EP4112754A1

EP4112754A1 - Precipitation-hardening martensitic stainless steel

Info

Publication number: EP4112754A1
Application number: EP20920783.6A
Authority: EP
Inventors: Taiki Maeda; Fugao Wei
Original assignee: Nippon Yakin Kogyo Co Ltd
Current assignee: Nippon Yakin Kogyo Co Ltd
Priority date: 2020-02-27
Filing date: 2020-10-27
Publication date: 2023-01-04
Also published as: CN115210389A; WO2021171698A1; JP6776467B1; JP2021134395A

Abstract

A precipitation hardening martensite stainless steel can maintain improved strength and toughness by performing aging heat treatment, the precipitation hardening martensite stainless steel comprising: in mass%, indicated by "%", C: 0.01 to 0.05%, Si: 1.0 to 2.0%, Mn: 0.70 to 1.50%, P: not more than 0.04%, S: not more than 0.01%, Ni: 6.0 to 8.0%, Cr: 12.0 to 15.0%, Mo: 0.50 to 1.50%, Cu: 0.40 to 1.20%, Ti: 0.20 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.005%, Al: 0.001 to 0.2%, O: 0.0001 to 0.01%, with inevitable impurities and Fe being the remainder, wherein Cu phase and Ni16(Ti, Nb)6Si7 type intermetallic compounds phase are distributed, and Nb in the intermetallic compounds phase is 0.2 to 3.0 (at%).

Description

Technical Field

The present invention relates to a precipitation hardening martensite stainless steel exhibiting high strength and ductility after aging heat treatment.

Background Art

A precipitation hardening type stainless steel is used for a steel belt, press plate, or the like, because strength thereof can be increased by performing aging heat treatment. As an example, SUS630, SUS631, or the like, may be mentioned.
The above SUS631 is a semi-austenitic stainless steel, and it is a metastable austenitic stainless steel under a solid solution condition.
After a deformation-induced martensite structure is formed by performing cold rolling on this steel, NiAl is precipitated by aging heat treatment so as to strengthen it; however, there is a problem in that productivity is not satisfactory. Furthermore, there is also a problem in that a δ ferrite phase is easily precipitated under high temperatures because Al is contained, and hot processing workability is not satisfactory.
The above SUS630 is a martensitic stainless steel, and has a martensite structure after solution heat treatment. It is strengthened by precipitation of ε-Cu phase by aging heat treatment; however, achievable strength is about 1500 MPa (Vickers hardness about 400).
Furthermore, among martensitic stainless steels, the same as for SUS630, a kind of steel exists in which Ti and Si are added so as to precipitate Ni₁₆Ti₆Si₇ type intermetallic compound phase (hereinafter referred to as a G phase) in addition to the ε-Cu phase, thereby causing strengthening.

Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-73783
Patent document 2: Japanese Unexamined Patent Application Publication No. 1999 (Heisei 11)-256282
Patent document 3: Japanese Unexamined Patent Application Publication No. 2017-155317

Summary of Invention

Precipitation hardening martensite stainless steels, stronger than those disclosed in the techniques of the above Patent documents, are widely used. However, as purposes of use for the precipitation hardening martensite stainless steels increased, demands associated with the purpose of use increased, and there may be a case in which properties are not sufficient, depending on conditions for use.
Therefore, an object of the present invention is to provide a precipitation hardening martensite stainless steel in which even greater strength and toughness can be maintained by performing aging heat treatment.
In order to solve the above problem, the inventors have researched focusing on a strengthened phase which is precipitated by alloy elements and aging heat treatment. In order to study effects by each of the elements, various components were solved by an experimental laboratory level, hot forged, and cold rolled so as to prepare a cold rolled material having a plate thickness of 2 mm. With respect to this material, solution heat treatment and aging heat treatment were performed, and then, mechanical properties were evaluated by tension test, Vickers hardness test, and the like, and a nanoscale precipitation hardening phase was evaluated by observing by a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM).
In particular, the observation was performed carefully and in detail by STEM having high resolution, and the precipitation phase was measured by EDS, and the following information was obtained. It became obvious that X in the G phase (Ni₁₆X₆Si₇) precipitated by aging heat treatment can be substituted by Fe, Mn, or Nb, in addition to Ti.
In particular, it was obvious that Ti was confirmed to be an element forming a skeleton of a G phase, and Mn was solid-solved at an X-site in a case in which Nb was not added. However, in this case, particle diameter of a G phase was large, being 4 to 20 nm, and at the same time, a Cu phase was also large, being 4 to 50 nm, and there was a tendency for the G phase and Cu phase to be distributed unevenly at a grain boundary, and as a result, precipitation hardening was not improved.
On the other hand, in a material in which Nb was added, it was obvious that Nb, instead of Mn, was solid-solved at the X-site. In addition, it was clear that precipitation of a precipitation hardening phase such as a G phase and a Cu phase was promoted, high strength could be obtained by shorter aging heat treatment time compared to the case in which Nb was not added, and the precipitation hardening phase such as a G phase and a Cu phase was fined, having a particle diameter of 1 to 20 nm. Furthermore, it was obvious that these G phase and Cu phase were not distributed unevenly at grain boundaries, and were dispersed finely in crystalline grains and grain boundaries. It was clear that precipitation hardening was extremely improved by these fine precipitation effects.
That is, the present invention is as follows:
An aspect of the present invention is a precipitation hardening martensite stainless steel including: in mass%, indicated by "%", C: 0.01 to 0.05%, Si: 1.0 to 2.0%, Mn: 0.70 to 1.50%, P: not more than 0.04%, S: not more than 0.01%, Ni: 6.0 to 8.0%, Cr: 12.0 to 15.0%, Mo: 0.50 to 1.50%, Cu: 0.40 to 1.20%, Ti: 0.20 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.005%, Al: 0.001 to 0.2%, O: 0.0001 to 0.01%, with the remainder being inevitable impurities and Fe, in which Cu phase and Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compounds phase are distributed, and Nb in the intermetallic compounds phase is 0.2 to 3.0 (at%).
In addition, in precipitation hardening, distribution condition and size of precipitation hardening phase may greatly affect strength. Therefore, another aspect of the precipitation hardening martensite stainless steel of the present invention is that not less than 50 number% of the Cu phase and the Ni₁₆(Ti, Nb)₆Si₇type intermetallic compounds phase is distributed in crystal grains, when a thin layer sample is prepared using a focused ion beam, an element mapping image is obtained by EDS by using an energy dispersive X-ray analyzer installed in a scanning transmission electron microscope (STEM), and the image is image-analyzed by observing and evaluating precipitation hardening phase at the nanoscale so as to obtain the distribution.
Another aspect of the precipitation hardening martensite stainless steel of the present invention is that average diameter of the Cu phase and the Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compounds phase is 1 to 20 nm.
Furthermore, another aspect of the present invention is that elongation is 2 to 15% and hardness is 400 to 600 Hv as a mechanical property.

Brief Description of Drawings

Fig. 1 is a conceptual diagram showing precipitation conditions of a Cu phase and a G phase in the stainless steel of the present invention, and showing neighbors at the grain boundary of three crystal grains.

Embodiments of the Invention

Reasons for limitations on chemical composition of the stainless steel of the present invention are explained. It should be noted that hereinafter "%" means "mass%" unless particularly noted.

C: 0.01 to 0.10%

C is austenite forming element, and reduces generation of δ ferrite phase at high temperatures. Furthermore, it solid-solves in a martensite phase so as to increase strength; however, a residual austenite phase may easily be increased after solution heat treatment, and sufficient strength may not be obtained after aging heat treatment. Furthermore, in a case in which C amount is large, Ti and Nb, which are constituent components of the G phase contributing to precipitation hardening, may be easily consumed by formation of TiC and NbC. Therefore, in order to reduce precipitation hardening ability by aging heat treatment, the content of C is set to be 0.01 to 0.10%. Furthermore, it is desirably set to be 0.03 to 0.05%.

Si: 1.0 to 2.0%

Since a G phase is generated by aging heat treatment and strength is greatly increased by precipitation hardening, Si is set to be not less than 1.0%. On the other hand, content amount of Si is set to be not more than 2.0% since Si is a ferrite generating element, δ ferrite phase may be easily generated by a large content amount of Si, and hot workability and strength around a welded portion may be decreased. Furthermore, it is desirably set to be 1.30 to 1.90%.

Mn: 0.50 to 1.50%

Since Mn is an austenite forming element, generation of δ ferrite phase at high temperatures is restrained. Furthermore, residual austenite phase may be easily increased after solution heat treatment, and toughness may be increased; but on the other hand, strength may be decreased after aging heat treatment. Furthermore, MnO and MnS are formed so that corrosion resistance is decreased. Therefore, the range of Mn is set to be 0.50 to 1.50%. Furthermore, it is desirably set to be 0.70 to 1.20%.

P: Not more than 0.04%

P is segregated at a crystal grain boundary so that solidification crack susceptibility is increased and hot workability is decreased. Therefore, content amount of P is desirably as small as possible, and it is set to be not more than 0.04%.

S: Not more than 0.01%

S is a harmful component since MnS is formed so that corrosion resistance is decreased and since S segregates at a grain boundary so that hot workability is decreased. Therefore, content amount of S is desirably as small as possible, and it is set to be not more than 0.01%.

Ni: 6.0 to 8.0%

Ni is set to be not less than 6.0% since it is an austenite forming element, a constituent element of the G phase, and an important element for precipitation hardening. However, it is set to be not more than 8.0% since a residual austenite phase after solution heat treatment may be easily increased, and strength may be decreased if content amount of Ni is too high.

Cr: 12.0 to 15.0%

Content amount of Cr is set to be not less than 12.0% in order to maintain corrosion resistance of the stainless steel. However, it is set to be not more than 15.0% since it is a ferrite forming element, δ ferrite phase may be easily generated at high temperatures, and hot workability may be decreased.

Mo: 0.50 to 1.50%

Mo is an element effective for increasing corrosion resistance; however, it may promote generation of δ ferrite phase. Therefore, content amount of Mo is set to be 0.50 to 1.50%. Furthermore, it is desirably set to be 0.50 to 1.00%.

Cu: 0.40 to 1.20%

Cu is an element effective for precipitation hardening since a Cu phase is generated by aging heat treatment. However, excess addition may cause deterioration of strength by increasing residual austenite phase and cause generation of cracks by decrease of hot workability. Therefore, content amount of Cu is set to be 0.40 to 1.20%. Furthermore, it is desirably set to be 0.50 to 1.00%.

Ti: 0.20 to 0.50%

Ti is a necessary element for formation of a G phase and is an element effective for increasing strength by precipitation hardening. However, since it may easily form oxides and nitrides and may cause defects, the range of Ti content is set to be 0.20 to 0.50%.

Nb: 0.05 to 0.40%

Nb is a constituent element for a G phase, and it is a very important element. Nb is an effective element since it has actions for controlling the G phase to be Ni₁₆(Ti, Nb)₆Si₇ type and for promoting generation of nucleus. Furthermore, it also has an effect of dispersing Cu phase finely, and ability for precipitation hardening by a Cu phase and a G phase is extremely improved. Furthermore, although it is not limited in particular, it is also effective for fining crystal grains since it has an effect of inhibiting coarsening of crystal grains by forming Nb carbides having sizes of about 0.3 to 1 µm. Therefore, the content amount of Nb is set to be not less than 0.05%. However, it is set to be not more than 0.40% since excessive addition of Nb causes formation of excess NbC, decreasing solid-solved C amount, and decreasing elongation. Furthermore, it is desirably set to be 0.10 to 0.30%.

N: 0.001 to 0.005%

N is an austenite generating element similar to C, and it solid-solves in a martensite phase so as to increase strength. However, Ti and Nb, which are constituent elements of a G phase, contributing to precipitation hardening, may be easily consumed by forming TiN and NbN, and precipitation hardening ability by aging heat treatment is decreased. Therefore, the range of N is set to be 0.001 to 0.02%.

Al: 0.001 to 0.2%

Al is an effective deoxidizing agent element for decreasing amount of O. Furthermore, since Nb is an element which is relatively easily oxidized, by decreasing oxygen concentration by deoxidizing by Al, Nb can be reliably controlled in a range of the present invention. However, in a case in which an excessive amount is contained, generation of δ ferrite phase is promoted and hot workability and toughness are decreased. Therefore, the range of Al is set to be 0.001 to 0.2%.

O: 0.0001 to 0.01%

Since O forms non-metallic inclusions by combining with Si and Ti, which are constituent elements of a G phase, contributing to precipitation hardening, strength after aging heat treatment is decreased. Furthermore, the oxide type inclusions may decrease cleanliness level of steel and cause defects. However, since excess deoxidizing may increase cost, the range of O is set to be from 0.0001 to 0.01%.
The steel of the present invention is a precipitation hardening type martensite stainless steel having superior strength, which is realized by precipitating simultaneously a Cu phase and G phase Ni₁₆X₆Si₇. Distribution conditions of these precipitation hardening phases and size of the precipitation hardening phase itself have a large effect on mechanical properties such as hardness and elongation.
For example, in a case in which precipitations exist more at a crystal grain boundary and less inside a crystal grain, the precipitations may easily grow to be coarse and brittle. On the other hand, strength is increased in a case in which precipitations are distributed uniformly regardless of whether it is inside a crystal grain or at a crystal grain boundary. Therefore, precipitations are uniformly dispersed by solution heat treatment and aging heat treatment under appropriate conditions. The appropriate heating treatment conditions here means, although not limited thereto in particular, performing solution heat treatment at 1000 to 1150 °C for 1 to 5 minutes, and then performing aging heat treatment at 400 to 600 °C for 30 minutes to 10 hours.
It should be noted that there is an optimal value of ratio of Nb in the G phase. That is, in a case in which Nb content is too low, distribution of the G phase becomes uneven and the G phase is distributed more at crystal grain boundaries, so that hardness required in the present invention is not satisfied. On the other hand, in a case in which Nb content is too high, elongation may be less than 2%, which is the lower limit value of the range of the present invention, and the alloy does not elongate sufficiently, so that the alloy cannot be processed. In a case in which content of Nb is set to be x in Ni₁₆(Ti, Nb)₆Si₇, the G phase is described as Ni₁₆(Ti(_1-x), Nb_x)₆Si₇. Therefore, ratio of Nb atom (at%) in the G phase can be described as x/(16+6+7). Although not limited in particular, hardness, and elongation can be controlled within the present invention by setting Nb (at%) in the G phase 0.2 to 3.0. To realize this range, content amount of Nb is set to be 0.05 to 0.40%.
In addition, in the steel of the present invention, by adding Nb, generation of nucleus of the precipitation phase can be promoted, and precipitations can be dispersed uniformly. Therefore, by setting Nb within the range of the present invention, it is possible that not less than 50% of Cu phase and Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compound phase are distributed inside crystal grains.
Furthermore, since sizes of these precipitation hardening phases themselves have great effect on strength, the value is very important in the present invention. Even if rearrangement progresses, strength can be maintained high as long as precipitations can stop the rearrangement.
This action of precipitations as a blockade varies depending on size of precipitation phase, and there is an optimal size of the precipitation phase. In the steel of the present invention, since precipitation phase of a size of 1 nm to 20 nm has maximal action of precipitation phase as a blockade against rearrangement, it is necessary to optimize the size of the precipitation phase. Therefore, by performing solution heat treatment and aging heat treatment under appropriate conditions, and by setting Nb within the range of the present invention, it is possible for the average particle diameter of a Cu phase and Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compound phase to be 1 to 20 nm.
Since strength of precipitation hardening type stainless steel can be increased by performing aging heat treatment, it is used for a steel belt or a press plate. These require strength and fatigue properties, and hardness of HV 400 or more is necessary to increase these properties. On the other hand, since elongation is reduced if it has very high hardness, hardness is set to be not more than HV 600. In addition, since toughness is required, elongation is set to be 2 to 15 % in view of balance with hardness.

Examples

Next, structure, action, and effect of the present invention are explained with reference to Examples; however, the present invention is not limited only to the following Examples.

Table 1 shows chemical compositions, existence of precipitation hardening phase, Nb amount in G phase, ratio of precipitations inside of a grain, Vickers hardness, and elongation of each of sample materials. A bracketed value of a chemical composition is outside the range of the present invention. In addition, Examples 2 and 5 are Reference Examples.

Table 1

Section	Steel No.	Chemical composition mass%														Exis tence of Cu phase	Exis tence of G phase	Evalua tion	Nb in G phase (at%)	Evalua tion	Ratio of precipita tion inside of grain (%)	Evalua tion	Average particle diameter of precipita tion phase (nm)	Evalua tion	Hard ness Hv (10kg Load)	Evalua tion	Elonga tion (%)	Evalua tion	Overall Evalua tion
Section	Steel No.	C	Si	Mn	P	S	\| Ni	Cr	Mo	Cu	Ti	Nb	N	Al	O	Exis tence of Cu phase	Exis tence of G phase	Evalua tion	Nb in G phase (at%)	Evalua tion	Ratio of precipita tion inside of grain (%)	Evalua tion	Average particle diameter of precipita tion phase (nm)	Evalua tion	Hard ness Hv (10kg Load)	Evalua tion	Elonga tion (%)	Evalua tion	Overall Evalua tion
Examples	1	0.042	1.45	0.72	0.003	0.0009	6.14	12.8	1.32	0.62	0.22	0.14	0.005	0.022	0,0044	Y	Y	Superior	0.2	Superior	77	Superior	10.8	Superior	488	Superior	9.6	Superior	A
	2	0.042	1.78	(0.51)	0.003	0.0006	6.92	14.6	0.65	0.83	0.30	0.18	0.003	0.017	0,0052	Y	Y	Superior	0.3	Superior	86	Superior	3.6	Superior	502	Superior	8.6	Superior	A
	3	0.042	1.28	0.99	0.003	0.0008	7.86	12.5	0.74	0.76	0,27	0.06	0.003	0.032	0.0005	Y	Y	Superior	0.2	Superior	66	Superior	9.8	Superior	399	Inferior	10.1	Superior	B
	4	0.043	1.97	1.42	0.007	0.0008	7.52	13.7	1.33	0.44	0,49	0.26	0.005	0.020	0,0050	Y	Y	Superior	0.5	Superior	52	Superior	24.2	Inferior	443	Superior	3.8	Superior	B
	5	0.043	1,83	(0.63)	0.004	0.0008	6.91	14.2	0.92	0.83	0,29	0.39	0.002	0.019	0,0049	Y	Y	Superior	2.8	Superior	49	Inferior	3.3	Superior	537	Superior	1.9	Inferior	B
Compara tve Examples	6	0.044	1.11	1.29	0.003	0.0008	(9.51)	13.8	0.75	(1.62)	0.21	(0.01)	0.003	0.023	0.0047	Y	Y	Superior	0.1	Inferior	20	Inferior	23.2	Inferior	323	Inferior	14.8	Superior	C
	7	0.043	1.92	(0.27)	0.004	0.0008	6.93	13.2	0.85	0.66	(0.03)	0.26	0.002	0.019	0.0041	Y	N	Inferior	-	Inferior	62	Superior	25.6	Inferior	378	Inferior	11.2	Superior	C
	8	0.041	1.73	1.23	0.004	0.0008	6.91	13.8	0.74	(0.05)	0.48	0.39	0.005	0.018	0,0053	N	Y	Inferior	2.1	Superior	78	Superior	15.8	Superior	383	Inferior	17.5	Inferior	C
	9	0.042	1.79	(0.38)	0.004	0.0008	7.98	13.7	0.51	0.82	(0,96)	(0.83)	(0.006)	0.019	0,0043	Y	Y	Superior	8.5	Inferior	40	Inferior	24.5	Superior	620	Inferior	1.6	Inferior	C
	10	0.042	(0.23)	(2.31)	0.004	0.0008	6.94	12.3	0.83	0.75	0.26	(0.03)	(0.006)	0.012	0.0062	Y	Y	Superior	0.1	Inferior	35	Inferior	4.6	Superior	280	Inferior	32.2	Inferior	C

To prepare each of the steels, raw materials were melted in a high-frequency induction furnace, and the melt was casted in a cast-iron mold so as to prepare an ingot of about 20 kg. The ingot was hot-forged at 1000 to 1200 °C so as to obtain a forged plate having a thickness of 12 mm. Then, the forged plate was cold-rolled to obtain cold rolled material having a thickness of 2 mm, and solution heat treatment and aging heat treatment were performed with respect to this. The solution heat treatment was performed to solid-solve precipitations existing in a steel, and martensite transformation may occur by rapid cooling after the heat treatment. With respect to the cold-rolled material above, solution heat treatment was performed at 1050 °C for 2 minutes.
The aging heat treatment is a treatment in which precipitation hardening phase, that is a Cu phase and a G phase in the steel of the present invention, is finely dispersed and precipitated after the solution heat treatment. With respect to the cold-rolled material above, aging heat treatment was performed at 480 °C for 1 hour.
Evaluation of mechanical properties such as a tensile test and a Vickers hardness test, evaluation of structure by an optical microscope and an SEM, and evaluation of nanoscale precipitation hardening phase by TEM and STEM observation of these sample materials were performed.
Existence, distribution, and size of Cu phase and G phase, which are the precipitation hardening phases, were measured by preparing a thin film sample using a focused ion beam (FIB), obtaining an element mapping image by energy dispersive X-ray spectroscopy (EDS) installed in a STEM, and analyzing the image.
Hereinafter, bases for evaluation of each evaluation item shown in Table 1 are explained.

Existence of Cu phase and G phase

In a case in which both a Cu phase and a G phase exist (both "Yes" in Table 1), evaluation was "Superior". In a case in which one of the phases does not exist (one "No" in Table 1), evaluation was "Inferior". As a basis for deciding whether each phase exists or not, observing a freely selected view, a case in which precipitations are not less than 0.001 (pieces/nm²) was regarded as the phase of "existing".

Nb (at%) in G phase

Nb (at%) in the G phase Ni₁₆(Ti, Nb)₆Si₇ can be described as x/(16+6+7), in a case of Ni₁₆(Ti(_1-x), Nb_x)₆Si₇. This Nb amount in a G phase during aging heat treatment was calculated by using a thermodynamic calculating software (trade name: Thermo-Calc). Furthermore, Nb amount in a G phase obtained by this thermodynamic calculation matches well with the result of STEM-EDS analysis. In view of mechanical properties such as hardness and elongation, Nb (at%) in G phase being 0.2 to 3.0 was evaluated as "Superior".

Ratio of precipitations inside of grains (%)

Ratio of precipitations inside of grains being not less than 50% was evaluated as "Superior".

Average particle diameter of precipitation phase (nm)

Average particle diameter of a precipitation phase being 1 to 20 nm was evaluated as "Superior".

Hardness Hv (10 kg load)

In the Vickers hardness test, heat treatment was performed on the above cold-rolled material with thickness of 2 mm, the rolled surface was polished by #800, five points were measured with a 10 kg load with respect to the surface, and average value thereof was calculated. Hardness being 400 to 600 Hv was evaluated as "Superior".

Elongation (%)

In the tensile test, heat treatment was performed on the above cold-rolled material of thickness of 2 mm, a flat type tensile test piece, defined by JIS (Japanese Industrial Standards) No. 13B, in which tensile direction matches rolled direction, is cut out, and measurement was performed. From the measurement results, elongation being 2 to 15% was evaluated as "Superior".

Overall evaluation

In view of the above results, overall evaluation was "A" in a case in which all of the evaluations were "Superior", overall evaluation was "B" in a case in which one or two instances of "Inferior" were included, and overall evaluation was "C" in a case in which not less than three instances of "Inferior" were included. The overall evaluation was "A" or "B" in Examples; in contrast, the overall evaluation was "C" in Comparative Examples.
In Comparative Example 6, the Nb amount was low, and the ratio of precipitations inside of grains was low. Furthermore, residual γ amount could easily be greater since Ni amount and Cu amount were high, and hardness was low.
Comparative Example 7 is out of the range of the present invention and hardness was low since Ti amount was low and no G phase existed.
In Comparative Example 8, although a G phase existed, no Cu phase existed, and hardness and elongation were low since the Cu amount was low.
In Comparative Example 9, although a Cu phase and a G phase existed, elongation was low since Ti and Nb amounts were high.
In Comparative Example 10, residual γ amount was extremely high, similar to the case of Comparative Example 6, since the Mn amount was high. Furthermore, Nb in the G phase was low since Nb amount was low, and hardness was low and elongation was high since ratio of precipitations inside of grains was low.
As explained so far, mechanical properties of both of, or of one of, hardness and elongation, were inferior in Comparative Examples compared to in Examples.

Claims

A precipitation hardening martensite stainless steel comprising:
in mass%, indicated by "%", C: 0.01 to 0.05%, Si: 1.0 to 2.0%, Mn: 0.70 to 1.50%, P: not more than 0.04%, S: not more than 0.01%, Ni: 6.0 to 8.0%, Cr: 12.0 to 15.0%, Mo: 0.50 to 1.50%, Cu: 0.40 to 1.20%, Ti: 0.20 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.005%, Al: 0.001 to 0.2%, O: 0.0001 to 0.01%, with inevitable impurities and Fe being the remainder,

wherein Cu phase and Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compounds phase are distributed, and Nb in the intermetallic compounds phase is 0.2 to 3.0 (at%).
The precipitation hardening martensite stainless steel according to claim 1, wherein in a case in which distribution of the Cu phase and the Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compounds phase are calculated by observing and evaluating precipitation hardening phase at the nanoscale by using image analysis of an element mapping image obtained by EDS by an energy dispersive X-ray analyzer installed in a scanning transmission electron microscope (STEM) with respect to a thin layer sample prepared by a focused ion beam (FIB), not less than 50 number% is distributed in crystal grains.
The precipitation hardening martensite stainless steel according to claim 1, wherein average particle diameter of the Cu phase and the Ni₁₆(Ti, Nb)₆Si₇ type intermetallic compounds phase is 1 to 20 nm.
The precipitation hardening martensite stainless steel according to claim 1, wherein elongation is 2 to 15%.
The precipitation hardening martensite stainless steel according to claim 1, wherein hardness is 400 to 600 Hv.