JP2011219864A - Heat resistant steel for exhaust valve - Google Patents

Abstract

A heat resistant steel for exhaust valves that has a relatively low Ni content, high mechanical properties at high temperatures (for example, tensile strength, fatigue strength, wear resistance, hardness, etc.) and excellent corrosion resistance. To provide.
A heat-resistant steel for exhaust valves having the following configuration. (1) The heat resistant steel for exhaust valves is 0.50 <C <0.80 mass%, 0.30 <N <0.60 mass%, 17.0 ≦ Cr <25.0 mass%, 4.0 ≦ Ni <. 12.0 mass%, 7.0 ≦ Mn <14.0 mass%, 2.0 ≦ Mo <6.0 mass%, and 0.5 <Si <1.5 mass%, 0.025 ≦ Nb <1.0 mass% The remainder is composed of Fe and inevitable impurities, and has a composition regulated to P <0.03 mass%. (2) The heat resistant steel for exhaust valve satisfies 0.85 ≦ C + N ≦ 1.3 mass%. (3) The heat resistant steel for exhaust valve satisfies 0.05 ≦ Nb / C <1.8.
[Selection figure] None

Description

  The present invention relates to heat resistant steel for exhaust valves.

  An engine uses an intake valve for introducing a mixed gas of fuel and air into a cylinder and an exhaust valve for discharging combustion gas outside the cylinder. Of these, exhaust valves are exposed to high-temperature combustion gases, so materials with high-temperature characteristics (for example, high-temperature hardness, fatigue characteristics, high-temperature strength, wear resistance, and oxidation resistance) are used for the exhaust valves. It has been. Known exhaust valve materials include Ni-based superalloys (for example, NCF751), austenitic heat-resistant steel (for example, SUH35), and the like.

The Ni-base superalloy is a material in which the γ ′ phase is precipitated by an aging treatment, thereby increasing the strength and wear resistance at high temperatures. Ni-base superalloys are expensive but have extremely high heat resistance. Therefore, a valve using this is mainly used in a high-power engine that is exposed to a temperature of 800 ° C. or higher.
On the other hand, austenitic heat-resisting steel is a material in which M ₂₃ C ₆ type carbide is precipitated, thereby increasing the strength and wear resistance at high temperatures. Austenitic heat-resisting steel is inferior in high-temperature characteristics as compared with Ni-base superalloys, but is inexpensive. Therefore, the valve using this is mainly used for engines that do not require high heat resistance.

Various proposals have been made for materials suitable for such exhaust valves.
For example, Patent Document 1 discloses that by weight, C: 0.01 to 0.2%, Si: 1% or less, Mn: 1% or less, Ni: 30 to 62%, Cr: 13 to 20%, W : 0.01 to 3.0%, Al: 0.7% or more and less than 1.6%, Ti: 1.5 to 3.0%, and B: 0.001 to 0.01%, P : 0.02% or less, S: 0.01% or less, and a heat-resistant alloy for exhaust valves consisting of Fe and unavoidable impurities in the balance is disclosed.

  Further, Patent Document 2 discloses that by weight%, C: 0.01 to 0.10%, Si: 2% or less, Mn: 2% or less, Cr: 14 to 18%, Nb + Ta: 0.5 to 1. 5%, Ti: 2.0 to 3.0%, Al: 0.8 to 1.5%, Ni: 30 to 35%, B: 0.001 to 0.01%, Cu: 0.5% or less , P: 0.02% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, with the balance being Fe and inevitable impurities, and a predetermined component A balanced Fe—Cr—Ni heat-resistant alloy is disclosed.

Further, Patent Document 3 discloses Fe group having a composition of Fe-0.53% C-0.2% Si-9.2% Mn-3.9% Ni-21.5% Cr-0.43% N. A method for manufacturing an automotive engine valve is disclosed in which heat-resistant steel is subjected to solution heat treatment at 1100 to 1180 ° C., the umbrella portion is forged at 700 to 1000 ° C., and subjected to aging treatment.
The document describes that the hardness of the valve face portion can be increased to HV400 or more by solution treatment, forging and aging treatment of Fe-base heat-resisting steel having a predetermined composition under predetermined conditions. .

JP 2004-277860 A JP-A-9-279309 JP 2001-323323 A

Due to soaring raw material costs in recent years, the manufacturing cost of exhaust valves greatly affects fluctuations in raw material costs. In particular, since the Ni-based superalloy has a high Ni content, the raw material cost and manufacturing cost of the Ni-based superalloy exhaust valve are greatly affected by the Ni price. Therefore, a material in which the amount of Ni is further reduced and the fluctuation range of the raw material cost is reduced is desired. However, in the Ni-base superalloy, since Ni is a γ ′ phase generation element that is a strengthening phase, further reduction in Ni content makes it difficult to increase the strength using the γ ′ phase.
On the other hand, carbide precipitation type austenitic heat resistant steels are less susceptible to Ni price, but have a problem that they are inferior in high-temperature characteristics as compared to γ ′ precipitation type Ni-base superalloys. In order to solve this problem, a material in which SUH35 is strengthened (for example, overseas standard LV21-43 steel (SUH35 + 1W, 2Nb)) is also known. However, LV21-43 steel still has problems such as difficult structure control and poor hot workability.

  The problem to be solved by the present invention is that the Ni content is relatively low, the mechanical properties at high temperatures (for example, tensile strength, fatigue strength, wear resistance, hardness, etc.) are high, and the corrosion resistance is excellent. The object is to provide heat resistant steel for exhaust valves.

In order to solve the above problems, the heat-resistant steel for exhaust valves according to the present invention is summarized as having the following configuration.
(1) The heat resistant steel for the exhaust valve is
0.50 <C <0.80 mass%,
0.30 <N <0.60 mass%,
17.0 ≦ Cr <25.0 mass%,
4.0 ≦ Ni <12.0 mass%,
7.0 ≦ Mn <14.0 mass%,
2.0 ≦ Mo <6.0 mass%,
0.5 <Si <1.5 mass%, and
0.025 ≦ Nb <1.0 mass%
And the balance consists of Fe and inevitable impurities,
In the inevitable impurities,
P <0.03 mass%,
It has a regulated composition.
(2) The heat resistant steel for exhaust valve satisfies 0.85 ≦ C + N ≦ 1.3 mass%.
(3) The heat resistant steel for exhaust valve satisfies 0.05 ≦ Nb / C <1.8.

C and N are both austenite stabilizing elements and at the same time forming elements of MX type carbonitrides (including MC type carbides). In the present invention, since the amount of C + N and the Nb / C ratio are optimized, an appropriate size and an appropriate amount of MX type carbonitride (including MC type carbide) are generated in the material after the solution heat treatment. . Therefore, after the solution heat treatment, the crystal grains do not become coarse and coarse primary crystal MX carbonitride does not remain. Further, since an appropriate amount of M ₂₃ C ₆ type carbide is precipitated in the material by the aging treatment, the high temperature characteristics are improved. Furthermore, since the solid solution strengthening element is limited to Mo, the high temperature characteristics are improved.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Heat-resistant steel for exhaust valves]
The heat-resistant steel for exhaust valves according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

[1.1. Main constituent elements]
(1) 0.50 <C <0.80 mass%.
C is an austenite stabilizing element and suppresses the generation of sigma phase and Laves phase which are harmful phases. Further, C preferentially bonds with Nb to generate MC type carbide. The MC type carbide having an appropriate size and an appropriate amount suppresses crystal grain coarsening during the solution heat treatment and improves strength characteristics. In addition, the MC type carbide having an appropriate size and an appropriate amount works as a hard phase and improves wear resistance. Furthermore, C improves the wear resistance and strength characteristics by combining with Cr to form M ₂₃ C ₆ type carbide. In order to obtain such an effect, the C content needs to be more than 0.50 mass%.
On the other hand, when the C content is excessive, the amount of carbide is excessive and the workability is reduced. Therefore, the C content needs to be less than 0.80 mass%. The C content is more preferably less than 0.70 mass%.

(2) 0.30 <N <0.60 mass%.
N is an austenite stabilizing element and acts as an alternative element for austenite-generating elements such as Ni and Mn. Further, since N has a small atomic radius, it works to strengthen the matrix as an interstitial solid solution strengthening element. Further, N works in combination with substitutional solid solution strengthening elements such as Mo and W, and contributes to improvement in strength. Further, N substitutes for the C site of MC type carbide to form MX type carbonitride. In order to obtain such an effect, the N content needs to be more than 0.30 mass%. The N content is more preferably more than 0.35 mass%.
On the other hand, when the N content is excessive, it is difficult to make a solid solution in the parent phase. Therefore, the N content needs to be less than 0.60 mass%. The N content is more preferably less than 0.50 mass%.

(3) 17.0 ≦ Cr <25.0 mass%.
Cr acts to form a protective oxide film of Cr ₂ O ₃ in the operating temperature range of the exhaust valve. Therefore, Cr is an indispensable element for improving corrosion resistance and oxidation resistance. Further, Cr combines with C to form Cr ₂₃ C ₆ carbide, which contributes to improvement of strength characteristics. In order to acquire such an effect, Cr content needs to be 17.0 mass% or more. The Cr content is more preferably 18.0 mass% or more.
On the other hand, since Cr is a ferrite stabilizing element, if the Cr content is excessive, austenite is destabilized. Moreover, excessive addition of Cr promotes the formation of a sigma phase or a Laves phase, which are embrittled phases, and causes a decrease in hot workability and strength characteristics. Therefore, the Cr content needs to be less than 25.0 mass%. The Cr content is more preferably less than 23.5 mass%.

(4) 4.0 ≦ Ni <12.0 mass%.
Ni is added as an austenite stabilizing element. In order to stabilize austenite, the Ni content needs to be 4.0 mass% or more. The Ni content is more preferably 5.1 mass% or more.
On the other hand, when the Ni content is excessive, the cost is increased. Therefore, the Ni content needs to be less than 12.0 mass%. The Ni content is more preferably less than 11.5 mass%.

(5) 7.0 ≦ Mn <14.0 mass%.
Mn is added as an austenite stabilizing element. Mn not only works as an alternative element for expensive Ni, but also has an effect of increasing the solubility of N. In order to obtain such an effect, the Mn content needs to be 7.0 mass% or more. The Mn content is more preferably 7.5 mass% or more.
On the other hand, when the Mn content is excessive, the high temperature characteristics are lowered due to the lowering of the melting point. Therefore, the Mn content needs to be less than 14.0 mass%. The Mn content is more preferably less than 11.0 mass%.

(6) 2.0 ≦ Mo <6.0 mass%.
Mo works as a solid solution strengthening element of the parent phase γ phase and is an effective element for improving the high temperature strength. In order to acquire such an effect, Mo content needs to be 2.0 mass% or more. The Mo content is more preferably 2.9 mass% or more.
On the other hand, when the Mo content is excessive, the deformation resistance is increased. Further, it promotes the generation of a sigma phase and a Laves phase, which are embrittled phases, and reduces hot workability and fatigue characteristics. Therefore, the Mo content needs to be less than 6.0 mass%. The Mo content is more preferably less than 5.1 mass%.

  In addition, as a solid solution strengthening element, there is a method by addition of W in addition to Mo, but in the present invention, it is limited to addition of Mo. The amount of solid solution strengthening by solid solution strengthening elements such as Mo and W greatly depends on the atomic weight. Mo has a small atomic weight as compared with W and a large number of atoms per unit mass%, so that the solid solution strengthening amount is large. Therefore, when an equivalent solid solution strengthening amount is obtained by adding W, the precipitation of the Laves phase becomes dominant, and the same effect as Mo cannot be obtained. Therefore, in this invention, in order to acquire the effect of solid solution strengthening to the maximum, it limited to Mo addition.

(7) 0.5 <Si <1.5 mass%.
Si is an element effective for imparting a deoxidizer at the time of dissolution and oxidation resistance in a high temperature range. Si has an effect of improving strength as a solid solution strengthening element. In order to obtain such an effect, the Si content needs to be more than 0.5 mass%. The Si content is more preferably 0.55 mass% or more.
On the other hand, when the Si content is excessive, the strength characteristics are reduced. Also, Si oxides are easy to peel off. When a large amount of Si oxide is generated, the oxide layer is peeled off, resulting in poor oxidation resistance. Therefore, the Si content needs to be less than 1.5 mass%. The Si content is more preferably less than 1.1 mass%.

  Note that Fe-based alloys containing Si generally have a problem of being easily corroded in a high-temperature environment in which Pb coexists. For this reason, materials that do not contain Si have been conventionally used for exhaust valve steel. However, due to the current fuel situation (lead-free gasoline), lead corrosion resistance is no longer a problem. Therefore, in the present invention, Si is positively added and effectively used to improve oxidation resistance and strength characteristics. This is one of the major features of the present invention.

(8) 0.025 ≦ Nb <1.0 mass%.
Nb combines with C and N to precipitate MX type carbonitride (including MC type carbide, the same applies hereinafter). An appropriate size and an appropriate amount of MX type carbonitride suppresses the coarsening of crystal grains after the solution heat treatment and is effective in improving the high-temperature strength characteristics. In order to acquire such an effect, Nb content needs to be 0.025 mass% or more.
On the other hand, excessive addition of Nb promotes the formation of ferrite and generates a large amount of coarse MX type carbonitride. A part of the coarse carbonitride remains even after the solution heat treatment, which causes a decrease in hot workability. In addition, fatigue properties are also reduced. Therefore, the Nb content needs to be less than 1.0 mass%. The Nb content is more preferably less than 0.9 mass%.

In addition, as a production element of MX type carbide, there are Ti, V, etc. in addition to Nb, but in the present invention, it is limited to Nb. The reason is as follows.
Ti has strong bonding strength with C and N, and a relatively coarse primary crystal MX carbonitride (primary carbide) is crystallized in a large amount. Since these primary carbides are not dissolved even by solution heat treatment, coarse carbonitrides greatly affect the deterioration of fatigue characteristics and impact characteristics.
V is effective in improving strength characteristics. However, since V has a strong bonding force with O, V oxide is generated, and the oxidation resistance of the material is significantly reduced.
Therefore, the MX type carbonitride-forming element is limited to Nb from the balance of strength characteristics and oxidation resistance.

(9) P <0.03 mass%.
Addition of P promotes a refinement effect of carbide and is effective in improving high temperature strength characteristics. However, excessive addition of P significantly lowers the melting point and lowers the high temperature strength and hot workability. Further, depending on the aging treatment conditions, the precipitated carbide is coarsened. In the fatigue characteristics, these coarse carbides serve as starting points for breakage, leading to deterioration of the characteristics. Therefore, the P content needs to be regulated to less than 0.03 mass%. In this application, since the improvement of the high-temperature strength characteristics is aimed at by increasing the amount of solid solution strengthening element and the amount of carbide, the smaller the P content, the better in order to suppress the deterioration of workability as much as possible.

[1.2. Sub-constituent elements]
The heat resistant steel for exhaust valves according to the present invention may further contain any one or more of the following elements in addition to the elements described above.

(1) 0.001 ≦ (Mg, Ca) <0.01 mass%.
Both Mg and Ca can be added as a deoxidizing / desulfurizing agent during the melting of the alloy. Mg and / or Ca contribute to the improvement of hot workability of the alloy. In order to obtain such effects, the total content of Mg and Ca needs to be 0.001 mass% or more.
On the other hand, when the content of Mg and / or Ca is excessive, the workability tends to be lowered. Therefore, the total content of Mg and Ca needs to be less than 0.01 mass%.

(2) 0.001 ≦ B <0.03 mass%.
(3) 0.001 ≦ Zr <0.1 mass%.
Both B and Zr segregate at the grain boundaries to strengthen the grain boundaries. In order to obtain such an effect, the contents of B and Zr need to be 0.001 mass% or more, respectively.
On the other hand, when the contents of B and Zr are excessive, hot workability is impaired. Therefore, the B content needs to be less than 0.03 mass%. Moreover, Zr content needs to be less than 0.1 mass%.
One of B and Zr may be added, or both may be added.

(4) 0.01 ≦ Co <5.0 mass%.
Co acts as an austenite stabilizing element and is used as an alternative element for Ni. Co contributes to improvement of strength characteristics. In order to obtain such an effect, the Co content needs to be 0.01 mass% or more.
On the other hand, when the Co content is excessive, the cost is increased. Therefore, the Co content needs to be less than 5.0 mass%.

[1.3. Ingredient balance]
The heat-resisting steel for exhaust valves according to the present invention is characterized in that, in addition to the component elements being in the above-described range, the following conditions are satisfied.

(1) 0.85 ≦ C + N ≦ 1.3 mass%.
As described above, C and N are both strong austenite stabilizing elements and effectively work to reduce costs as expensive alternative elements for Ni. Moreover, both C and N have the effect | action which produces | generates MX type carbonitride.
An appropriate size and an appropriate amount of MX type carbonitride suppresses the coarsening of crystal grains after the solution heat treatment and is effective in improving the high-temperature strength characteristics. In order to obtain such an effect, C + N needs to be 0.85 mass% or more. C + N is more preferably 0.9 mass% or more.
On the other hand, when C + N is excessive, a large amount of coarse MX carbonitride is generated. A part of the coarse carbonitride remains even after the solution heat treatment, which causes a decrease in hot workability. Therefore, C + N needs to be 1.3 mass% or less. C + N is more preferably 1.15 mass% or less.

(2) 0.05 ≦ Nb / C <1.8.
An appropriate size and appropriate amount of MX carbonitride has a role of preventing crystal grain coarsening due to the pinning effect. In order to obtain such an effect, the ratio (= Nb / C) of the Nb content (mass%) to the C content (mass%) needs to be 0.05 or more. Nb / C is more preferably 0.1 or more.
On the other hand, when Nb is relatively excessive with respect to C, Nb is preferentially bonded to C, and a large amount of coarse primary crystal MX carbonitride is crystallized. Coarse primary crystal MX carbonitride does not disappear completely even after the solution heat treatment, which causes a decrease in fatigue characteristics. Further, C is depleted, and the amount of precipitation of M ₂₃ C ₆ type carbide effective for improving wear resistance and strength characteristics is reduced. Therefore, Nb / C needs to be less than 1.8. Nb / C is more preferably 1.3 or less.

[2. Manufacturing method of heat-resistant steel for exhaust valves]
The manufacturing method of the heat-resistant steel for exhaust valves according to the present invention includes a dissolution casting process, a homogenization heat treatment process, a forging process, a solution heat treatment process, and an aging process.

[2.1. Melting and casting process]
The melting and casting process is a process of melting and casting raw materials blended to have a predetermined composition. The raw material melting method and the molten metal casting method are not particularly limited, and various methods can be used. The melting conditions may be any conditions as long as the components are uniform and a castable molten metal is obtained.

[2.2. Homogenization heat treatment process]
The homogenizing heat treatment step is a step of homogenizing heat treatment of the ingot obtained in the melt casting step. The homogenization heat treatment is performed to homogenize the components of the ingot.
As the homogenization heat treatment conditions, optimum conditions are selected according to the components. Usually, the heat treatment temperature is 1100 to 1250 ° C. The heat treatment time is 5 to 25 hours.

[2.3. Forging process]
The forging step is a step of plastically deforming the ingot subjected to the solution heat treatment into a predetermined shape. The forging method and forging conditions are not particularly limited as long as the target shape can be efficiently manufactured.

[2.4. Solution heat treatment process]
The solution heat treatment step is a step for solution heat treatment of the material obtained in the forging step. The solution heat treatment is performed to eliminate coarse primary crystal MX carbonitride.
As the solution heat treatment conditions, optimum conditions are selected according to the components. In general, the higher the temperature of the solution heat treatment, the lower the amount of primary carbide remaining, and the more the amount of fine carbide in the grains precipitated during the aging treatment increases, which is effective in improving fatigue characteristics. However, when the treatment is performed at a temperature higher than 1200 ° C., precipitation of grain boundary reaction carbide is promoted, resulting in deterioration of characteristics. Accordingly, the solution heat treatment condition is preferably 1000 to 1200 ° C. × 20 minutes or more + oil cooling treatment.

[2.5. Aging process]
The aging step is a step of aging the material after the solution heat treatment. The aging process is performed to precipitate M ₂₃ C ₆ type carbide.
As the aging treatment conditions, optimum conditions are selected according to the components. Although it depends on the components, the aging treatment condition is preferably 700 to 850 ° C. × 2 hours or more + air cooling treatment.

[3. Action of heat-resistant steel for exhaust valves]
C and N are both austenite-stabilizing elements and MX-carbonitride forming elements. In the present invention, since the amount of C + N and the Nb / C ratio are optimized, an appropriate size and an appropriate amount of MX carbonitride are generated in the material after the solution heat treatment. Therefore, after the solution heat treatment, the crystal grains do not become coarse and coarse primary crystal MX carbonitride does not remain. Further, since an appropriate amount of M ₂₃ C ₆ type carbide is precipitated in the material by the aging treatment, the high temperature characteristics are improved. Furthermore, since the solid solution strengthening element is limited to Mo, the high temperature characteristics are improved.
Moreover, since the addition amount of Si was optimized, oxidation resistance is improved and solid solution strengthening is also achieved. Furthermore, since the amount of Ni is increased as compared with the conventional austenitic heat resistant steel, the γ phase is stabilized and the toughness is improved.

(Examples 1-24, Comparative Examples 1-16)
[1. Preparation of sample]
Alloys having the compositions shown in Table 1 and Table 2 were melted in a high frequency induction furnace to obtain a 50 kg ingot. The melted ingot was subjected to a homogenization heat treatment at 1180 ° C. for 16 hours, and then forged into a bar with a diameter of 18 mm. The forged material was further subjected to solution heat treatment (ST). The solution heat treatment conditions were 1050 ° C. × 30 minutes-oil cooling (Examples 1 to 24) or 1050 ° C. × 30 minutes-oil cooling (Comparative Examples 1 to 16). Furthermore, aging treatment (AG) of 750 ° C. × 4 hours—air cooling was performed on the material after ST.

[2. Test method]
[2.1. Hardness]
The hardness at room temperature was measured on a C scale using a Rockwell hardness meter. Moreover, the hardness in 800 degreeC was measured by the measurement load of 5 kg using the high temperature Vickers hardness meter.
[2.2. Tensile test]
A test piece having a test part diameter of 8 mm and a test piece length of 90 mm was cut out from each material. Using this test piece, a tensile test was performed at 800 ° C. to measure the tensile strength.
[2.3. Fatigue test]
A test piece having a parallel part diameter of 8 mm and a test piece length of 90 mm was cut out from each material. Using this test piece, an Ono type rotating bending fatigue test was performed at 800 ° C., and the fatigue strength was measured 10 ⁷ times.
[2.4. Oxidation test]
A cylindrical sample having a diameter of 8 mm and a length of 17 mm was prepared from each material. This test piece was subjected to continuous heating for 400 hours in an air atmosphere at 850 ° C. and air-cooled. The increase in oxidation was calculated from the weight difference before and after the test and used as an index of oxidation resistance.

[3. result]
Tables 3 and 4 show the results. Table 3 and Table 4 show the following.
(1) Each of Examples 1 to 24 has a high temperature hardness at 800 ° C. of about 200 HV or more, and has sufficient high temperature wear resistance required for use in exhaust valve applications. Moreover, all Examples 1-24 have the tensile strength of 370 Mpa or more in 800 degreeC. These are affected by solid solution strengthening by Mo and strengthening by carbides (particularly, M ₂₃ C ₆ type carbides).

(2) Examples 1 to 24 all have 10 ⁷ times fatigue strength of 240 MPa or more, and are excellent in high temperature fatigue characteristics as carbide precipitation type austenitic heat resistant steel used for exhaust valve materials. This is because the coarsening of crystal grains is suppressed by the pinning effect of the Nb-based MX type carbonitride, and the size and amount of coarse carbides (primary crystal carbides) that cause early fracture are Nb / This is considered to be due to optimization by the C standard.

(3) Comparative Examples 1 and 2 are existing steels. Comparative Example 1 is SUH35, and Comparative Example 2 is LV21-43, but all have low mechanical properties, fatigue properties, and oxidation resistance at high temperatures.
Since Comparative Example 3 has a low Mo content, and Comparative Example 4 has an excessive Nb content, both have low mechanical properties and fatigue strength at high temperatures.
Comparative Examples 5 and 6 in which part or all of Mo is replaced with W have low mechanical properties at high temperatures, and in particular, fatigue strength is low.
Ti and V are MX-type carbonitride-forming elements like Nb, and are considered to have the same effect as Nb. However, Comparative Examples 7 and 8 to which these are added have a larger oxidation gain than Examples 1 to 24. This is presumably because Ti and V have higher bonding strength with O than Nb, and oxide formation easily occurred. Further, Comparative Example 8 has a low fatigue strength of 10 ⁷ times. This is presumably because the bond strength between Ti, C, and N is large, so that stable and coarse carbonitrides are generated, causing early breakage under repeated stress. Therefore, Ti and V cannot be substitute elements for Nb.
In Comparative Example 9, since the P content is excessive, the fatigue strength is low.
Since Comparative Example 10 has a low Si content, and Comparative Example 11 has an excessive Si content, both have low oxidation resistance.
Since Nb / C is small in Comparative Example 12, and Nb / C is too large in Comparative Example 13, both have low fatigue strength.
In Comparative Example 14, since C + N is small, mechanical properties and fatigue strength at high temperatures are low. Moreover, since C + N is too large in Comparative Example 15, the fatigue strength is low.
Furthermore, since the comparative example 16 has excessive Mo, the fatigue strength is low.

On the other hand, it turned out that Examples 1-24 have favorable oxidation resistance in the oxidation resistance test in the air | atmosphere atmosphere of 850 degreeC x 400 hours compared with Comparative Examples 1-16.
In addition, as shown in Tables 3 and 4, in a more preferable range of components, the high temperature hardness, fatigue characteristics, and oxidation resistance required for the exhaust valve are optimized at a satisfactory level. Specifically, the high-temperature hardness at 800 ° C. is 210 or more, the 10 ⁷ times fatigue strength at 800 ° C. is 260 MPa or more, and the oxidation increase after the 850 ° C. × 400 h oxidation test is 1.3 mg / cm ² or less.

  From the above results, it was found that the heat-resistant steel according to the present invention is excellent in high temperature characteristics and useful as a material for exhaust valves.

  The embodiment of the present invention has been described in detail above, but the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

  The heat resistant steel for exhaust valves according to the present invention can be used for exhaust valves of various engines.

Claims (4)

Heat-resistant steel for exhaust valves with the following configuration.
(1) The heat resistant steel for the exhaust valve is
0.50 <C <0.80 mass%,
0.30 <N <0.60 mass%,
17.0 ≦ Cr <25.0 mass%,
4.0 ≦ Ni <12.0 mass%,
7.0 ≦ Mn <14.0 mass%,
2.0 ≦ Mo <6.0 mass%,
0.5 <Si <1.5 mass%, and
0.025 ≦ Nb <1.0 mass%
And the balance consists of Fe and inevitable impurities, and
In the inevitable impurities,
P <0.03 mass%
It has a regulated composition.
(2) The heat resistant steel for exhaust valve satisfies 0.85 ≦ C + N ≦ 1.3 mass%.
(3) The heat resistant steel for exhaust valve satisfies 0.05 ≦ Nb / C <1.8. 0.001 ≦ (Mg, Ca) <0.01 mass%
The heat-resistant steel for exhaust valves according to claim 1, further comprising: 0.001 ≦ B <0.03 mass%, and
0.001 ≦ Zr <0.1 mass%
The heat resistant steel for exhaust valves according to claim 1 or 2, further comprising at least one selected from the group consisting of: 0.01 ≦ Co <5.0 mass%
The heat-resistant steel for exhaust valves according to any one of claims 1 to 3, further comprising: