JP7131225B2 - Precipitation Hardening Martensitic Stainless Steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a precipitation hardening martensitic stainless steel, and more particularly to a precipitation hardening martensitic stainless steel having excellent corrosion resistance while maintaining suitable strength and toughness.

Precipitation hardening stainless steel refers to steel obtained by adding a small amount of Al, Cu, Mo, Ti, etc. to Cr-Ni stainless steel and precipitating an intermetallic compound in the matrix phase by heat treatment. Precipitation hardening stainless steels are classified into martensitic, semi-austenitic, and austenitic types according to the structure of the parent phase.
Among these, precipitation hardening martensitic stainless steels such as SUS630 are used for aerospace structural members and the like because they are excellent in corrosion resistance, strength and toughness. However, conventional precipitation hardening martensitic stainless steels have a problem of poor balance between strength and toughness.

For example, SUS630, which uses Cu as the main hardening element, exhibits relatively low yield strength. On the other hand, alloys using Al and Ti as hardening elements (eg, PH13-8Mo, Custom 450, etc.) have been designed to increase strength. However, it is difficult for these alloys to obtain high impact properties while maintaining high tensile strength (>1450 MPa).

In order to solve this problem, various proposals have been conventionally made.
For example, in Patent Document 1,
(a) in mass %, C: ≤ 0.2%, 7% ≤ Ni ≤ 14%, 0% ≤ Co ≤ 3.5%, 9.5% ≤ Cr ≤ 14%, 0.5% ≤ Mo ≤ 3%, 0.25%<Al<1%, and 0.75%<Ti≦2.5%, the balance being Fe and impurities, and
(b) A precipitation-strengthened stainless steel is disclosed that satisfies a predetermined relational expression.
In the same document, when Mo is added to a precipitation-strengthened martensitic stainless steel with a low C content, and furthermore, by optimizing the ratio and amount of Al and Ti that form compounds with Ni, corrosion resistance, tensile strength and It is described that the 0.2% yield strength can be greatly improved while maintaining ductility.

In Patent Document 2,
(a) By weight %, Cr: 10 to 19%, Ni: 5.5 to 10%, Si: 0.4% or less, Mn: 2.0% or less, Al: 1.10 to 2.00% , Ti: 0.5 to 2.0%, C: 0.03% or less, and N: 0.04% or less, the balance being Fe and unavoidable impurities, and further,
(b) A martensitic stainless steel is disclosed that satisfies a predetermined relational expression.
In the same document,
(a) A positive combined addition of Al, Ti, and Ni provides strength superior to that of conventional steel, and
(b) It is described that the workability is remarkably improved by reducing C and N and limiting the amount of Si.

In Patent Document 3,
(a) 9% ≤ Cr ≤ 13%, 1.5% ≤ Mo ≤ 3%, 8% ≤ Ni ≤ 14%, 1% ≤ Al ≤ 2%, Al + Ti ≥ 2.25% in terms of composition by weight 0.5% ≤ Ti ≤ 1.5%, measurement limit ≤ Co ≤ 2%, Mo + (W/2) ≤ 3%, measurement limit ≤ W ≤ 1%, measurement limit ≤ P ≤ 0.02%, measurement limit ≤ S ≤ 0.0050%, measurement limit ≤ N ≤ 0.0060%, measurement limit ≤ C ≤ 0.025%, measurement limit ≤ Cu ≤ 0.5% , measurement limit ≤ Mn ≤ 3%, measurement limit ≤ Si ≤ 0.25%, measurement limit ≤ O ≤ 0.0050%, and
(b) A martensitic stainless steel is disclosed that satisfies a predetermined relationship.
The document describes that such martensitic stainless steel is excellent in corrosion resistance, strength and toughness.

In Patent Document 4, by mass, C of 0.1% or less, N of 0.1% or less, Cr of 9.0% or more and 14.0% or less, Ni of 9.0% or more and 14.0% or less , 0.5% to 2.5% Mo, 0.5% or less Si, 1.0% or less Mn, 0.25% to 1.75% Ti, and 0.25% or more A precipitation hardening martensitic stainless steel containing 1.75% or less of Al with the balance being Fe and incidental impurities is disclosed.
The document describes that such a precipitation hardening martensitic stainless steel is excellent in structural stability, strength, toughness, corrosion resistance, and productivity.

In Patent Document 5,
(a) in weight percent, trace ≤ C ≤ 0.03%, trace ≤ Si ≤ 0.25%, trace ≤ Mn ≤ 0.25%, trace ≤ S ≤ 0.020%, trace ≤ P ≤ 0.040 %, 8%≦Ni≦14%, 8%≦Cr≦14%, 1.5%≦Mo+W/2≦3.0%, 1.0%≦Al≦2.0%, 0.5%≦Ti ≤ 2.0%, 2% ≤ Co ≤ 9%, trace ≤ N ≤ 0.030%, and trace ≤ O ≤ 0.020%, the balance being iron and impurities, and
(b) A martensitic stainless steel is disclosed that satisfies a predetermined relationship.
The document describes that such martensitic stainless steels have high mechanical strength properties and toughness, as well as high corrosion resistance.

In Patent Document 6,
(a) By mass, 0.1% or less of C, 0.1% or less of N, 9.0% or more and 14.0% or less of Cr, 9.0% or more and 14.0% or less of Ni, 0. 5% to 2.5% Mo, 0.5% or less Si, 1.0% or less Mn, 0.25% to 1.75% Ti, 0.25% to 1.75% a steam turbine low-pressure final stage long blade made of precipitation hardening martensitic stainless steel containing Al with the balance being Fe and unavoidable impurities,
(b) A steam turbine rotor is disclosed in which a disc having a predetermined composition is joined to the final stage of a turbine rotor made of low alloy steel.
The document describes that a high-efficiency, large-capacity steam turbine can be manufactured by such a method.

Furthermore, in Patent Document 7,
(a) In mass %, C: 0.02 to 0.10%, Si: ≤0.25%, Mn: 0.001 to 0.10%, P: ≤0.010%, S: ≤0. 010%, Ni: 8.5-10.0%, Cr: 10.5-13.0%, Mo: 2.0-2.5%, N: 0.001-0.010%, Al: 1 .15 to 1.50%, Cu: <0.10%, and Ti: ≤0.20%, with the balance consisting of Fe and unavoidable impurities, and
(b) A steam turbine blade steel is disclosed that satisfies a predetermined relationship.
The document describes that such steel for steam turbine blades can achieve both a 0.2% proof stress of 1450 MPa or more and a Charpy impact property of 15 J or more.

Precipitation hardening martensitic stainless steels are required to have a better balance between strength and toughness in the future. For example, aircraft members and high-strength fasteners are required to have a tensile strength of ≧1550 MPa and an impact value of ≧30 J/cm ² .
In precipitation hardening martensitic stainless steel, the strength is affected by the type and amount of precipitates. In particular, recent precipitation hardening stainless steels actively use intermetallic compounds to increase strength. On the other hand, toughness is also presumed to be related to the type and amount of precipitates, but details have not been clarified. Therefore, in precipitation hardening stainless steel, if a strengthening element is simply added excessively, strength characteristics can be improved, but there is a problem that this is accompanied by a significant decrease in toughness.
Furthermore, in aerospace members, etc., excellent strength, toughness, and corrosion resistance are required. However, there has been no proposal of a precipitation hardening martensitic stainless steel that exhibits high corrosion resistance while maintaining adequate strength and toughness.

WO2012/002208 JP-A-02-310339 Japanese Patent Publication No. 2008-546912 JP 2013-147698 A Japanese Patent Publication No. 2017-503083 JP 2013-209742 A JP 2013-241670 A

The problem to be solved by the present invention is to provide a precipitation hardening martensitic stainless steel exhibiting high corrosion resistance while maintaining appropriate strength and toughness.

In order to solve the above problems, a precipitation hardening martensitic stainless steel according to the present invention has the following constitution.
(1) The precipitation hardening martensitic stainless steel is
C<0.10 mass%,
0.01 ≤ Si ≤ 0.10 mass%,
0.01≦Mn≦0.10 mass%,
P≤0.010 mass%,
S ≤ 0.010 mass%,
7.5≦Ni≦11.0 mass%,
10.0≦Cr≦14.0 mass%,
1.0≤Mo≤2.5 mass%,
0.001≦N≦0.010 mass%,
0.40≦Al≦1.40 mass %,
Cu<0.10 mass%,
0.30 ≤ Ti ≤ 1.40 mass%, and
0≦Nb<0.50 mass%
with the remainder consisting of Fe and unavoidable impurities.
(2) The precipitation hardening martensitic stainless steel satisfies the following formulas (1) to (4).
1.00≦[Al]+[Ti]+[Nb]≦2.00 (1)
4.00≦[Ni]/([Al]+[Ti]+[Nb])≦8.00 (2)
8.00≦Ni _eq ≦12.00 (3)
16.00≦Cr _eq ≦21.00 (4)
however,
Ni _eq =[Ni]+0.11[Mn]−0.0086[Mn] ² +0.44[Cu]+18.4[N]+24.5[C],
Cr _eq =[Cr]+1.21[Mo]+0.48[Si]+2.2[Ti]+2.48[Al],
[X] represents the content of the element X (mass%).

In precipitation hardening martensitic stainless steel, optimizing the amount of Cr and the amount of Ni can moderately improve the strength and toughness of the matrix while maintaining the high corrosion resistance of the matrix. Furthermore, by simultaneously adding an appropriate amount of Al and an appropriate amount of Ti to a precipitation hardening martensitic stainless steel with an optimized amount of Cr and Ni, strength and toughness are further improved while maintaining high corrosion resistance. can be done. This is considered to be due to composite strengthening by two phases of B2 phase (NiAl) and η phase (Ni ₃ Ti).

An embodiment of the present invention will be described in detail below.
[1. Precipitation Hardening Martensitic Stainless Steel]
[1.1. Main constituent element]
The precipitation hardening martensitic stainless steel according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

(1) C<0.10 mass%:
C precipitates M ₂ X type carbonitrides and contributes to improving the strength of the base material. C also contributes to the refinement of the prior austenite grain size. However, when the amount of C becomes excessive, a large amount of M ₂ X carbonitrides precipitate, so it becomes necessary to raise the solid-solution temperature. As a result, the austenite grains become coarse during solution treatment, causing variations in properties. In addition, (Cr, Mo)-based carbides are excessively precipitated during the aging treatment, which lowers toughness and corrosion resistance. Furthermore, the martensite transformation start temperature (Ms point) is lowered, and the austenite phase is stabilized. Therefore, the C content should be less than 0.10 mass%. The amount of C is preferably 0.08 mass% or less, more preferably 0.05 mass% or less.

(2) 0.01≦Si≦0.10 mass%:
Si acts as a deoxidizing agent. If the amount of Si is too small, deoxidation during dissolution will be insufficient, resulting in a decrease in cleanliness. Therefore, the Si content should be 0.01 mass % or more.
On the other hand, if the amount of Si becomes excessive, oxide-based inclusions are formed and the toughness is lowered. Therefore, the Si content should be 0.10 mass% or less.

(3) 0.01≦Mn≦0.10 mass%:
Mn has the effect of suppressing the grain boundary segregation of S. In order to obtain such effects, the Mn content should be 0.01 mass % or more.
On the other hand, when the amount of Mn becomes excessive, sulfides increase and the toughness decreases. Therefore, the Mn content should be 0.10 mass% or less. The amount of Mn is preferably 0.05 mass% or less.

(4) P ≤ 0.010 mass%:
P segregates at grain boundaries and deteriorates hot workability. Therefore, the P content should be 0.010 mass% or less.

(5) S ≤ 0.010 mass%:
S segregates at grain boundaries and lowers hot workability. Also, S combines with Ti to form sulfide-based inclusions. Therefore, the S content should be 0.010 mass% or less.

(6) 7.5≦Ni≦11.0 mass%:
Ni is an important element that precipitates intermetallic compound phases such as NiAl and Ni ₃ (Al, Ti) and contributes to improving the strength of the base material. In addition, Ni has the effect of suppressing the formation of the δ ferrite phase. Furthermore, Ni lowers the ductile-brittle transition temperature (DBTT) of the parent phase and contributes to the improvement of toughness at room temperature. In order to obtain such effects, the amount of Ni should be 7.5 mass % or more. The amount of Ni is preferably 8.0 mass% or more.
On the other hand, when the amount of Ni becomes excessive, the Ms point decreases. Therefore, retained austenite increases and strength decreases. Therefore, the Ni content should be 11.0 mass% or less. The Ni content is preferably 10.0 mass% or less, more preferably 9.0 mass% or less.

(7) 10.0≦Cr≦14.0 mass%:
Cr is an element necessary to ensure corrosion resistance. Also, when the amount of Cr is small, the M ₂₃ C ₆ -type carbide, which is coarser than the M ₂ X-type carbonitride, is stabilized and the 0.2% proof stress is lowered. Therefore, the Cr content should be 10.0 mass % or more. The Cr content is preferably 11.0 mass% or more, more preferably 12.0 mass% or more.

Cr also contributes to the adjustment of the Ms point, and the lower the amount of Cr, the higher the Ms point. Therefore, the smaller the amount of Cr, the smaller the amount of retained austenite after solution heat treatment or subzero treatment. This also improves the homogeneity of the microstructure and increases the 0.2% proof stress.
Conversely, as the amount of Cr increases, the Ms point decreases, so the amount of retained austenite increases. Moreover, when the amount of Cr becomes excessive, the amount of retained austenite before aging treatment becomes excessive, and the 0.2% yield strength is lowered. Furthermore, when the amount of Cr becomes excessive, the delta ferrite phase tends to be formed. Therefore, the Cr content should be 14.0 mass% or less. The Cr content is preferably 13.5 mass% or less, more preferably 13.0 mass% or less.

(8) 1.0 ≤ Mo ≤ 2.5 mass%:
Mo contributes to improvement in corrosion resistance. In addition, Mo precipitates M ₂ X type carbonitrides and contributes to improving the strength of the base material. Furthermore, Mo also contributes to the refinement of the prior austenite grain size. In order to obtain such effects, the amount of Mo needs to be 1.0 mass% or more. The Mo content is preferably 1.5 mass% or more, more preferably 2.0 mass% or more.
On the other hand, when the amount of Mo becomes excessive, a large amount of M ₂ X type carbonitride precipitates, so it becomes necessary to raise the solid solution temperature. As a result, the austenite grains become coarse during solution treatment, causing variations in properties. Furthermore, when the amount of Mo becomes excessive, the delta ferrite phase is likely to be formed. Therefore, the Mo content should be 2.5 mass% or less. The Mo content is preferably 2.4 mass% or less.

(9) 0.001≦N≦0.010 mass%:
N is included in M ₂ X type carbonitrides. However, N combines with Al, which is added as a strengthening element, to form nitrides and lower toughness. Also, N lowers the Ms point and stabilizes austenite. Therefore, the amount of N should be 0.010 mass% or less.
On the other hand, even if the amount of N is reduced more than necessary, there is little effect on strength and toughness, and rather it causes an increase in manufacturing cost. Therefore, the amount of N should be 0.001 mass% or more.

(10) 0.40≦Al≦1.40 mass%:
Al is an important element that forms an intermetallic compound (spherical NiAl of 2 to 5 nm) with Ni, and contributes to improving the strength of the base material. Al also functions as a deoxidizing element. In order to obtain such effects, the amount of Al needs to be 0.40 mass% or more. The Al content is preferably 0.50 mass% or more, more preferably 0.60 mass% or more.
On the other hand, when the amount of Al becomes excessive, the toughness decreases. Also, when the amount of Al becomes excessive, the δ ferrite phase is likely to be formed. Therefore, the Al content should be 1.40 mass% or less. The Al content is preferably 1.35 mass% or less, more preferably 1.30 mass% or less.

(11) Cu<0.10 mass%:
A small amount of Cu has the effect of improving the strength without greatly impairing the toughness. However, when the amount of Cu becomes excessive, the toughness and hot workability deteriorate. Therefore, the Cu content should be less than 0.10 mass%.

(12) 0.30 ≤ Ti ≤ 1.40 mass%:
Ti, like Al, is an important element that forms an intermetallic compound (a rod-like Ni ₃ Ti with a width of 2 to 5 nm and a length of several tens of nm) with Ni, and contributes to improving the strength of the base material. Moreover, when the amount of Ti is sufficient, grain boundaries are covered with Ni ₃ Ti precipitates. As a result, the grain boundary strength is improved, which contributes to the improvement of toughness. In order to obtain such effects, the amount of Ti should be 0.30 mass% or more. The Ti content is preferably 0.50 mass% or more, more preferably 0.60 mass% or more.
On the other hand, when the amount of Ti becomes excessive, inclusions increase and the toughness is lowered. Also, when the amount of Ti becomes excessive, the δ ferrite phase is likely to be formed. Therefore, the Ti content should be 1.40 mass% or less. The Ti content is preferably 1.35 mass% or less, more preferably 1.30 mass% or less.

(13) 0≦Nb<0.50 mass%:
Nb, like Al and Ti, is Ni(Al,Nb), Ni(Al,Nb), Ni ₍ Al,Nb), Ni ₍ Al,Ti) in which part of Al and Ti in Ni and intermetallic compounds (NiAl and Ni3(Al,Ti) are substituted with Nb). Al, Ti, Nb), etc.) and contributes to the strength improvement of the base material. In addition, Nb forms carbonitrides and contributes to refinement of crystal grains. Therefore, Nb can be added as needed.
On the other hand, if the amount of Nb becomes excessive, carbonitrides increase and the toughness is lowered. Moreover, when the amount of Nb becomes excessive, the δ ferrite phase is likely to be formed. Therefore, the Nb content should be less than 0.50 mass%. The Nb content is preferably 0.40 mass% or less, more preferably 0.30 mass% or less.

[1.2. Ingredient balance]
The precipitation hardening martensitic stainless steel according to the present invention satisfies the following formulas (1) to (4) in addition to having the main constituent elements within the ranges described above.
1.00≦[Al]+[Ti]+[Nb]≦2.00 (1)
4.00≦[Ni]/([Al]+[Ti]+[Nb])≦8.00 (2)
8.00≦Ni _eq ≦11.00 (3)
16.00≦Cr _eq ≦21.00 (4)
however,
Ni _eq =[Ni]+0.11[Mn]−0.0086[Mn] ² +0.44[Cu]+18.4[N]+24.5[C],
Cr _eq =[Cr]+1.21[Mo]+0.48[Si]+2.2[Ti]+2.48[Al],
[X] represents the content of the element X (mass%).

[1.2.1. Formula (1)]
Formula (1) represents the range of the total amount of Al, Ti, and Nb. As the total amount of these elements increases, the amount of precipitation of intermetallic compounds such as B2 phase (NiAl) and η phase (Ni ₃ (Al, Ti), Ni ₃ (Al, Ti, Nb)) increases, improving strength. contribute to Further, by adding Al and Ti in combination, precipitates of both B2 phase and η phase are formed, which contributes to improvement of strength and toughness. In order to obtain such effects, the total amount of these elements should be 1.00 mass % or more. The total amount is preferably 1.10 mass% or more, more preferably 1.20 mass% or more.
On the other hand, when the total amount of these elements is excessive, intermetallic compounds are excessively precipitated, or the δ ferrite phase is likely to be formed, which causes deterioration of characteristics. Therefore, the total amount of these elements should be 2.00 mass% or less. The total amount is preferably 1.90 mass% or less, more preferably 1.85 mass% or less.

[1.2.2. Formula (2)]
Formula (2) represents the range of the ratio of the amount of Ni to the total amount of Al, Ti, and Nb (hereinafter also simply referred to as "Ni ratio"). If the Ni ratio is too small, the amount of precipitated intermetallic compound phases (B2 phase, η phase) becomes excessive, or the strength of the matrix phase becomes insufficient, resulting in a decrease in toughness. In order to achieve both strength and toughness, the Ni ratio should be 4.00 or more. The Ni ratio is preferably 4.50 or higher.
On the other hand, if the Ni ratio becomes excessive, the amount of retained austenite increases significantly, and it becomes difficult to reduce the amount of retained austenite even if Cr and Mo are reduced. Therefore, the Ni ratio should be 8.00 or less. The Ni ratio is preferably 7.50 or less, more preferably 7.00 or less.

[1.2.3. Formula (3), Formula (4)]
Equation (3) represents the range of Ni equivalents (Ni _eq ). Equation (4) represents the range of Cr equivalents (Cr _eq ). Optimizing the combination of Ni _eq and Cr _eq suppresses the residual δ ferrite phase after homogenization heat treatment (~1240 ° C), and the residual before aging treatment (after solution heat treatment and after subzero treatment) Less austenite (ie more martensite formed). As a result, the strength of steel can be increased.

[A. Ni equivalent]
Ni _eq needs to be 8.00 or more in order to obtain moderate strength and toughness at Cr _eq necessary for obtaining excellent corrosion resistance. Ni _eq is preferably 8.50 or more.
On the other hand, if Ni _eq is excessive in Cr _eq to be described later, retained austenite before aging treatment increases and strength decreases. Therefore, Ni _eq must be 12.00 or less. Ni _eq is preferably 11.00 or less, more preferably 9.50 or less.

[B. Cr equivalent]
If Cr _eq is too low, sufficient oxidation resistance and corrosion resistance cannot be obtained. Therefore, Cr _eq must be greater than or equal to 16.00. Cr _eq is preferably 17.00 or more, more preferably 18.00 or more.
On the other hand, if Cr _eq is excessive, the δ ferrite phase remains even after the homogenization heat treatment, and the impact value decreases. On the other hand, when Cre _eq becomes excessive, the retained austenite before aging treatment increases and the strength decreases. Therefore, Cr _eq must be 21.00 or less. Cr _eq is preferably 20.00 or less.

[2. Manufacturing method of precipitation hardening martensitic stainless steel]
The precipitation hardening martensitic stainless steel according to the present invention is
(a) melting and casting a raw material blended to have a predetermined composition;
(b) subjecting the obtained ingot to homogenization heat treatment,
(c) hot forging the material after the homogenization heat treatment;
(d) performing solution heat treatment on the hot forged material,
(e) subjecting the material after the solution heat treatment to sub-zero treatment as necessary,
(f) It can be manufactured by subjecting the material after subzero treatment to aging treatment.

[2.1. Melting and casting process]
First, raw materials blended to have a predetermined composition are melted and cast. The melting/casting method and conditions are not particularly limited, and the optimum method and conditions can be selected according to the purpose.

[2.2. Homogenization heat treatment process]
Next, the obtained ingot is subjected to homogenization heat treatment. Homogenization heat treatment is performed to remove segregation generated during casting. The conditions for the homogenization heat treatment are not particularly limited as long as such effects are achieved. The homogenization heat treatment is usually carried out by heating and holding the ingot under conditions of a temperature of 1150 to 1240° C. and a time of 10 hours or more.

[2.3. Hot forging process]
Next, the material after the homogenization heat treatment is hot forged. Hot forging is performed to destroy coarse cast structures and refine the structures. The conditions for hot forging are not particularly limited as long as such effects are achieved. Usually, hot forging is carried out by heating the raw material under conditions of 900 to 1240° C.×1 hour or more, forging under conditions of a forging final temperature of 900° C., and then air cooling. The hot forging may be performed continuously without cooling the material to room temperature after the homogenization heat treatment.

[2.4. Solution heat treatment step]
Next, solution heat treatment is performed on the material after hot forging. The solution heat treatment is performed to convert the material into a single austenite phase and then martensite transformation. Conditions for the solution heat treatment are not particularly limited as long as such an effect can be obtained. Usually, the solution heat treatment is performed by heating the material under the conditions of temperature: 900 to 1100° C. and heating time: 1 to 10 hours, followed by cooling. Cooling methods include, for example, air cooling, blast cooling, oil cooling, and water cooling.

[2.5. Sub-zero treatment process]
Next, the material after the solution heat treatment is subjected to sub-zero treatment as necessary. The sub-zero treatment is performed to transform the austenite remaining after the solution heat treatment into martensite. Conditions for the sub-zero treatment are not particularly limited as long as such effects are achieved. Sub-zero treatment is usually carried out by holding the material at a temperature of 0° C. or below for 1 to 10 hours.

[2.6. Aging treatment process]
Next, aging treatment is performed on the material after the subzero treatment. Aging treatment is performed to precipitate intermetallic compound phases such as B2 phase and η phase in the matrix phase. Conditions for the aging treatment are not particularly limited as long as such effects are exhibited. Generally, the aging treatment is carried out by heating the material at 400-600° C. for 1-24 hours. After the heat treatment, cooling is performed by air cooling.

[3. action]
Precipitation hardening martensitic stainless steel is a material with excellent strength, toughness, and corrosion resistance, but it is known that it is difficult to balance strength and toughness. In precipitation hardening martensitic stainless steel, strength is increased mainly by adding strengthening elements such as Cu and Al. However, simply adding an excessive amount of strengthening elements improves the strength properties, but significantly reduces the toughness.
Furthermore, it is difficult for conventional precipitation-hardening martensitic stainless steels to further improve corrosion resistance while appropriately balancing strength and toughness.

On the other hand, in the precipitation hardening martensitic stainless steel, if the Cr content and Ni content are optimized, the strength and toughness of the matrix can be moderately improved while maintaining the high corrosion resistance of the matrix. Furthermore, by simultaneously adding an appropriate amount of Al and an appropriate amount of Ti to a precipitation hardening martensitic stainless steel with an optimized amount of Cr and Ni, strength and toughness are further improved while maintaining high corrosion resistance. can be done. This is considered to be due to composite strengthening by two phases of B2 phase (NiAl) and η phase (Ni ₃ Ti).

(Examples 1 to 13, Comparative Examples 1 to 8)
[1. Preparation of sample]
In a vacuum induction furnace, 50 kg of steel having the composition shown in Table 1 was melted and cast into an ingot. After that, a homogenization heat treatment was performed under the conditions of 1220° C.×20 hours and air cooling. Further, a φ24 mm round bar was forged under conditions of a start temperature of 1220° C. and an end temperature of 900° C., and then air-cooled.
Next, each steel ingot was subjected to solution heat treatment under the conditions of 1000° C.×1 hour and water cooling. Subsequently, sub-zero treatment was performed under conditions of -30°C x 3 hours. Furthermore, aging treatment was performed under the conditions of 530° C.×4 hours and air cooling.

[2. Test method]
[2.1. Tensile test (measurement of 0.2% yield strength)]
A tensile test was performed according to the metal tensile test method specified in ASTM A370 to measure the 0.2% yield strength. A test piece having a test portion diameter of φ12.5 mm and a gauge length of 50 mm according to ASTM E8 was used. The test temperature was room temperature.
[2.2. Charpy impact test]
A 2 mm V-notch test piece was taken so that the longitudinal direction coincided with the forging direction. Using this test piece, the impact properties (absorbed energy) were measured according to the ASTM A370 standard. The test temperature was room temperature.
[2.3. Evaluation test of pitting corrosion potential]
According to the pitting potential measurement method specified in JIS G 0577, the surface of the sample was covered with an insulating coating so that the measurement area was 1 cm ² . A 3.5 mass % NaCl aqueous solution at 30±1° C. was used as the test liquid, and the pitting potential was measured at a sweep rate of 20 mV/min to evaluate the corrosion resistance.

[3. result]
Table 1 shows the results. Table 1 also shows the composition of each sample. Table 1 shows the following.

(1) Comparative Example 1, which has an excessive amount of C, has a high 0.2% proof stress (>1400 MPa) and moderate corrosion resistance (100 to 150 mV), but low toughness (<10 J).
(2) Comparative Example 2 has an excessive amount of Al + Ti + Nb, so the 0.2% yield strength is high (>1400 MPa) and exhibits moderate corrosion resistance (100 to 150 mV), but the toughness is low (<10J).
(3) Comparative Example 3 exhibits moderate 0.2% yield strength (1300 to 1400 MPa) and moderate corrosion resistance (100 to 150 mV) because of its small amount of Ti, but its toughness is low (<10J). This is probably because the amount of Ti is so small that the effect of composite precipitation of the B2 phase and the η phase is not sufficiently exhibited.

(4) Comparative Example 4 has a low Al content, so it has high toughness (>20J) and moderate corrosion resistance (100-150mV), but has a low 0.2% proof stress (<1300MPa). This is probably because the Al content is so small that the effect of composite precipitation of the B2 phase and the η phase is not sufficiently exhibited.
(5) Comparative Example 5, which has an excessive amount of Ni, has high toughness (>20J) and moderate corrosion resistance (100-150mV), but has a low 0.2% proof stress (<1300MPa). This is probably because the amount of retained austenite increased.
(6) Comparative Example 6 has a small amount of Al and Ti, and a small amount of Al + Ti + Nb, so it has high toughness (>20 J) and moderate corrosion resistance (100 to 150 mV), but 0.2% Low yield strength (<1300 MPa).

(7) Comparative Example 7 has an excessive Cr amount and an excessive Cr _eq , so the 0.2% yield strength is high (>1400 MPa) and the corrosion resistance is also high (>150 mV), but the toughness is low (<10 J). This is considered to be due to the generation of the δ ferrite phase.
(8) Comparative Example 8 has a small amount of Ni and a small Ni _eq , so it exhibits moderate corrosion resistance (100 to 150 mV), but fractures before 0.2% proof stress is exhibited, and toughness is also low ( <10 J).

(9) Examples 1 to 13 are all high in 0.2% yield strength, toughness and corrosion resistance.
(10) Examples 8 to 13 have particularly high corrosion resistance. It is considered that this is because the amount of Cr and the amount of Mo are relatively large.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The precipitation hardening martensitic stainless steel according to the present invention can be used for steam turbine blades, aerospace structural members, high-strength fasteners, and the like.

Claims (1)

A precipitation hardening martensitic stainless steel comprising:
(1) The precipitation hardening martensitic stainless steel is
C<0.10 mass%,
0.01 ≤ Si ≤ 0.10 mass%,
0.01≦Mn≦0.10 mass%,
P≤0.010 mass%,
S ≤ 0.010 mass%,
7.5≦Ni≦11.0 mass%,
10.0≦Cr≦14.0 mass%,
1.0≤Mo≤2.5 mass%,
0.001≦N≦0.010 mass%,
0.40≦Al≦1.40 mass %,
Cu<0.10 mass%,
0.30 ≤ Ti ≤ 1.40 mass%, and
0≦Nb<0.50 mass%
with the remainder consisting of Fe and unavoidable impurities.
(2) The precipitation hardening martensitic stainless steel satisfies the following formulas (1) to (4).
1.00≦[Al]+[Ti]+[Nb]≦2.00 (1)
4.00≦[Ni]/([Al]+[Ti]+[Nb])≦8.00 (2)
8.00≦Ni _eq ≦12.00 (3)
16.00≦Cr _eq ≦21.00 (4)
however,
Ni _eq =[Ni]+0.11[Mn]−0.0086[Mn] ² +0.44[Cu]+18.4[N]+24.5[C],
Cr _eq =[Cr]+1.21[Mo]+0.48[Si]+2.2[Ti]+2.48[Al],
[X] represents the content of the element X (mass%).