WO2020054540A1 - Precipitation hardening-type martensite-based stainless steel and underground excavation drill component - Google Patents

Abstract

This precipitation hardening-type martensite-based stainless steel and this underground excavation drill using the same, contain C<0.10 mass%, 0.01≤Si≤0.10 mass%, 0.01≤Mn≤0.10 mass%, P≤0.010 mass%, S≤0.010 mass%, 10.0≤Ni≤16.0 mass%, 8.0≤Cr≤10.9 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%, the remaining portion being Fe and unavoidable impurities, wherein 1.00 ≤ [Al]+[Ti]+[Nb] ≤ 2.00, 5.50 ≤ [Ni]/([Al]+[Ti]+[Nb]) ≤ 12.00, 10.00≤Nieq≤17.00, and 12.00≤Creq≤17.00 are satisfied.

Description

Precipitation hardening martensitic stainless steel and drill parts for underground drilling

The present invention relates to a precipitation-hardened martensitic stainless steel and a drill part for underground drilling, and more particularly, to a precipitation-hardened martensitic stainless steel excellent in strength and toughness, and a drill part for underground drilling using the same. About.

The precipitation hardening type stainless steel is a steel obtained by adding a small amount of Al, Cu, Mo, Ti, or the like to a Cr—Ni stainless steel, and precipitating an intermetallic compound in a parent phase by heat treatment. Precipitation hardening stainless steels are classified into martensite, semi-austenite, and austenite, depending on the structure of the matrix.
Among these, precipitation hardening martensitic stainless steels such as SUS630 are used for aerospace structural members and the like because of their excellent corrosion resistance, strength, and toughness. However, the conventional precipitation hardening type martensitic stainless steel has a problem that the balance between strength and toughness is poor.

For example, SUS630 uses Cu as a main hardening element, but shows relatively low yield strength. On the other hand, in order to increase the strength, alloys using Al or Ti as a hardening element (for example, PH13-8Mo, Custom450, etc.) have been designed. However, it is difficult for these alloys to obtain high impact properties while maintaining high tensile strength (> 1450 MPa).

Therefore, various proposals have conventionally been made to solve this problem.
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%, with the balance being Fe and impurities,
(B) A precipitation-strengthened stainless steel satisfying a predetermined relational expression is disclosed.
According to the document, when Mo is added to a precipitation-strengthened martensitic stainless steel in which C is kept low and the ratio and amount of Al and Ti which form a compound with Ni are optimized, corrosion resistance, tensile strength and It describes that the 0.2% proof stress can be greatly improved while maintaining ductility.

In Patent Document 2,
(A) In 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.
(B) A martensitic stainless steel satisfying a predetermined relational expression is disclosed.
The document states that
(A) The point that superior strength is obtained compared with the conventional steel by the active complex addition of Al, Ti, and Ni; and
(B) It is described that workability is remarkably improved by reducing C and N and limiting the amount of Si.

Patent Document 3 states that
(A) When the composition is weight%, 9% ≦ Cr ≦ 13%, 1.5% ≦ Mo ≦ 3%, 8% ≦ Ni ≦ 14%, 1% ≦ Al ≦ 2%, and Al + Ti ≧ 2.25% Under the conditions of 0.5% ≦ Ti ≦ 1.5%, measurement limit value ≦ Co ≦ 2%, Mo + (W / 2) ≦ 3%, measurement limit value ≦ W ≦ 1%, measurement limit value ≦ P ≦ 0.02%, measurement limit value ≦ S ≦ 0.0050%, measurement limit value ≦ N ≦ 0.0060%, measurement limit value ≦ C ≦ 0.025%, measurement limit value ≦ Cu ≦ 0.5% , Measurement limit ≦ Mn ≦ 3%, measurement limit ≦ Si ≦ 0.25%, measurement limit ≦ O ≦ 0.0050%, and
(B) A martensitic stainless steel satisfying a predetermined relational expression is disclosed.
The document describes that such martensitic stainless steel is excellent in corrosion resistance, strength, and toughness.

Patent Document 4 discloses that, by mass, C is 0.1% or less, N is 0.1% or less, Cr is 9.0% or more and 14.0% or less, and Ni is 9.0% or more and 14.0% or less. , 0.5% or more and 2.5% or less Mo, 0.5% or less Si, 1.0% or less Mn, 0.25% or more and 1.75% or less Ti, and 0.25% or more A precipitation-hardening martensitic stainless steel containing 1.75% or less Al and the balance being Fe and inevitable 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%, with the balance being iron and impurities,
(B) A martensitic stainless steel satisfying a predetermined relational expression is disclosed.
The document describes that such a martensitic stainless steel has high mechanical strength characteristics and toughness, and 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, 0.1% or less. 5% or more and 2.5% or less Mo, 0.5% or less Si, 1.0% or less Mn, 0.25% or more and 1.75% Ti, 0.25% or more and 1.75% or less Comprising a precipitation-hardened martensitic stainless steel steam turbine low-pressure final stage long blade comprising Al and the balance consisting of Fe and unavoidable impurities,
(B) There is disclosed a steam turbine rotor in which a disk 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.

Further, 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 to 10.0%, Cr: 10.5 to 13.0%, Mo: 2.0 to 2.5%, N: 0.001 to 0.010%, Al: 1 .15 to 1.50%, Cu: <0.10%, and Ti: ≤0.20%, with the balance being Fe and unavoidable impurities.
(B) A steam turbine blade steel satisfying a predetermined relational expression is disclosed.
The document describes that such steam turbine blade steel can achieve both 0.2% proof stress of 1450 MPa or more and Charpy impact characteristics of 15 J or more.

In the future, precipitation-hardening martensitic stainless steels require a further balance of strength and toughness. For example, a drill component for underground excavation, a high-strength fastener, and the like 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 the precipitate. In particular, in recent precipitation hardening stainless steels, intermetallic compounds are actively used for increasing strength. On the other hand, toughness is also presumed to be related to the type and amount of precipitates, but details are not disclosed. Therefore, when precipitation-hardening stainless steel is simply added with an excessive amount of a strengthening element, although the strength characteristics are improved, there is a problem that the toughness is significantly reduced.
Furthermore, aerospace members are required to have excellent strength, toughness, and corrosion resistance. However, there is no example in which a precipitation hardening martensitic stainless steel exhibiting high corrosion resistance while maintaining appropriate strength and toughness has been proposed.

International Publication No. WO2012 / 002208 Japanese Patent Application Laid-Open No. 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

An object of the present invention is to provide a precipitation-hardened martensitic stainless steel excellent in balance between strength and toughness.
Another object of the present invention is to provide a drill part for underground drilling using such a precipitation hardening type martensitic stainless steel.

In order to solve the above problems, a gist of the present invention is to provide a precipitation-hardening martensitic stainless steel having the following configurations <1> and <2>.
<1> The precipitation hardening type 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%,
10.0 ≦ Ni ≦ 16.0 mass%,
8.0 ≦ Cr ≦ 10.9 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%
And the balance consists of Fe and inevitable impurities.
<2> The precipitation hardening martensitic stainless steel satisfies the following equations (1) to (4).
1.00 ≦ [Al] + [Ti] + [Nb] ≦ 2.00 (1)
5.50 ≦ [Ni] / ([Al] + [Ti] + [Nb]) ≦ 12.00 (2)
10.00 ≦ Ni _eq ≦ 17.00 (3)
12.00 ≦ Cr _eq ≦ 17.00 (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 (mass%) of the element X.
The drill part for underground drilling according to the present invention is made of the precipitation hardening martensitic stainless steel according to the present invention.

By optimizing the amounts of Cr and Ni in the precipitation hardening type martensitic stainless steel, the strength and toughness of the matrix can be improved while maintaining the corrosion resistance of the matrix in an appropriate level. Furthermore, when an appropriate amount of Al and an appropriate amount of Ti are simultaneously added to the precipitation hardening type martensitic stainless steel in which the Cr amount and the Ni amount are optimized, the strength and the toughness are further improved while maintaining the appropriate corrosion resistance. be able to. This, B2 phase and (NiAl), believed to be due to a composite reinforced by 2-phase η phase (Ni ₃ Ti).

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

(1) C <0.10 mass%:
C precipitates the M ₂ X-type carbonitride and contributes to the improvement of the strength of the base material. Further, C also contributes to miniaturization of the prior austenite grain size. However, when the amount of C is excessive, a large amount of M ₂ X carbonitride precipitates, so that it is necessary to raise the solid solution temperature. For this reason, austenite grains are coarsened during solid solution, which causes characteristic variations. In addition, during aging treatment, (Cr, Mo) -based carbides precipitate excessively, thereby lowering toughness and corrosion resistance. Further, the martensite transformation start temperature (Ms point) is lowered, and the austenite phase is stabilized. Therefore, the C content needs to be less than 0.10 mass%. C content 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 at the time of dissolution will be insufficient, and the cleanliness will decrease. Therefore, the amount of Si needs to be 0.01 mass% or more.
On the other hand, when the amount of Si becomes excessive, oxide inclusions are formed, and toughness is reduced. Therefore, the amount of Si needs to be 0.10 mass% or less.

(3) 0.01 ≦ Mn ≦ 0.10 mass%:
Mn has an effect of suppressing grain boundary segregation of S. In order to obtain such an effect, the amount of Mn needs to be 0.01 mass% or more.
On the other hand, when the amount of Mn is excessive, sulfide increases and toughness decreases. Therefore, the Mn content needs to 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 the grain boundaries and reduces hot workability. Therefore, the amount of P needs to be 0.010 mass% or less.

(5) S ≦ 0.010 mass%:
S segregates at the grain boundaries and reduces hot workability. S combines with Ti to form sulfide-based inclusions. Therefore, the amount of S needs to be 0.010 mass% or less.

(6) 10.0 ≦ Ni ≦ 16.0 mass%:
Ni is an important element that precipitates an intermetallic compound phase such as NiAl or Ni ₃ (Al, Ti) and contributes to improving the strength of the base material. Ni has the effect of suppressing the formation of the δ ferrite phase. Further, Ni lowers the ductile-brittle transition temperature (DBTT) of the parent phase and contributes to improvement in toughness at room temperature. In order to obtain such an effect, the amount of Ni needs to be 10.0 mass% or more. The Ni content is preferably at least 11.0 mass%, more preferably at least 12.0 mass%.
On the other hand, when the amount of Ni becomes excessive, the Ms point decreases. Therefore, the retained austenite increases and the strength decreases. Therefore, the amount of Ni needs to be 16.0 mass% or less. The amount of Ni is preferably 15.0 mass% or less, and more preferably 14.0 mass% or less.

(7) 8.0 ≦ Cr ≦ 10.9 mass%:
Cr is an element necessary for ensuring corrosion resistance. Further, when the amount of Cr is small, M ₂ coarse M ₂₃ C ₆ type carbides than X type carbonitride is stabilized, reduced 0.2% yield strength. Therefore, the amount of Cr needs to be 8.0 mass% or more. The amount of Cr is preferably 8.5 mass% or more.

Cr also contributes to the adjustment of the Ms point, and the smaller the Cr amount, the higher the Ms point. Therefore, the smaller the Cr content, the smaller the retained austenite after the solution heat treatment or the sub-zero treatment. This also improves the homogeneity of the microstructure and improves the 0.2% proof stress.
Conversely, as the amount of Cr increases, the Ms point decreases, so the amount of retained austenite increases. On the other hand, when the Cr content is excessive, the residual austenite content before the aging treatment is excessive, and the 0.2% proof stress is reduced. Further, when the Cr content is excessive, a δ ferrite phase is easily formed. Therefore, the amount of Cr needs to be 10.9 mass% or less. The amount of Cr is preferably 10.0 mass% or less, and more preferably 9.5 mass% or less.

(8) 1.0 ≦ Mo ≦ 2.5 mass%:
Mo contributes to improvement of corrosion resistance. Mo precipitates the M ₂ X-type carbonitride and contributes to the improvement of the strength of the base material. Further, Mo also contributes to miniaturization of the prior austenite grain size. In order to obtain such an effect, the Mo amount needs to be 1.0 mass% or more. The Mo amount is preferably at least 1.1 mass%, more preferably at least 1.2 mass%.
On the other hand, when the amount of Mo becomes excessive, a large amount of M ₂ X-type carbonitride precipitates, so that it is necessary to raise the solid solution temperature. For this reason, austenite grains are coarsened during solid solution, which causes characteristic variations. Further, when the amount of Mo is excessive, a δ ferrite phase is easily formed. Therefore, the amount of Mo needs to be 2.5 mass% or less. The Mo content is preferably 2.0 mass% or less, more preferably 1.5 mass% or less.

(9) 0.001 ≦ N ≦ 0.010 mass%:
N is contained in the M ₂ X type carbonitride. However, N combines with Al added as a strengthening element to form a nitride and lowers toughness. Also, N lowers the Ms point and stabilizes austenite. Therefore, the amount of N needs to be 0.010 mass% or less.
On the other hand, even if the N content is reduced more than necessary, the influence on the strength and toughness is small, and rather, it causes the production cost to increase. Therefore, the amount of N needs to 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 improvement of the strength of the base material. Al also functions as a deoxidizing element. To obtain such an effect, the amount of Al needs to be 0.40 mass% or more. The amount of Al is preferably 0.50 mass% or more, and more preferably 0.60 mass% or more.
On the other hand, when the amount of Al is excessive, the toughness decreases. When the amount of Al is excessive, a δ ferrite phase is easily formed. Therefore, the amount of Al needs to be 1.40 mass% or less. The amount of Al is preferably 1.35 mass% or less, and more preferably 1.30 mass% or less.

(11) Cu <0.10 mass%:
A small amount of Cu has an effect of improving the strength without significantly impairing the toughness. However, when the amount of Cu becomes excessive, toughness and hot workability decrease. Therefore, the amount of Cu needs to be less than 0.10 mass%.

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

(13) 0 ≦ Nb <0.50 mass%:
Nb is, like Al and Ti, Ni and an intermetallic compound (Ni (Al, Nb), Ni ₃ (Ni or Al _{3 in} Ni ₃ (Al, Ti) in which part of Al or Ti is substituted with Nb)). Al, Ti, Nb)) to contribute to the improvement of the strength of the base material. Nb forms carbonitrides and contributes to the refinement of crystal grains. Therefore, Nb can be added as needed.
On the other hand, if the amount of Nb is excessive, carbonitrides increase and the toughness decreases. When the Nb content is excessive, a δ ferrite phase is easily formed. Therefore, the Nb amount needs to be less than 0.50 mass%. The Nb amount is preferably 0.40 mass% or less, more preferably 0.30 mass% or less.

[1.2. Component balance]
The precipitation-hardening martensitic stainless steel according to the present invention satisfies the following equations (1) to (4) in addition to the main constituent elements being in the above-described range.
1.00 ≦ [Al] + [Ti] + [Nb] ≦ 2.00 (1)
5.50 ≦ [Ni] / ([Al] + [Ti] + [Nb]) ≦ 12.00 (2)
10.00 ≦ Ni _eq ≦ 17.00 (3)
12.00 ≦ Cr _eq ≦ 17.00 (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 (mass%) of the element X.

[1.2.1. Equation (1)]
Equation (1) represents the range of the total amount of Al, Ti, and Nb. As the total amount of these elements increases, B2 phase (NiAl), eta phase _{(Ni 3 (Al, Ti)} , Ni 3 (Al, Ti, Nb)) precipitation amount of intermetallic compounds such as increased strength improvement To contribute. Further, by adding Al and Ti in combination, precipitates of both the B2 phase and the η phase are formed, which contributes to improvement in strength and toughness. In order to obtain such an effect, the total amount of these elements needs to be 1.00 mass% or more. The total amount is preferably 1.10 mass% or more, and more preferably 1.20 mass% or more.
On the other hand, when the total amount of these elements is excessive, an intermetallic compound is excessively precipitated, or a δ ferrite phase is easily formed, which causes deterioration of characteristics. Therefore, the total amount of these elements needs to 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. Equation (2)]
Equation (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 precipitation amount of the intermetallic compound phase (B2 phase, η phase) becomes excessive, or the strength of the parent phase becomes insufficient, so that the toughness decreases. In order to achieve both strength and toughness, the Ni ratio needs to be 5.50 or more. The Ni ratio is preferably at least 6.00, more preferably at least 7.00.
On the other hand, when the Ni ratio is excessive, the amount of retained austenite increases remarkably, and it becomes difficult to reduce the amount of retained austenite even if Cr and Mo are reduced. Therefore, the Ni ratio needs to be 12.00 or less. The Ni ratio is preferably 11.00 or less, more preferably 10.00 or less.

[1.2.3. Equation (3), Equation (4)]
Equation (3) represents the range of Ni equivalent (Ni _eq ). Equation (4) represents the range of Cr equivalent (Cr _eq ). By optimizing the combination of Ni _eq and Cr _eq , it is possible to suppress the δ ferrite phase from remaining after the homogenization heat treatment (up to 1240 ° C.), and to reduce the residual before the aging treatment (after the solution heat treatment and after the sub-zero treatment). Less austenite (ie, more martensite formed). As a result, the strength of the steel can be increased.

[A. Ni equivalent]
In order to obtain a precipitation-hardened martensitic stainless steel having excellent strength and toughness, Ni _eq needs to be 10.00 or more. Ni _eq is preferably at least 11.50, and more preferably at least 13.00.
On the other hand, when Ni _eq becomes excessive, the retained austenite before the aging treatment increases, and the strength decreases. Therefore, Ni _eq needs to be 17.00 or less. Ni _eq is preferably 16.50 or less, more preferably 15.50 or less.

[B. Cr equivalent]
If the _Ceq is too small, the strength will be insufficient. On the other hand, if _Creq is too small, sufficient oxidation resistance and corrosion resistance cannot be obtained. Therefore, _Creq needs to be 12.00 or more. _Creq is preferably 12.50 or more, and more preferably 13.00 or more.
On the other hand, in the above-mentioned Ni _eq , when Cr _eq becomes excessive, the retained austenite before the aging treatment increases, and the strength decreases. Therefore, Cr _eq is required to be 17.00 or less. _Creq is preferably 16.50 or less, more preferably 15.50 or less.

[1.3. Characteristic]
[1.3.1. 0.2% proof stress]
The precipitation-hardening martensitic stainless steel according to the present invention exhibits a relatively high 0.2% proof stress when the components are optimized and an appropriate heat treatment is performed.
Specifically, when the components and the heat treatment conditions are optimized, the 0.2% proof stress becomes 1300 MPa or more. If the components and heat treatment conditions are further optimized, the 0.2% proof stress will exceed 1400 MPa.

[1.3.2. Absorbed energy]
The precipitation-hardened martensitic stainless steel according to the present invention exhibits relatively high absorbed energy when its components are optimized and appropriate heat treatment is performed.
Specifically, when the components and the heat treatment conditions are optimized, the absorbed energy becomes 10 J or more. If the components and heat treatment conditions are further optimized, the absorbed energy will exceed 30J.

[2. Production method of precipitation hardening martensitic stainless steel]
Precipitation hardening type martensitic stainless steel according to the present invention,
(A) melting and casting a raw material blended to have a predetermined composition;
(B) performing a homogenizing heat treatment on the obtained ingot,
(C) hot forging the material after the homogenization heat treatment,
(D) A solution heat treatment is performed on the hot forged material,
(E) Sub-zero treatment is performed on the solution after solution heat treatment as necessary,
(F) It can be manufactured by subjecting the material after the sub-zero treatment to an aging treatment.

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

[2.2. Homogenization heat treatment process]
Next, a homogenization heat treatment is performed on the obtained ingot. The homogenization heat treatment is performed to remove segregation generated during casting. The conditions for the homogenizing heat treatment are not particularly limited as long as such effects are exhibited. Usually, the homogenization heat treatment is performed by heating and maintaining the ingot under the conditions of a temperature of 1150 to 1240 ° C. and a time of 10 hr or more.

[2.3. Hot forging process]
Next, the material after the homogenization heat treatment is hot forged. Hot forging is performed in order to destroy a coarse cast structure and refine the structure. The conditions of the hot forging are not particularly limited as long as such effects are exerted. Normally, hot forging is performed by heating a material under the conditions of 900 to 1240 ° C. × 1 hr or more, forging at a forging end temperature of 900 ° C., and then air cooling. The hot forging may be performed continuously after the homogenizing heat treatment is performed, without cooling the material to room temperature.

[2.4. Solution heat treatment step]
Next, a solution heat treatment is performed on the hot forged material. The solution heat treatment is performed to transform the material into an austenitic single phase and then transform the material into martensite. The conditions for the solution heat treatment are not particularly limited as long as such effects are exhibited. Usually, the solution heat treatment is performed by heating and cooling the material under the conditions of temperature: 900 to 1100 ° C. × heating time: 1 to 10 hours. Examples of the cooling method include air cooling, blast cooling, oil cooling, and water cooling.

[2.5. Sub-zero processing step]
Next, the material after the solution heat treatment is subjected to a sub-zero treatment as necessary. The sub-zero treatment is performed to transform austenite remaining after the solution heat treatment into martensite. The condition of the sub-zero processing is not particularly limited as long as such an effect is achieved. Usually, the sub-zero treatment is performed by holding the material at a temperature of 0 ° C. or less for 1 to 10 hours.

[2.6. Aging treatment process]
Next, aging processing is performed on the material after the sub-zero processing. The aging treatment is performed to precipitate an intermetallic compound phase such as a B2 phase and an η phase in the mother phase. The condition of the aging treatment is not particularly limited as long as such an effect is exerted. Usually, the aging treatment is performed by heating the material at 400 to 600 ° C. for 1 to 24 hours. After the heat treatment, cooling is performed by air cooling.

[3. Drill parts for underground drilling]
The drill part for underground drilling according to the present invention is made of the precipitation hardening martensitic stainless steel according to the present invention. The details of the precipitation hardening type martensitic stainless steel are as described above, and thus the description is omitted.

As drill parts for underground drilling, for example,
(A) a rotor and a stator of a muddy water motor component in an underground drill, which are rotated by hydraulic power of a fluid;
(B) a drive shaft for transmitting the rotation of the rotor and the stator,
(C) a structural member of a bearing for holding the drive shaft,
(D) Measurement-while-drilling tools (MWD) structural members for measuring the drilling depth, tilt angle, and azimuth of the drill string of the underground drill.
(E) Logging-while-drilling tools (LWD) structural members for analyzing geology,
(F) MWW or LWD housing member,
and so on.

[4. Action]
Precipitation hardened martensitic stainless steel is a material having excellent strength, toughness, and corrosion resistance, but it is known that it is difficult to balance strength and toughness. In precipitation hardening martensitic stainless steels, high strength is achieved mainly by adding reinforcing elements such as Cu and Al. However, simply adding an excessive amount of a strengthening element improves the strength properties but significantly lowers the toughness.

On the other hand, in the precipitation hardening type martensitic stainless steel, when the Cr amount and the Ni amount are optimized, the strength and toughness of the matrix can be improved while maintaining the corrosion resistance of the matrix at an appropriate level. Furthermore, when an appropriate amount of Al and an appropriate amount of Ti are simultaneously added to the precipitation hardening type martensitic stainless steel in which the Cr amount and the Ni amount are optimized, the strength and the toughness are further improved while maintaining the appropriate corrosion resistance. be able to. This, B2 phase and (NiAl), believed to be due to a composite reinforced by 2-phase η phase (Ni ₃ Ti).

(Examples 1 to 16, 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 ingot. Thereafter, a homogenizing heat treatment was performed under the conditions of 1200 ° C. × 24 hours and air cooling. Further, a round bar having a diameter of 24 mm was forged under the conditions of a start temperature of 1200 ° 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 hr and water cooling. Subsequently, a sub-zero treatment was performed under the condition of −76 ° C. × 6 hours. Further, aging treatment was performed under the condition of 530 ° C. × 4 hours and air cooling.

[2. Test method]
[2.1. Tensile test (measurement of 0.2% proof stress)]
A tensile test was performed according to a metal tensile test method specified in ASTM A370, and a 0.2% proof stress was measured.
[2.2. Charpy impact test]
A 2 mm V notch test piece was sampled so that the longitudinal direction coincided with the forging direction. Using this test piece, impact characteristics (absorbed energy) were measured in accordance with ASTM A370 standard. The test temperature was room temperature.

[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 has a high 0.2% proof stress (> 1400 MPa) but a low toughness (<10 J) due to an excessive amount of C.
(2) In Comparative Example 2, since the amount of Al + Ti + Nb is excessive, there are many precipitates. Therefore, the 0.2% proof stress is high (> 1400 MPa), but the toughness is low (<10 J).
(3) Comparative Example 3 shows a moderate 0.2% proof stress (1300 to 1400 MPa) due to a small amount of Ti, but has low toughness (<10 J). This is presumably because the effect of the combined precipitation of the B2 phase and the η phase is not sufficiently exhibited due to the small amount of Ti.

(4) Comparative Example 4 has high toughness (> 30 J) but low 0.2% proof stress (<1300 MPa) because of a small amount of Al. This is presumably because the effect of composite precipitation of the B2 phase and the η phase is not sufficiently exhibited because the Al content is small.
(5) Comparative Example 5 has a high toughness (> 30 J) but a low 0.2% proof stress (<1300 MPa) due to an excessive amount of Ni. This is probably because the amount of retained austenite increased.
(6) Comparative Example 6 has high toughness (> 30 J) but low 0.2% proof stress (<1300 MPa) because both the Al content and the Ti content are small and the Al + Ti + Nb content is also small.

(7) Comparative Example 7 has a high 0.2% proof stress (> 1400 MPa) but a low toughness (<10 J) because the amount of Cr and the amount of Cr _eq are also excessive. This is presumably because a δ ferrite phase was formed.
(8) In Comparative Example 8, since the amount of Ni and the amount of Ni _eq were small, it was broken before exhibiting 0.2% proof stress, and the toughness was low (<10 J).

(9) Examples 1 to 16 all have high 0.2% proof stress and high toughness.
(10) Examples 9 to 11 and 16 have particularly high 0.2% proof stress and toughness. This is considered to be due to the optimization of the Ni amount and the Al, Ti, and Nb amounts.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

Precipitation hardening type martensitic stainless steel according to the present invention,
(A) a rotor and a stator of a muddy water motor component in an underground drill, which are rotated by hydraulic power of a fluid;
(B) a drive shaft for transmitting the rotation of the rotor and the stator,
(C) a structural member of a bearing for holding the drive shaft,
(D) Measurement-while-drilling tools (MWD) structural members for measuring the drilling depth, tilt angle, and azimuth of the drill string of the underground drill.
(E) Logging-while-drilling tools (LWD) structural members for analyzing geology,
(F) It can be used as a housing member of MWD or LWD.
In addition, it can be used for steam turbine blades, aerospace structural members, high-strength fasteners, and the like.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on September 13, 2018 (Japanese Patent Application No. 2018-171958) and a Japanese patent application filed on August 9, 2019 (Japanese Patent Application No. 2019-147851), and the contents thereof are as follows. Is hereby incorporated by reference.

Claims (4)

A precipitation hardening martensitic stainless steel having the following configurations <1> and <2>.
<1> The precipitation hardening type 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%,
10.0 ≦ Ni ≦ 16.0 mass%,
8.0 ≦ Cr ≦ 10.9 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%
And the balance consists of Fe and inevitable impurities.
<2> The precipitation hardening martensitic stainless steel satisfies the following equations (1) to (4).
1.00 ≦ [Al] + [Ti] + [Nb] ≦ 2.00 (1)
5.50 ≦ [Ni] / ([Al] + [Ti] + [Nb]) ≦ 12.00 (2)
10.00 ≦ Ni _eq ≦ 17.00 (3)
12.00 ≦ Cr _eq ≦ 17.00 (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 (mass%) of the element X. The precipitation hardening martensitic stainless steel according to claim 1, wherein the 0.2% proof stress is 1300 MPa or more. (3) The precipitation-hardening martensitic stainless steel according to claim 1 or 2, having an absorption energy of 10 J or more. A drill part for underground drilling, comprising the precipitation hardening type martensitic stainless steel according to any one of claims 1 to 3.