained in a certain amount or more, they adversely affect fatigue characteristics, room temperature ductility, and hot workability. As a result of investigating the content that does not adversely affect the fatigue characteristics, room temperature ductility, and hot workability, it was found that Si was less than 0.1% and C was less than 0.01%, and the upper limit was set. Since Si and C are inevitable to be contained as inevitable impurities, the lower limit of the substantial content is usually 0.005% or more for Si and 0.0005% or more for C.

  In the present invention according to claim 2, a part of Fe is replaced with one or more of Ni of less than 0.15%, Cr of less than 0.25%, and Mn of less than 0.25%. This replaces a part of Fe with an inexpensive element that works in the same way as Fe.

Here, the upper limit of the addition amount of Ni, Cr, and Mn is set to less than 0.15%, less than 0.25%, and less than 0.25%, respectively. Then, an intermetallic compound (Ti ₂ Ni, TiCr ₂ , TiMn) which is an equilibrium phase is generated, and fatigue strength and room temperature ductility deteriorate. The total amount of Ni, Cr, Mn, and Fe needs to be 1.4% or more and less than 2.3%. This is because if it is less than 1.4%, the room temperature tensile strength is reduced, and if it is 2.3% or more, the room temperature ductility is lowered.

  In this invention of Claim 3, about 500 Hv or more thickness is 5-25 micrometers from the surface layer about the thickness of the oxidation hardening layer formed in a part of engine valve surface or the whole surface. If the thickness is less than 5 μm, the oxide-cured layer may be lost during use, and if it exceeds 25 μm, the surface roughness deteriorates and the fatigue strength deteriorates. More preferably, it is 10-20 micrometers.

  In this invention of Claim 4, the thickness of the hard film formed in a part of engine valve surface or the whole surface is 1-10 micrometers. This is because if it is thinner than 1 μm, it may be worn away during use, and if it is thicker than 10 μm, it tends to crack or chip. Among these, 2 to 6 μm is desirable. Examples of the material of the hard coating include CrN, TiN, TiAlN, and the like. As a means for forming the coating, the ion plating method is preferable because it can suppress the temperature rise of the base material as compared with other means.

  In the present invention described in claim 5, a peening treatment to be applied to a part of or the entire surface of the engine valve is defined. In order to improve the fatigue strength by the peening treatment, it is preferable to apply a large compressive residual stress in the vicinity of the surface layer using a projection material having a small particle size, and it is preferable to use a material having a particle size of 200 μm or less. The projection material may be metal particles or non-metal particles.

  In the present invention described in claim 6, it is defined that at least the microstructure of the shaft portion is a structure composed of an acicular α phase having a length of 50 μm or less. The acicular α phase includes those formed during solution treatment / cooling, aging, annealing, or oxidation treatment after rough forming, and those in which the pro-eutectoid α phase formed before rough forming is elongated by processing. . Portions other than the acicular α phase are residual β phases. The reason why the acicular α phase is set to 50 μm or less is that remarkably high strength and fatigue strength are obtained.

  The peening treatment can be added only to a portion requiring particularly fatigue strength. Similarly, the oxidation hardened layer or the hard film formation can be added only to a portion particularly requiring abrasion resistance. For example, an oxidation hardened layer can be formed on the entire engine valve shown in FIG. 1, and a hard coating can be added only to the shaft end 1, and further, peening can be performed only on the neck 3.

  Hereinafter, the present invention will be described more specifically with reference to examples.

  Titanium alloys having the components shown in Table 1 were melted by a vacuum arc melting (VAR) method and cast into an ingot of about 0.5 tonnes. A wire having a diameter of 13 mm manufactured from these ingots by forging and hot rolling was used as a raw material.

  The automotive engine valve prototyped here has a shape as shown in FIG. 1. First, the shaft portion 2 and the umbrella portion 4 are roughly formed into a shape of an engine valve hot from a titanium alloy material, and the β transformation is performed. A solution treatment was performed at a temperature higher than the temperature, and the solution was cooled at a speed higher than that of air cooling, followed by cutting and oxidation treatment.

Table 2 shows the evaluation results of automobile engine valves manufactured with titanium alloys having the components shown in Table 1. The strength was evaluated by taking a tensile test piece from the shaft portion of the manufactured automobile engine valve and performing a tensile test in the atmosphere at a strain rate of 1 × 10 ⁻⁴ s ⁻¹ . In any case, the elongation was 5% or more, which is practically acceptable. Hot workability was evaluated by the presence or absence of cracks during rough forming.

  Sample No. of Comparative Example The alloys of 8 to 10 are equivalent to the α + β titanium alloy described in Patent Document 4 (including only Al and Fe). The tensile strength of automobile engine valves manufactured with these alloys is less than 1050 MPa, and the strength is insufficient. On the other hand, Sample No. of the present invention example to which Mo was added. The engine valves for automobiles manufactured with the alloys 1 to 7 have a tensile strength of 1050 MPa or more, no cracks are generated during rough forming, and have sufficient strength and excellent hot workability.

  In addition, Sample No. In the alloys 11 to 13, a part of Fe is replaced with an appropriate amount of Ni, Cr, or Mn. Automotive engine valves made of these alloys also have sufficient strength and room temperature ductility and excellent hot workability. On the other hand, the sample No. of the comparative example in which the amount of Ni, Cr, Mn exceeded the appropriate amount. The engine valve for automobiles manufactured with an alloy of 14 to 16 has cracks during rough forming and has low hot workability.

  In addition, Sample No. Alloys 17 and 18 are obtained by replacing a part of Fe with a composite of an appropriate amount of Ni, Cr, and Mn. Automotive engine valves made of these alloys also have sufficient strength and excellent hot workability. On the other hand, sample No. of the comparative example in which the total amount of Fe, Ni, Cr, and Mn exceeded an appropriate amount. The engine valve for automobiles manufactured with the alloy of 19 has cracks during rough forming and has low hot workability. Moreover, sample No. of the comparative example whose total amount of Fe, Ni, Cr, and Mn is less than an appropriate amount. The engine valve for automobiles manufactured with the alloy of 20 does not reach the tensile strength of 1050 MPa.

  In addition, sample No. The alloys of Nos. 21, 22, 23 and 24 are sample Nos. This is an alloy obtained by adding 0.1% or more of Si to the alloys of 4, 5, and 17. All automotive engine valves manufactured with these alloys are cracked during rough forming.

  Table 3 shows the specifications and various test results of the automotive engine valve of the present invention (hereinafter referred to as the present product) and the conventional Ti-6Al-4V automotive engine valve (hereinafter referred to as the conventional product).

  Here, the engine valve for automobiles is air-cooled by roughly forming the shaft portion and the umbrella portion into the shape of the engine valve hot from each titanium alloy material, and performing solution treatment at a temperature of β transformation temperature −100 ° C. or higher. After cooling at the above speed, cutting was performed, aging treatment or annealing treatment was performed, and then oxidation treatment, hard film treatment, or peening treatment was performed. In addition, a solution treatment may be abbreviate | omitted among said processes. The aging treatment or annealing treatment was performed in the range of 500 to 750 ° C. for 1 to 4 hours. The oxidation treatment was performed at a treatment temperature of 670 to 820 ° C. for 1 to 16 hours, and the oxidation hardened layer was adjusted to an appropriate thickness. When the microstructure is a needle-like structure, the solution treatment is performed at a β transformation temperature or higher, and when the microstructure is made of a fine needle-like α phase, the solution treatment is performed at a β transformation temperature to a β transformation temperature of −100 ° C. The solution treatment was omitted or only the annealing treatment was performed. In the case of an acicular structure, the solution temperature of the product of the present invention is 1010 ° C. and the conventional product is different from 1050 ° C. because the β transformation temperature is different.

The wear resistance is determined by applying a tensile load in the axial direction of the engine valve material, causing the SCM435 material to collide with the shaft surface under the conditions of a load of 98 N (10 kgf) and a vibration frequency of 500 Hz. ^The evaluation was based on the presence or absence of cracks on the surface after ⁷ times. Fatigue characteristics are the same as those for engine valves, but specimens that have undergone solution treatment, aging treatment, oxidation treatment, hard coating treatment or peening treatment are prepared, and rotational bending fatigue is performed under conditions of stress ratio R = -1, 3600 rpm and room temperature. The test was performed, and the strength that did not break at a repetition number of 1 × 10 ⁷ times was defined as fatigue strength.

  No. 1 and No. 10, the conventional product has a fatigue strength of 500 MPa, whereas the product of the present invention has a very high fatigue strength of 640 MPa.

  No. 2 and No. No. 13 is a comparison of fatigue strength between the product of the present invention and the conventional product when an oxidation hardened layer is formed. Even when an oxidation hardened layer is formed, the product of the present invention has a very high fatigue strength. No. In No. 3, even when the thickness of the oxide hardened layer is increased to 25 μm, No. 3 It has the same fatigue strength as that of the conventional 13 products with an oxide hardened layer thickness of 15 μm, and even if the thickness of the oxide hardened layer decreases during use, the remaining thickness is secured and the wear resistance is maintained. Show. However, no. As shown in FIG. 4, when the thickness of the oxide hardened layer is 30 μm or more, No. 4 is obtained. This is unsuitable because it is below the fatigue strength of the conventional product No. 3.

  No. 5 and no. No. 14 compares the product of the present invention with a conventional product when a hard coating is formed, and shows that the fatigue strength of the product of the present invention is high.

  No. 6 is a product of the present invention in which both an oxide hardened layer and a hard film are formed, and has high fatigue strength and wear resistance.

  No. 7 and no. 15 is a comparison of the fatigue strength between the product of the present invention and the conventional product when peening is performed. When the peening treatment is performed, the fatigue strength of the conventional product is only 520 MPa, while the product of the present invention has a very high fatigue strength of 690 MPa.

  No. 8 and no. Reference numeral 16 denotes a case where the peening treatment is performed after the oxidation hardened layer is formed. Even in that case, the fatigue strength of the conventional product is only 390 MPa, whereas the product of the present invention has a very high fatigue strength of 670 MPa.

  No. 9, 10, among the products of the present invention, the solution treatment is performed in the range of β transformation temperature to β transformation temperature −100 ° C., and a structure composed of acicular α phase having a length of 30 μm or less is formed. It has a very high fatigue strength of 560 MPa when no peening treatment is performed, and 700 MPa when a peening treatment is performed.

  No. 11 is a product obtained by omitting the solution treatment and performing a hard coating treatment in the product of the present invention, but even in that case, sufficiently high fatigue strength is obtained as compared with the conventional product. It has shown that it has.

It is a figure which shows the engine valve for motor vehicles with a front view.

Explanation of symbols

1 Shaft end 2 Shaft 3 Neck 4 Umbrella

Claims (6)

  It contains 4.4% or more and less than 5.5% Al, 1.4% or more and less than 2.3% Fe, 1.5% or more and less than 5.5% Mo in terms of mass%, and as impurities, Si A high-strength titanium alloy engine valve for automobiles, characterized in that is less than 0.1%, C is less than 0.01%, and is made of a titanium alloy consisting of the remaining titanium and inevitable impurities.   A part of the Fe is replaced by one or more of Ni of less than 0.15% by mass, Cr of less than 0.25%, and Mn of less than 0.25%. Item 4. A high-strength titanium alloy automobile engine valve according to Item 1.   3. The oxide hardened layer having a thickness of 5 to 25 μm from the surface and a Vickers hardness Hv of 500 or more is formed on a part of or the entire surface of the engine valve. 4. High-strength titanium alloy automotive engine valve.   4. The high-strength titanium alloy automobile engine valve according to claim 1, wherein a part or the whole of the surface is covered with a hard coating having a thickness of 1 to 10 μm.   The engine valve for automobiles made of high-strength titanium alloy according to any one of claims 1 to 4, wherein a part or the whole of the surface is subjected to shot peening treatment.   The engine valve for automobiles made of high-strength titanium alloy according to any one of claims 1 to 5, characterized in that at least the α phase of the microstructure of the shaft portion is composed of a needle-like α phase having a length of 50 µm or less. .

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