JP2024008729A - Martensitic stainless steel for nitrogen-enriching treatment and martensitic stainless steel member - Google Patents

Abstract

To provide a martensitic stainless steel for nitrogen-enriching treatment that has excellent hot workability and excellent cold workability, and a martensitic stainless steel member.SOLUTION: The present invention relates to a martensitic stainless steel for nitrogen-enriching treatment, comprising 0.10≤C≤0.30 mass%, Si≤0.20 mass%, 0.20≤Mn≤1.50 mass%, P≤0.05 mass%, S≤0.01 mass%, Cu≤0.3 mass%, 10.5≤Cr≤17.0 mass%, 0.50≤Ni≤3.00 mass%, 0.50≤Mo≤3.00 mass%, and 0.1≤Nb≤0.5 mass%, with the balance being Fe and inevitable impurities, satisfying 1.00≤[Ni]*[Mo]≤9.00 and [Nieq]/[Creq]≥1.00, and having a ferrite phase area ratio of 50% or more. A martensitic stainless steel member comprises a base part composed of the martensitic stainless steel and a nitrogen enriched layer formed on the surface of the base part.SELECTED DRAWING: None

Description

The present invention relates to a martensitic stainless steel for nitrogen enrichment treatment and a martensitic stainless steel member, and more specifically to a martensite stainless steel for nitrogen enrichment treatment that has excellent hot workability, cold workability, and fatigue properties. This invention relates to site-based stainless steel and martensitic stainless steel members using the same.

Martensitic stainless steel refers to stainless steel that has a martensitic structure at room temperature. Martensitic stainless steel is manufactured by quenching and tempering the austenitic region. Martensitic stainless steel has excellent corrosion resistance, strength, wear resistance, etc., and is therefore used in cutlery, turbine blades, bearings, etc.

It is known that when nitrogen is added as a solid solution to such martensitic stainless steel, corrosion resistance, hardness, wear resistance, etc. are further improved. However, when a method of dissolving nitrogen in molten steel is used, blowholes may occur in the ingot. Therefore, after producing a base material made of low-nitrogen martensitic stainless steel, nitrogen is dissolved only in the surface layer of the base material, and a layer (hereinafter referred to as , also called a "nitrogen-enriched layer") has been proposed. By forming a nitrogen-enriched layer on the surface layer of a low-nitrogen base material, the corrosion resistance and abrasion resistance of the base material can be improved without causing blowholes.

Various proposals have been made regarding methods for forming such a nitrogen-enriched layer.
For example, in Patent Document 1,
(a) Quenching and tempering a steel wire made of martensitic stainless steel containing predetermined amounts of Mo, V, and Al;
(b) A method for manufacturing martensitic stainless steel is disclosed in which the steel wire is further subjected to gas nitriding treatment.
This document describes that by such a method, a thick spring material with excellent delayed fracture resistance and fatigue resistance can be obtained.

In Patent Document 2,
(a) Producing a member (annealed material) made of martensitic stainless steel containing a predetermined amount of C, Si, Mn, and Cr,
(b) heating the member in nitrogen gas (99% purity) at atmospheric pressure, then cooling it in a furnace to 600°C or less,
(c) Furthermore, a method for manufacturing a martensitic stainless steel member is disclosed in which the member is hardened and tempered.
This document describes that a martensitic stainless steel member having excellent corrosion resistance and wear resistance can be obtained by such a method.

Although Patent Document 3 does not describe a method for forming a nitrogen-enriched layer,
(a) Producing a steel bar made of martensitic stainless steel containing predetermined amounts of C, Si, Mn, S, P, Ni, Cr, Mo, N, and Al;
(b) Softening annealing the steel bar under predetermined conditions,
(c) cold working the softened and annealed steel bar,
(d) A method for manufacturing martensitic stainless steel is disclosed in which a cold-worked member is hardened by quenching to produce a final product.
In the same document,
(A) If carbonitrides are finely dispersed in the steel after softening annealing, the fine carbonitrides will pin the movement of dislocations and grain boundaries, resulting in a decrease in cold workability;
(B) Softening annealing at high temperatures reduces fine carbonitrides, improving cold workability;
is listed.

Wear resistance can be improved by subjecting the surface of a member made of low-nitrogen martensitic stainless steel to a treatment for forming a nitrogen-enriched layer (hereinafter also referred to as "nitrogen-enrichment treatment"). However, if the treatment conditions are inappropriate, abnormal grain growth may occur during the nitrogen enrichment treatment. Abnormal grain growth causes deterioration of the fatigue properties of the member.
In order to solve this problem, it may be possible to add a carbonitride-forming element to precipitate a large amount of carbonitrides in the steel, and to suppress abnormal grain growth by the carbonitrides. However, when the amount of carbonitride precipitation becomes excessive, hot workability and/or cold workability may deteriorate. Furthermore, when the amount of carbonitride precipitation becomes excessive, the corrosion resistance of the member may deteriorate.

JP 2021-143388 Publication JP2019-167630A Japanese Patent Application Publication No. 2020-050916

The problem to be solved by the present invention is to provide a martensitic stainless steel for nitrogen enrichment treatment that has excellent hot workability and cold workability, and a martensitic stainless steel member using the same. be.
Further, other problems to be solved by the present invention are a martensitic stainless steel for nitrogen enrichment treatment that can produce members with excellent fatigue properties, and a martensitic stainless steel member using the same. Our goal is to provide the following.
Furthermore, other problems to be solved by the present invention are a martensitic stainless steel for nitrogen enrichment treatment that can produce members with excellent fatigue properties and corrosion resistance, and a martensitic stainless steel using the same. Our objective is to provide steel members.

In order to solve the above problems, the martensitic stainless steel for nitrogen enrichment treatment according to the present invention has the following features:
0.10≦C≦0.30mass%,
Si≦0.20mass%,
0.20≦Mn≦1.50 mass%,
P≦0.05mass%,
S≦0.01mass%,
Cu≦0.3mass%,
10.5≦Cr≦17.0mass%,
0.50≦Ni≦3.00mass%,
0.50≦Mo≦3.00mass%, and
0.1≦Nb≦0.5mass%
with the remainder consisting of Fe and unavoidable impurities,
The following formulas (1) and (2) are satisfied,
The area ratio of the ferrite phase is 50% or more.

1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si].

The martensitic stainless steel member according to the present invention includes:
A base made of martensitic stainless steel,
a nitrogen-enriched layer formed on the surface of the base,
The martensitic stainless steel is
0.10≦C≦0.30mass%,
Si≦0.20mass%,
0.20≦Mn≦1.50 mass%,
P≦0.05mass%,
S≦0.01mass%,
Cu≦0.3mass%,
10.5≦Cr≦17.0mass%,
0.50≦Ni≦3.00mass%,
0.50≦Mo≦3.00mass%, and
0.1≦Nb≦0.5mass%
with the remainder consisting of Fe and unavoidable impurities,
The martensitic stainless steel satisfies the following formulas (1) and (2),
The base has a ferrite phase area ratio of 5% or less,
The thickness of the nitrogen-enriched layer is 100 μm or more.

1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si].

In martensitic stainless steel having a predetermined composition, if the Ni amount and Mo amount are optimized so as to satisfy formula (1), coarsening of crystal grains during nitrogen enrichment treatment can be suppressed. As a result, a martensitic stainless steel member with excellent fatigue properties is obtained.
Moreover, if the content of each element is optimized so as to satisfy formula (2), a martensitic stainless steel containing a relatively large amount of martensitic phase after quenching and tempering can be obtained. As a result, the fatigue properties of a member using this martensitic stainless steel are improved.

Further, in a martensitic stainless steel having a predetermined composition, optimizing the composition (particularly the amount of Ni) improves the cold workability of the martensitic stainless steel.
Furthermore, if martensitic stainless steel contains excessive amounts of Cu, P, and/or S, hot workability decreases. On the other hand, when the amount of Cu, the amount of P, and the amount of S are set to below the critical values, the hot workability of martensitic stainless steel is improved.
Furthermore, by optimizing the nitrogen enrichment treatment method and/or treatment conditions, excessive nitride formation within the nitrogen enrichment layer is suppressed. As a result, a martensitic stainless steel member with excellent corrosion resistance is obtained.

An embodiment of the present invention will be described in detail below.
[1. Martensitic stainless steel for nitrogen enrichment treatment]
[1.1. Main constituent elements]
The martensitic stainless steel for nitrogen enrichment treatment (hereinafter also simply referred to as "martensitic stainless steel") according to the present invention contains the following elements, with the remainder consisting of Fe and inevitable impurities. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

(1) 0.10≦C≦0.30mass%:
C is an element that increases the strength of martensitic stainless steel. In order to obtain a sufficient effect, the amount of C needs to be 0.10 mass% or more. The amount of C is preferably 0.15 mass% or more.
On the other hand, when the amount of C becomes excessive, the amount of undissolved Cr carbide increases during quenching, which may reduce the corrosion resistance of the matrix. Therefore, the amount of C needs to be 0.30 mass% or less.

(2) Si≦0.20mass%:
Si is an effective element as a deoxidizing element. However, when the amount of Si becomes excessive, ductility and toughness may be impaired. Therefore, the amount of Si needs to be 0.20 mass% or less. The amount of Si is preferably 0.15 mass% or less.

(3) 0.20≦Mn≦1.50mass%:
Mn is a deoxidizing element. Mn is also an element that increases the solid solubility limit of nitrogen during nitrogen enrichment treatment (particularly solid phase nitrogen absorption treatment). In order to obtain a sufficient effect, the amount of Mn needs to be 0.20 mass% or more. The amount of Mn is preferably 0.40 mass% or more.
On the other hand, when the amount of Mn becomes excessive, corrosion resistance may be impaired. Therefore, the amount of Mn needs to be 1.50 mass% or less. The amount of Mn is preferably 1.20 mass% or less.

(4) P≦0.05mass%:
P is an unavoidable impurity. When the amount of P becomes excessive, grain boundaries may become brittle and hot workability may deteriorate. Therefore, the amount of P needs to be 0.05% or less.

(5) S≦0.01mass%:
S is an unavoidable impurity. If the amount of S is excessive, sulfides may be formed and hot workability and/or corrosion resistance may deteriorate. Therefore, the amount of S needs to be 0.01 mass% or less.

(6) Cu≦0.3mass%:
Cu is an unavoidable impurity. When the amount of Cu becomes excessive, undissolved Cu may precipitate in the steel. Undissolved Cu melts during hot working, causing a reduction in hot workability. Therefore, the amount of Cu needs to be 0.3 mass% or less. The amount of Cu is preferably 0.2 mass% or less.

(7) 10.5≦Cr≦17.0mass%:
Cr is an element that has the effect of improving corrosion resistance. In order to obtain a sufficient effect, the amount of Cr needs to be 10.5 mass% or more. The amount of Cr is preferably 12.0 mass% or more.
On the other hand, when the amount of Cr is excessive, a large amount of ferrite phase is generated after quenching and tempering, which may reduce the strength. Therefore, the amount of Cr needs to be 17.0 mass% or less. The amount of Cr is preferably 16.0 mass% or less.

(8) 0.50≦Ni≦3.00mass%:
Ni is an element effective in improving corrosion resistance. In order to obtain a sufficient effect, the Ni amount needs to be 0.50 mass% or more. The amount of Ni is preferably 0.70 mass% or more.
On the other hand, when the amount of Ni becomes excessive, cold workability may deteriorate. Therefore, the amount of Ni needs to be 3.00 mass% or less. The amount of Ni is preferably 2.50 mass% or less.

(9) 0.50≦Mo≦3.00mass%:
Mo is an element that has the effect of improving corrosion resistance. In order to obtain a sufficient effect, the amount of Mo needs to be 0.50 mass% or more. The amount of Mo is preferably 0.70 mass% or more.
On the other hand, when the amount of Mo is excessive, a large amount of ferrite phase is generated after quenching and tempering, which may reduce the strength. Therefore, the amount of Mo needs to be 3.00 mass% or less. The amount of Mo is preferably 2.50 mass% or less.

(10) 0.1≦Nb≦0.5mass%:
Nb is an element that forms carbonitrides, refines crystal grains, and contributes to improving strength. For this purpose, the amount of Nb needs to be 0.1 mass% or more. The amount of Nb is preferably 0.15 mass% or more.
On the other hand, when the amount of Nb is excessive, coarse carbonitrides are formed, which may impair the grain refinement effect. Furthermore, carbonitrides may become a starting point for fracture, resulting in a decrease in strength. Therefore, the amount of Nb needs to be 0.5 mass% or less.

Similar elements having a crystal grain refining effect include Ti and V. Among these, Nb has the advantage that coarse crystallized carbides are less likely to be formed than Ti, and carbonitrides are more stable than V, and precipitated particles are less likely to grow. Therefore, in the present invention, Nb is an essential element.

[1.2. Unavoidable impurities]
"Unavoidable impurities" refer to trace components that are mixed in from raw materials and refractories during the production of martensitic stainless steel. Examples of unavoidable impurities include:
(a) P of 0.050 mass% or less,
(b) S of 0.010 mass% or less,
(c) Al of 0.05 mass% or less,
(d) O of 0.030 mass% or less,
and so on.

[1.3. Sub-constituent elements]
In addition to the above-mentioned elements, the martensitic stainless steel according to the present invention may further contain one or more of the following elements. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

(1) Ti≦0.5mass%:
When Ti is further added to martensitic stainless steel containing Nb, Ti plays an auxiliary role to Nb and has the effect of enhancing the grain refinement effect. Therefore, the martensitic stainless steel may further contain Ti.
However, Ti is more likely to form coarse crystallized carbides than Nb. Therefore, when the amount of Ti becomes excessive, fine carbonitrides may decrease and crystal grains may become coarse. Therefore, the amount of Ti is preferably 0.5 mass% or less.

(2) V≦1.0mass%:
Like Ti, V plays an auxiliary role to Nb and has the effect of enhancing the crystal grain refinement effect. Therefore, the martensitic stainless steel may further contain V.
However, V has lower stability of carbonitrides at high temperatures than Nb, and precipitated particles tend to grow. Therefore, when the amount of V becomes excessive, fine carbonitrides may decrease and crystal grains may become coarse. Therefore, the amount of V is preferably 1.0 mass% or less.

[1.4. Ingredient balance]
The martensitic stainless steel according to the present invention satisfies the following formulas (1) and (2).
1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si].

[1.4.1. Formula (1)]
[Ni]*[Mo] has a correlation with the average grain size of martensitic stainless steel after nitrogen enrichment treatment. If [Ni]*[Mo] becomes too small, the crystal grains may become coarse after the nitrogen enrichment treatment. As a result, if repeated stress is applied to the member after the nitrogen enrichment treatment, grain boundary fracture may progress. Therefore, [Ni]*[Mo] needs to be 1.00 or more. [Ni]*[Mo] is preferably 1.05 or more, more preferably 1.10 or more.

On the other hand, if [Ni]*[Mo] becomes too large, the crystal grains will become coarse before the nitrogen enrichment treatment, and fine crystal grains may not be obtained even after the nitrogen enrichment treatment. Therefore, [Ni]*[Mo] needs to be 9.00 or less. [Ni]*[Mo] is preferably 6.00 or less, more preferably 4.00 or less.

[1.4.2. Formula (2)]
[Nieq] is the Ni equivalent, which is an index of the austenite stabilizing element. [Creq] is the Cr equivalent which is an index of the ferrite stabilizing element. If [Nieq]/[Creq] becomes too small, the amount of ferrite phase remaining in the quenched structure increases, and the fatigue strength may decrease. Therefore, [Nieq]/[Creq] needs to be 1.00 or more. [Nieq]/[Creq] is preferably 1.02 or more, more preferably 1.05 or more.

[1.5. Area ratio of ferrite phase]
Since the martensitic stainless steel according to the present invention has optimized components, the area ratio of the ferrite phase at 23° C. after annealing (before nitrogen enrichment treatment) is 50% or more. When manufacturing conditions are optimized, the area ratio of the ferrite phase becomes 70% or more, or 90% or more. Further, when the components are optimized, the area ratio of the ferrite phase may reach 100%. The "area ratio of ferrite phase" will be described later.
The martensitic stainless steel according to the present invention has excellent cold workability because the area ratio of the ferrite phase is relatively large (that is, the area ratio of the martensitic phase is relatively small) in the state after annealing. There is.

[1.6. Use]
The martensitic stainless steel according to the present invention is used to produce a base by annealing and cold working, and then to perform a nitrogen enrichment treatment on the surface of the base. After the nitrogen enrichment treatment, quenching and tempering are further performed. When the nitrogen enrichment treatment is performed, a layer (nitrogen-enriched layer) having a higher nitrogen concentration than the core of the base is formed in the surface layer of the base. Furthermore, when quenching and tempering are performed, the nitrogen-enriched layer is hardened. As a result, a member with excellent fatigue properties can be obtained.

[2. Martensitic stainless steel parts]
The martensitic stainless steel member according to the present invention has the following configuration.
(1) The martensitic stainless steel member is
A base made of martensitic stainless steel,
and a nitrogen-enriched layer formed on the surface of the base.
(2) The martensitic stainless steel is
0.10≦C≦0.30mass%,
Si≦0.20mass%,
0.20≦Mn≦1.50 mass%,
P≦0.05mass%,
S≦0.01mass%,
Cu≦0.3mass%,
10.5≦Cr≦17.0mass%,
0.50≦Ni≦3.00mass%,
0.50≦Mo≦3.00mass%, and
0.1≦Nb≦0.5mass%
with the remainder consisting of Fe and unavoidable impurities,
The following equations (1) and (2) are satisfied.
(3) The base has a ferrite phase area ratio of 5% or less.
(4) The thickness of the nitrogen-enriched layer is 100 μm or more.

1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si].

[2.1. base]
[2.1.1. material]
The base is made of martensitic stainless steel according to the present invention. The details of the martensitic stainless steel are as described above, so the explanation will be omitted.

[2.1.2. Area ratio of ferrite phase]
"Area ratio (%) of ferrite phase" refers to the ratio of the area of ferrite phase to the cross-sectional area of martensitic stainless steel at 23°C.
In other words, "ferrite phase area ratio (%)" is
(a) cutting martensitic stainless steel after annealing or martensitic stainless steel member after nitrogen enrichment treatment + quenching and tempering in a direction perpendicular to the surface, polishing and etching the cross section,
(b) Observe the area near the center of the cross section (in the case of a member subjected to nitrogen enrichment treatment, the area (core) where the nitrogen concentration is at the level before nitrogen enrichment treatment) at room temperature with an optical microscope, Calculate the visual field area (S ₀ ) and the area (S) of the ferrite phase included in the visual field, respectively,
(c) Value obtained by dividing S by S ₀ (=S×100/S ₀ )
means.
In the present invention, S ₀ is 2 mm x 3 mm.

Since the composition of the martensitic stainless steel according to the present invention is optimized, the area ratio of the ferrite phase becomes relatively high in the state after annealing. On the other hand, after the nitrogen enrichment treatment + quenching and tempering, almost the entire surface of the base becomes a martensite phase, and the area ratio of the ferrite phase in the base becomes 5% or less. When the composition of martensitic stainless steel is optimized, the area ratio of the ferrite phase in the base after nitrogen enrichment treatment + quenching and tempering becomes 0.5% or less.

[2.1.3. Average grain size]
"Average grain size" is the average value of the grain size of prior austenite grains, and the grain size is measured from an optical microscope photograph by a cutting method using a straight test line described in JIS G0551. Table A in Book A. This is the value obtained by converting from the crystal grain size using 1.

The average grain size of the base core after nitrogen enrichment treatment affects the fatigue properties of the martensitic stainless steel member. Generally, the smaller the average crystal grain size of the base core after nitrogen enrichment treatment, the higher the fatigue characteristics of the martensitic stainless steel member. Since the martensitic stainless steel member according to the present invention contains an element that has the effect of refining crystal grains, grain growth during nitrogen enrichment treatment is suppressed. If the manufacturing conditions are optimized, the average crystal grain size of the base core after nitrogen enrichment treatment will be 50 μm or less. When the manufacturing conditions are further optimized, the average crystal grain size becomes 30 μm or less.

[2.2. Nitrogen-enriched layer]
[2.2.1. Definition]
"Nitrogen enriched layer" is a layer formed by applying nitrogen enrichment treatment to the base after annealing and cold working, and is a layer formed by applying nitrogen enrichment treatment to the base after annealing and cold working. This refers to an area where the nitrogen concentration is higher than the area where the nitrogen concentration is at the same level. More specifically, the "nitrogen-enriched layer" refers to a region where the amount of N is 0.1 mass% or more and 1.0 mass% or less.
In addition, when the amount of N in the nitrogen-enriched layer becomes excessive,
(a) Wear resistance decreases because martensitic transformation does not progress during quenching.
(b) Problems such as a decrease in corrosion resistance may occur due to the precipitation of a large amount of nitrides. Therefore, the amount of N in the nitrogen-enriched layer needs to be 1.0 mass% or less.

"Nitrogen enrichment treatment" refers to treatment in which nitrogen is dissolved in the surface layer of the base. Examples of the nitrogen enrichment treatment method include a solid phase nitrogen absorption method, a gas nitriding method, a soft nitriding method, and an ion nitriding method. Among these, the solid-phase nitrogen absorption method is suitable as a nitrogen enrichment treatment method because a member with excellent corrosion resistance can be obtained. The solid phase nitrogen absorption method has the advantage of suppressing the formation of chromium carbonitrides and improving wear resistance without reducing the corrosion resistance of the outermost surface. Details of the solid phase nitrogen absorption method will be described later.

[2.2.2. thickness]
"Thickness of the nitrogen-enriched layer" refers to the thickness of a region formed on the surface of the base by subjecting the base to nitrogen enrichment treatment and in which the amount of N is 0.1 mass% or more and 1.0 mass% or less. .
The thickness of the nitrogen-enriched layer affects the wear resistance of the component. If the thickness of the nitrogen-enriched layer becomes too thin, the wear resistance will be insufficient. Therefore, the thickness of the nitrogen-enriched layer is preferably 100 μm or more. The thickness is more preferably 150 μm or more, even more preferably 200 μm or more.

[3. Manufacturing method of martensitic stainless steel]
The martensitic stainless steel according to the present invention is
(a) Melting and casting raw materials blended to a predetermined composition,
(b) Hot forging the obtained ingot,
(c) It can be manufactured by annealing a hot-forged rough section.

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

[3.2. Hot forging process]
Next, the obtained ingot is hot forged. Hot forging is performed to destroy the casting structure and process the ingot into a rough shaped material having a predetermined shape. The hot forging method and conditions are not particularly limited, and optimal conditions can be selected depending on the purpose.

[3.3. Annealing process]
The martensitic stainless steel according to the present invention has too high hardness in the as-hot-forged state and has poor cold workability. This is because a martensitic phase is partially formed during the cooling process after hot forging. In order to improve cold workability, it is necessary to anneal the hot-forged rough shape material to increase the area ratio of the ferrite phase.

The annealing conditions are not particularly limited as long as the necessary cold workability is obtained.
Specifically, annealing is
(a) Maintained at a temperature of 850°C or higher and 950°C or lower for 1 hour or more and 10 hours or less,
(b) It is preferable to perform slow cooling in the temperature range from the annealing temperature to 650°C at an average cooling rate of 30°C/h or less.

[4. Manufacturing method of martensitic stainless steel parts]
The martensitic stainless steel member according to the present invention includes:
(a) Cold working is performed on the annealed rough shape material,
(b) Perform nitrogen enrichment treatment on the cold worked member,
(c) It can be manufactured by quenching and tempering a member that has been subjected to nitrogen enrichment treatment.

[4.1. Cold working process]
First, cold working is performed on the annealed rough section. The cold working method and conditions are not particularly limited as long as they can produce a member having a predetermined shape.

[4.2. Nitrogen enrichment treatment process]
Next, the cold worked member is subjected to nitrogen enrichment treatment. Examples of nitrogen enrichment treatment methods include solid phase nitrogen absorption, gas nitriding, soft nitriding, and ion nitriding. In the present invention, either method may be used. In order to obtain a member with excellent corrosion resistance, the nitrogen enrichment treatment method is preferably a solid phase nitrogen absorption method.

The solid phase nitrogen absorption treatment is performed by heating the member to a predetermined temperature in a predetermined nitrogen atmosphere. Examples of the nitrogen atmosphere include a nitrogen gas atmosphere.

The heating temperature affects the nitrogen concentration on the surface of the member. Therefore, it is preferable to select the optimum heating temperature depending on the composition of the member. Generally, the lower the heating temperature, the higher the equilibrium nitrogen concentration on the surface of the member. However, if the heating temperature becomes too low, a large amount of nitrides may precipitate and the corrosion resistance may deteriorate. Therefore, the heating temperature is preferably 900°C or higher.
On the other hand, if the heating temperature becomes too high, the equilibrium nitrogen concentration on the surface of the member may decrease and the hardness may decrease. Therefore, the heating temperature is preferably 1200°C or less.

The nitrogen partial pressure affects the nitrogen concentration on the surface of the member. Therefore, it is preferable to select an optimal value for the nitrogen partial pressure depending on the composition of the member. Generally, the higher the nitrogen partial pressure, the higher the equilibrium nitrogen concentration on the surface of the member. In order to obtain such an effect, the nitrogen partial pressure is preferably 0.1 atm (0.01 MPa) or higher.
On the other hand, when the nitrogen partial pressure becomes too high, the nitrogen concentration increases excessively and the martensitic transformation start temperature decreases. As a result, the austenite phase tends to remain after quenching, and the hardness may decrease. Therefore, the nitrogen partial pressure is preferably 3.0 atm (0.3 MPa) or less.

The nitrogen concentration on the surface of the member is determined by the heating temperature and nitrogen partial pressure. On the other hand, the treatment time affects the thickness of the nitrogen absorption layer (nitrogen enriched layer). Therefore, it is preferable to select an optimal treatment time depending on the required thickness of the nitrogen absorption layer. The treatment time is usually about 60 minutes to 600 minutes.

[4.3. Quenching and tempering process]
Next, the nitrogen-enriched member is quenched and tempered. This transforms the austenite phase into martensitic phase. The quenching conditions are not particularly limited as long as the conditions are such that the surface layer portion and the core portion can undergo martensitic transformation.

For example, when solid phase nitrogen absorption treatment is used as the nitrogen enrichment treatment method, quenching may be performed by rapidly cooling the member from the treatment temperature. Alternatively, quenching may be performed by performing solid phase nitrogen absorption treatment, cooling to near room temperature, reheating to the quenching temperature, and rapidly cooling. For rapid cooling, gas cooling, water cooling, ice water cooling, oil cooling, etc. can be used.
In order to make the entire core of the member have a martensitic structure, the faster the cooling rate during quenching, the better. Specifically, the average cooling rate from the quenching temperature to 500°C is preferably 200°C/min or more.

Further, before hardening, a nitrogen diffusion treatment may be performed to diffuse nitrogen into the inside of the member. Specifically, following the solid-phase nitrogen absorption treatment, the member is held at a high temperature of about 900 to 1200° C. in an inert atmosphere such as argon gas to diffuse nitrogen into the member. When nitrogen diffusion treatment is performed, the nitrogen absorption layer becomes thicker, so that the hardness of the surface of the member after quenching can be stably obtained.
Furthermore, sub-zero treatment may be performed to maintain the material at 0° C. or lower after quenching.

Next, after hardening (or after sub-zero treatment), tempering is performed. Tempering conditions are not particularly limited, and optimal conditions can be selected depending on the purpose. Specifically, the tempering is preferably maintained at a temperature of 150° C. to 500° C. for about 1 hour to 3 hours.

[5. Effect]
In martensitic stainless steel having a predetermined composition, if the Ni amount and Mo amount are optimized so as to satisfy formula (1), coarsening of crystal grains during nitrogen enrichment treatment can be suppressed. As a result, a martensitic stainless steel member with excellent fatigue properties is obtained.
Moreover, if the content of each element is optimized so as to satisfy formula (2), a martensitic stainless steel containing a relatively large amount of martensitic phase after quenching and tempering can be obtained. As a result, the fatigue properties of a member using this martensitic stainless steel are improved.

Further, in a martensitic stainless steel having a predetermined composition, optimizing the composition (particularly the amount of Ni) improves the cold workability of the martensitic stainless steel.
Furthermore, if martensitic stainless steel contains excessive amounts of Cu, P, and/or S, hot workability decreases. On the other hand, when the amount of Cu, the amount of P, and the amount of S are set to below the critical values, the hot workability of martensitic stainless steel is improved.
Furthermore, by optimizing the nitrogen enrichment treatment method and/or treatment conditions, excessive nitride formation within the nitrogen enrichment layer is suppressed. As a result, a martensitic stainless steel member with excellent corrosion resistance is obtained.

(Examples 1 to 14, Comparative Examples 1 to 16)
[1. Preparation of sample]
50 kg of steel having the composition shown in Tables 1 and 2 was melted in a vacuum induction furnace. The obtained ingot was hot forged to produce a bar with a diameter of 20 mm. Next, the bar was heated to 900°C and then softened to 650°C by slow cooling at 20°C/h. A test piece with a diameter of 15 mm x 8 mm was taken from this bar.

Next, each test piece was subjected to solid phase nitrogen absorption treatment and quenching. That is, first, a test piece was placed in a processing chamber, and the inside of the processing chamber was evacuated using a pressure reducing means. Next, nitrogen gas was introduced into the processing chamber to maintain the pressure and temperature within the processing chamber at predetermined values.
By appropriately adjusting the nitrogen partial pressure between 0.1 and 3.0 atm (0.01 MPa and 0.30 MPa) and the treatment temperature between 900 and 1200°C, the amount of N within the range of at least 100 μm from the surface of the member can be reduced to 0. A nitrogen absorption layer having a nitrogen content of .1 to 1.0 mass% was obtained. The heating time was adjusted so that the thickness of the nitrogen absorption layer after hardening was 100 μm or more.

Furthermore, after the solid phase nitrogen absorption treatment was completed, the test piece was quenched. Quenching was performed by gas cooling. Thereafter, sub-zero treatment at -80°C for 2 hours and tempering at 200°C were performed.

[2. Test method]
[2.1. Ferrite area ratio of the core of the quenched structure]
The test piece after solid-phase nitrogen absorption treatment and quenching and tempering was cut, and the area ratio of the ferrite phase in the core was calculated.

[2.2. Average grain size after nitrogen absorption treatment]
The test piece after the solid-phase nitrogen absorption treatment was cut, and the average crystal grain size of the core was calculated. The average crystal grain size was measured by the line segment method. First, a photograph of the structure of the core of the test piece after nitrogen absorption treatment was taken using an optical microscope (magnification: 100 times). Next, draw a total of 10 different straight lines, 5 vertically and 5 horizontally, on this photograph, and for each straight line, calculate the length (L) of the straight line and the number of grain boundaries that intersect with the straight line (n ) (=L/n) was calculated. Furthermore, the average crystal grain size was calculated by calculating their average value.

[2.3. Rotating bending fatigue test]
A rotating bending fatigue test was conducted using the test method specified in JIS Z2274. The fracture surface was observed and the origin of the fracture was confirmed.

[2.4. 5% sulfuric acid immersion test]
Corrosion loss was calculated for each sample after solid-phase nitrogen absorption treatment according to the following procedure. That is, the mass of the test piece after the solid-phase nitrogen absorption treatment before the corrosion test was measured. Next, the test piece was immersed for 6 hours in a 5% sulfuric acid aqueous solution maintained at 30° C. in a constant temperature bath. Thereafter, the corrosion products adhering to the test piece were removed by ultrasonic cleaning, and the mass of the test piece after the corrosion test was measured. Corrosion loss was calculated by calculating the mass loss of the test piece and dividing this by the surface area and time.

[2.5. Hot workability]
The presence or absence of cracks was visually confirmed for the bar material after hot working.
[2.6. Structure after softening annealing (cold workability)]
The presence or absence of a ferrite phase was confirmed by optical microscopic observation of the bar material after softening and annealing. Furthermore, the area ratio of the ferrite phase in the core was calculated.

[3. result]
Tables 3 and 4 show the results. From Tables 3 and 4, the following can be seen.
In addition, in Tables 3 and 4, regarding the "ferrite area ratio of the quenched structure core", "◎" indicates that the ferrite area ratio is 0.5% or less, and "○" indicates that the ferrite area ratio is 0.5% or less. It represents that the ferrite area ratio is more than 5% and 5% or less, and "x" represents that the ferrite area ratio is more than 5%.
Regarding the "average grain size after nitrogen absorption treatment,""◎" represents that the average grain size is 30 μm or less, "○" represents that the average grain size is more than 30 μm and 50 μm or less, and " ×” indicates that the average crystal grain size is more than 50 μm.

Regarding the "rotating bending fatigue test", "x" indicates fracture originating from the ferrite phase, intergranular fracture originating from the grain boundaries of crystal grains with an equivalent circle diameter of 100 μm or more, and/or originating from coarse carbonitrides. "○" indicates that grain boundary fracture occurred, and "○" indicates that no such fracture occurred.
Regarding the "5% sulfuric acid immersion test,""◎" indicates that the corrosion loss is 1 g/(m ²・hr) or less, and "○" indicates that the corrosion loss is over 1 g/(m ²・hr) and 10 g/( m ² ·hr) or less, and "x" indicates that the corrosion weight loss exceeds 10 g/(m ² ·hr).

Regarding "hot workability,""○" indicates that no cracking occurred during hot forging, and "x" indicates that cracking occurred during hot forging.
Furthermore, regarding the "structure after softening annealing", "◎" indicates a single ferrite phase (α) (area ratio of ferrite phase is over 90.0%), and "○" indicates a ferrite phase and martensite. It represents a mixed structure of phase (α') (area ratio of ferrite phase is 50.0% or more and 90.0% or less), and "x" indicates that the area ratio of martensitic phase is 50.0% or more (ferrite The area ratio of the phase is less than 50.0%).

(1) In Comparative Example 1, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, failure occurred starting from the ferrite phase. This is considered to be because [Nieq]/[Creq] was less than 1.0, and a large amount of ferrite phase remained even after quenching.
(2) In Comparative Example 2, the corrosion loss increased. This is considered to be because the amount of C was excessive and a large amount of undissolved Cr carbonitride was generated.
(3) In Comparative Example 3, the corrosion loss increased. This is considered to be because the amount of Mn is excessive.

(4) In Comparative Example 4, hot workability decreased. This is considered to be because the amount of Cu is excessive.
(5) In Comparative Example 5, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from the grain boundaries of coarse grains. This is considered to be because [Ni]*[Mo] is less than 1.0.
(6) In Comparative Example 6, the structure after softening annealing became α' single phase. This is considered to be because the amount of Ni is excessive.

(7) In Comparative Example 7, the corrosion loss exceeded 10 g/(m ² ·hr). This is considered to be due to the small amount of Cr.
(8) In Comparative Example 8, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, failure occurred starting from the ferrite phase. This is considered to be because the amount of Cr is excessive.
(9) In Comparative Example 9, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from the grain boundaries of coarse grains. Further, in Comparative Example 9, the corrosion loss exceeded 10 g/(m ² ·hr). This is considered to be because the amount of Mo is small and [Ni]*[Mo] is less than 1.0.

(10) In Comparative Example 10, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, failure occurred starting from the ferrite phase. This is considered to be because the amount of Mo is excessive and [Nieq]/[Creq] is less than 1.
(11) In Comparative Example 11, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from the grain boundaries of coarse grains. This is considered to be because the amount of Nb was small and abnormal grain growth occurred during the nitrogen absorption treatment.
(12) In Comparative Example 12, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from coarse carbonitrides. This is considered to be because coarse carbonitrides were formed due to the excessive amount of Nb.

(13) In Comparative Examples 13 and 14, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from the grain boundaries of coarse grains. This is considered to be because [Ni]*[Mo] is less than 1.0.
(14) In Comparative Example 15, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from coarse carbonitrides. This is considered to be because coarse carbonitrides were formed due to the excessive amount of V.
(15) In Comparative Example 16, the average crystal grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotating bending fatigue test, intergranular fracture occurred starting from coarse carbonitrides. This is considered to be because coarse carbonitrides were formed due to the excessive amount of Ti.

(16) In all of Examples 1 to 14, the ferrite area ratio in the core of the hardened structure was 0.5% or less, and the average crystal grain size after nitrogen absorption treatment was 50 μm or less. As a result, Examples 1 to 14 all showed good rotating bending fatigue properties. Furthermore, in all of Examples 1 to 14, the corrosion loss was 10 g/(m ² ·hr) or less, and no cracking occurred during hot working.
(17) The structures of Examples 1 to 4 and 6 to 14 after softening annealing were α+α' mixed structures. On the other hand, the structure after softening annealing in Example 5 was α single phase.

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.

The martensitic stainless steel for nitrogen enrichment treatment according to the present invention can be used for shafts, bearings, gears, pins, bolts, screws, turbine blades, valves, cutters, nozzles, etc.

Claims (4)

0.10≦C≦0.30mass%,
Si≦0.20mass%,
0.20≦Mn≦1.50 mass%,
P≦0.05mass%,
S≦0.01mass%,
Cu≦0.3mass%,
10.5≦Cr≦17.0mass%,
0.50≦Ni≦3.00mass%,
0.50≦Mo≦3.00mass%, and
0.1≦Nb≦0.5mass%
with the remainder consisting of Fe and unavoidable impurities,
The following formulas (1) and (2) are satisfied,
A martensitic stainless steel for nitrogen enrichment treatment in which the area ratio of ferrite phase is 50% or more.
1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si]. Ti≦0.5mass%, and/or
V≦1.0mass%
The martensitic stainless steel for nitrogen enrichment treatment according to claim 1, further comprising: A base made of martensitic stainless steel,
a nitrogen-enriched layer formed on the surface of the base,
The martensitic stainless steel is
0.10≦C≦0.30mass%,
Si≦0.20mass%,
0.20≦Mn≦1.50 mass%,
P≦0.05mass%,
S≦0.01mass%,
Cu≦0.3mass%,
10.5≦Cr≦17.0mass%,
0.50≦Ni≦3.00mass%,
0.50≦Mo≦3.00mass%, and
0.1≦Nb≦0.5mass%
with the remainder consisting of Fe and unavoidable impurities,
The martensitic stainless steel satisfies the following formulas (1) and (2),
The base has a ferrite phase area ratio of 5% or less,
A martensitic stainless steel member, wherein the nitrogen-enriched layer has a thickness of 100 μm or more.
1.00≦[Ni]*[Mo]≦9.00…(1)
[Nieq]/[Creq]≧1.00…(2)
however,
[X] is the content of element X (mass%),
[Nieq]=[Ni]+30[C]+0.5[Mn]+8,
[Creq]=[Cr”+[Mo]+1.5[Si]. The martensitic stainless steel is
Ti≦0.5mass%, and/or
V≦1.0mass%
The martensitic stainless steel member according to claim 3, further comprising: