JP2001279385A - Martensitic precipitation hardening stainless steel for machine structural use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic precipitation hardening stainless steel for machine structural use, superior in cold workability and machinability compared to the conventional one and excellent in aging hardenability and fatigue characteristic. SOLUTION: The martensitic precipitation hardening stainless steel for machine structural use is composed of an alloy steel which has a chemical composition containing, by weight, <=0.03% C, 0.5-0.95% Si, <=1.0% Mn, 1.5-3.5% Cu, 5.0-8.0% Ni, 14.0-17.0% Cr, 0.65-1.5% Ti, <=0.05% Al, 0.1-0.4% Nb, <=0.015% N, <=0.005% O and other inevitable impurities and satisfying Nb/C>=10 and Ti/N>=50. Moreover, the structure after solid solution heat treatment consists of martensite and <=5 vol.% ferrite and has <=300 Hv hardness. Further, hardness can be increased to >=510 Hv by subsequent aging treatment, and hardness increment ΔHv due to aging becomes >=230.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention improves the age hardening ability by adding Cu, Si, and Ti in combination, and finds an appropriate balance between Nb and C, and Ti and N, to thereby improve the hardness after solution treatment. The present invention relates to a martensitic precipitation-hardening stainless steel for machine structures, which has made it possible to achieve both suppression of ageing and improvement of age hardening ability.

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic precipitation hardening stainless steel for machine structures, for example,
The present invention relates to bearings, gears, and the like that are required to have corrosion resistance and are used under high surface pressure, and to shafts and the like that are required to have fatigue strength.

[0003]

2. Description of the Related Art Conventionally, SUS630 is generally used as a mechanical structural member which requires high corrosion resistance and high strength. However, in recent years, as mechanical structures have become smaller and lighter, bearings and shafts have been placed in severe operating environments such as high loads and high surface pressures. Has become. S
Materials that have higher hardness than US630 include SUS
However, since this steel type contains a large amount of Al, the productivity of the steel is poor and many oxide-based inclusions that cause fatigue fracture are generated. Furthermore, these steel grades have poor cold workability and machinability, and there is a demand for reducing the hardness after solution treatment as much as possible in order to reduce the cost of manufacturing parts.

[0004]

To solve the above-mentioned problems, Japanese Patent Application Laid-Open Nos. 60-36649, 60-177134, and 4-572 relate to obtaining high hardness by aging.
6, JP-A-4-36441, JP-A-7-157850, JP-A-8-74006, etc., but all lack consideration for hardness after solution treatment, that is, high solution hardening hardness, age hardening Noh (ΔHv)
Is low. Further, in Japanese Patent Application Laid-Open No. 8-19507 and Registration 2571949, a large amount of ferrite is generated in order to reduce the hardness after the solution treatment. However, this ferrite remains even after the aging treatment and causes not only the deterioration of the age hardening ability but also the deterioration of the fatigue properties. The present invention has been made as a result of intensive studies to solve such a problem, and provides a martensitic precipitation hardening stainless steel for mechanical structures.

[0005]

According to the present invention, in order to reduce the hardness after the solution treatment, Nb and Ti are added, and the ratio of Nb to C and Ti to N is limited. C and N contents are reduced, and Si, Cu, Cr, N
By limiting the amount of addition of i, Mo, etc., it is possible to suppress the formation of ferrite which adversely affects the fatigue properties, and to further improve the balance by adding Cu, Si, and Ti which exhibit age hardening ability in a well-balanced manner. It achieves age hardening ability.

Accordingly, the martensitic precipitation-hardening stainless steel for mechanical structures of the present invention has a chemical composition in weight%,
C: 0.03% or less, Si: 0.5 to 0.95%, Mn: 1.0% or less,
Cu: 1.5 to 3.5%, Ni: 5.0 to 8.0%, Cr: 14.0 to 17.0
%, Ti: 0.65 to 1.5%, Al: 0.05% or less, Nb: 0.1 to
0.4%, N: 0.015% or less, O: 0.005% or less, Nb /
C: 10 or more, Ti / N: 50 or more, or Mo: 1%
It may include the following, and is made of alloy steel containing other unavoidable impurities, and the structure after solution treatment is martensite and 5v
ol% or less ferrite, the hardness is Hv300 or less, and the hardness is Hv510 or more due to the subsequent aging treatment, and the hardness increase ΔHv due to aging is 230 or more. Next, the reasons for limitation such as chemical composition will be described.

C: 0.03% or less When C forms a solid solution in the martensite formed after the solution treatment, it not only increases its hardness but also deteriorates the corrosion resistance. And

Si: 0.5 to 0.95% Si is an element added as a deoxidizing agent in the production of steel, but is also an element that enhances high age hardening ability by adding it in combination with Cu and Ti. Requires an addition of 0.5% or more. However, if added in excess of 0.95%, not only does the solid solution strengthening of the martensite after solution treatment increase the hardness, but also promotes the formation of ferrite, so the upper limit was made 0.95%.

Mn: 1.0% or less Mn suppresses the formation of ferrite during the solution treatment.
At the same time, non-metallic inclusions of MnS, which adversely affect fatigue properties, are generated, further impairing hot workability.
0%.

Cu: 1.5 to 3.5% Cu is an important element for exhibiting age hardening ability as added to SUS630, and has an effect of further improving corrosion resistance. When added, the hot workability is significantly impaired, so the upper limit was made 3.5%. More preferably, it is 2.6 to 3.5%.

Ni: 5.0 to 8.0% Ni is an important element that improves corrosion resistance and further suppresses the formation of ferrite, and also exhibits high age hardening ability when combined with Si and Ti. 5.For effect
0% or more is necessary.If added excessively, not only does the effect saturate, but also increases the material cost, so the upper limit is set.
8.0%. More preferably, it is 6.5 to 8.0%.

Cr: 14.0-17.0% Cr is an essential element for imparting the corrosion resistance of stainless steel, and 14.0% or more is required to sufficiently bring out its effect, and if added excessively, the formation of ferrite is promoted. Therefore, the upper limit was set to 17.0%. More desirably 14.5-16.0%
It is.

Mo: 1.0% or less Mo is known as an element for improving corrosion resistance.
You may add up to 0%. However, if added in excess, not only increases the hardness by solid solution strengthening the martensite after solution treatment but also promotes the formation of ferrite, and further increases the material cost, so the upper limit was set to 1.0%. .

Ti: 0.65 to 1.5% Ti is an important element exhibiting high age hardening ability. In the present invention requiring high hardness, 0.65% or more is required. However, when added in excess, the age hardening ability further increases,
The upper limit is set to 1.5% because non-metallic inclusions with large TiN are likely to be generated, and as a result, the effect of improving the fatigue characteristics is saturated and possibly deteriorated. More preferably the upper limit
0.95%.

Al: 0.05% or less Al is added as a deoxidizing agent in the production of steel, and when added in a large amount, combines with Ni to exhibit age hardening ability.
At the same time, a large amount of oxide-based inclusions such as alumina is generated, which not only impairs the manufacturability, but also deteriorates the fatigue characteristics due to the inclusions. Therefore, in the present invention, the upper limit is set to 0.05% since Al is not used for the appearance of age hardening ability.

Nb: 0.1 to 0.4% Nb is an element having a very strong affinity for C, and is an element effective for preventing C from forming a solid solution in martensite at the time of solution treatment and reducing its hardness. It is. For that effect
Addition of 0.1% or more is necessary. Excessive addition not only saturates the effect but also increases the material cost, so the upper limit was made 0.4%.

N: 0.015% or less N is an element inevitably present in steel, but has an extremely strong affinity for Ti and generates TiN, which is a nonmetallic inclusion, and adversely affects fatigue properties. In order to secure Ti which contributes to age hardening, it is necessary that the content be at least 0.015% or less.

O: 0.005% or less O is mostly present as oxide inclusions typified by alumina in steel, and since these inclusions greatly deteriorate fatigue characteristics, the upper limit was made 0.005%. More preferably, it is 0.003% or less.

Nb / C: 10 or more Nb / C has been found to be a very important factor influencing the hardness after solution treatment, and it is necessary to make it 10 or more for its effect.

Ti / N: 50 or more Ti / N has been found to be an important factor influencing the hardness after the solution treatment and also affecting the age hardening ability. is necessary.

Ferrite content after solution treatment: 5 vol% or less Fatigue properties are very important as mechanical structural members. The ferrite after solution treatment does not disappear by aging, and the presence of this soft ferrite in martensite adversely affects the fatigue properties. Therefore, the upper limit is 5v
ol%.

Hardness after solution treatment: Hv 300 or less Martensitic precipitation hardening stainless steel such as SUS630 is formed into a component shape by cold working or machining in the process of forming a mechanical structural member. In order to improve the workability, the upper limit of the hardness after the solution treatment was set to Hv300.

Hardness after aging treatment: Hv 510 or more The lower limit is set to Hv 510 in order to obtain a high and stable effect in order to increase the strength of the mechanical structural member.

Age hardening ability ΔHv: 230 or more The lower limit of ΔHv was set to 230 in order to minimize the hardness after solution treatment and to express the age hardening ability in a straightforward manner.

In the present invention, in addition to these alloying elements, impurities inevitable in producing steel are included. Among them, B is not an element that affects the solution hardening hardness or the aging hardness, which is a feature of the present invention, but rather may improve hot workability, and may be added as necessary. .

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION In Table 1, A to H indicate chemical components of steel according to the present invention, I to P indicate comparative steels, and Q indicates a chemical component of SUS630 which is a conventional steel. At the same time, Nb /
Also shown are C and Ti / N.

[Table 1] Table 2 shows the hardness and the amount of ferrite after solution treatment, the hardness after the aging treatment, and the age hardening ability ΔHv of the steels A to Q having the chemical components shown in Table 1.

[Table 2] Furthermore the Table 3 10 ⁷ times endurance stress obtained by rotating bending fatigue tests carried out in the test conditions shown in Table 4, the average lifetime obtained from the thrust-type rolling contact fatigue life tests performed in the test conditions shown in Table 5 And the ratio based on the conventional steel SUS630. Table 4 shows the rotating bending fatigue test method. Table 5 shows the thrust rolling life test method.

[Table 3]

[Table 4]

[Table 5]

Here, the hardness of all the steels of the present invention after solution treatment is Hv300 or less, the ferrite content is 5 vol% or less, and the hardness after aging treatment is Hv510 or more and Δv
Hv is 230 or more, and both the rotating bending fatigue property and the rolling fatigue property are good as the aging hardness increases.

On the other hand, Comparative Steel I has a high C, Comparative Steel J has a low Nb / C even if C and Nb are within the scope of the present invention, and the hardness after solution treatment is low. Since the comparative steel K is not sufficiently reduced, the comparative steel K has a large amount of Si, a high hardness after solution treatment, and a large amount of ferrite. Furthermore, the comparative steel L has a low Ti content, and does not have sufficient hardness and age hardening ability after aging treatment. Further, the comparative steel M has a low Ti / N, a high hardness after solution treatment, and does not have a sufficient age hardening ability even if Ti and N are within the scope of the present invention. Further, the comparative steel N has an excessive amount of Ti, and although the hardness and the like after the aging treatment are sufficient, the improvement of the fatigue properties is not recognized due to TiN.

Further, the comparative steel O is excessively doped with Al, and does not show improvement in fatigue properties due to oxide inclusions such as alumina regardless of the aging hardness and the like. Furthermore, the comparative steel P has a low Cu content, and the hardness after the aging treatment and the age hardening ability are not sufficiently obtained. The comparative steels K, N, and O do not have a balance between hardness and fatigue properties, which not only increases the material cost unnecessarily, but is unsuitable for use as a machine structure,
That is, it is unacceptable in designing mechanical structural members. In addition, it turns out that conventional steel does not have sufficient hardness after aging treatment because Si is low and Ti is not added.

Therefore, the steel of the present invention has a low hardness after solution treatment and a high hardness is obtained after aging treatment, and the fatigue properties are improved in proportion to the hardness. Stainless steel. Although the embodiment of the present invention has been described in detail above, this is an example, and the present invention can be applied to a mechanical structural member requiring high corrosion resistance and high hardness without departing from the purpose of the present invention.

[0031]

As described above, according to the present invention, martensitic precipitation hardening for machine structures having better cold workability and machinability than before and excellent age hardening ability and fatigue properties. Mold stainless steel can be obtained.

Claims (2)

[Claims] 1. The chemical composition in weight%, C: 0.03% or less, S
i: 0.5 to 0.95%, Mn: 1.0% or less, Cu: 1.5 to 3.5%,
Ni: 5.0 to 8.0%, Cr: 14.0 to 17.0%, Ti: 0.65 to 1.
5%, Al: 0.05% or less, Nb: 0.1 to 0.4%, N: 0.015% or less, O: 0.005% or less, Nb / C: 10 or more, Ti /
N: 50 or more, made of alloy steel containing other unavoidable impurities, the structure after solution treatment consists of martensite and 5 vol% or less ferrite, and the hardness is Hv300 or less,
Further, a martensitic precipitation-hardening stainless steel for machine structures, whose hardness is increased to Hv 510 or more by the subsequent aging treatment and the hardness increase ΔHv due to aging is 230 or more. 2. The chemical composition in weight%, C: 0.03% or less, S
i: 0.5 to 0.95%, Mn: 1.0% or less, Cu: 1.5 to 3.5%,
Ni: 5.0 to 8.0%, Cr: 14.0 to 17.0%, Ti: 0.65 to 1.
5%, Al: 0.05% or less, Nb: 0.1 to 0.4%, N: 0.015% or less, O: 0.005% or less, Mo: 1.0% or less, Nb /
C: 10 or more, Ti / N: 50 or more, made of alloy steel containing other unavoidable impurities, the structure after solution treatment consists of martensite and 5 vol% or less ferrite, and the hardness is H
A martensitic precipitation-hardening stainless steel for machine structural use having a v300 or less and a hardness of Hv510 or more due to a subsequent aging treatment and a hardness increase ΔHv of 230 or more due to aging.