JP2022006584A - Stainless steel powder, stainless steel member and method for manufacturing stainless steel member - Google Patents

Abstract

To provide stainless steel powder, a stainless steel member and a method for manufacturing a stainless steel member.SOLUTION: Stainless steel powder comprises, by mass%, 0.001-0.06% C, 0.01-1.0% Si, 0.01-2.0% Mn, 0.05% or less P, less than 0.005% S, more than 15.0 to 19.0% Cr, 2.5-6.0% Ni, 0.005-0.5% V, 0.1% or less Al, 0.100% or less N, and 0.3% or less O, further contains one or more selected from 3.5% or less Mo, 3.5% or less Cu, 3.0% or less W and 0.5% or less Nb, and the balance including Fe with inevitable impurities, and has a particle diameter D50 of 10-200 μm and apparent density of 3.5-5.0 Mg/m3.SELECTED DRAWING: None

Description

The present invention relates to stainless steel powder suitable for manufacturing accessories such as parts and accessories of 17Cr-based high-strength stainless steel used in oil wells of crude oil or natural gas, gas wells (hereinafter, simply referred to as oil wells) and the like. The present invention also relates to a stainless steel member molded using this stainless steel powder and a method for manufacturing the stainless steel member.

In recent years, from the viewpoint of the depletion of energy resources expected in the near future, deep oil fields that have not been omitted in the past, and severe corrosive environments under so-called sour environment containing carbon dioxide gas and hydrogen sulfide, etc. Development is being actively carried out in oil fields and gas fields. In such oil and gas fields, the depth is generally extremely deep, the atmosphere is _high , and the environment is severely corroded containing CO ₂ , Cl ⁻ , and H2 S. Steel pipes for oil wells used in such an environment are required to have high strength and high corrosion resistance. Furthermore, since development is active in cold regions and deep seas, low temperature toughness is also required.

In addition to oil country tubular goods, special members of various shapes, commonly known as accessories, are used in the production of oil and natural gas. Therefore, accessories are also required to have high strength, and to further improve corrosion resistance and low temperature toughness, as in the case of steel pipes for oil wells.

Recently, the development of oil wells in a high-temperature corrosive environment up to 200 ° C. has been promoted, and 17Cr-based stainless steel is used in such an environment. The accessory (member) is usually made of a round bar-shaped steel material or a thick pipe (steel pipe), and is made into a desired shape by machining this material. As the material of the accessory, a round bar-shaped material (billet) or a stainless steel pipe can be used. For example, there are Patent Documents 1 to 3 as a 17Cr-based stainless steel pipe.

In Patent Document 1, mass% is C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less. , S: 0.005% or less, Cr: 15.5 to 18%, Ni: 1.5 to 5%, Mo: 1 to 3.5%, V: 0.02 to 0.2%, N: 0 It contains 0.01 to 0.15%, O: 0.006% or less, Cr, Ni, Mo, Cu, C satisfy a specific relational expression, and Cr, Mo, Si, C, Mn, Ni, It has a composition in which Cu and N are contained so as to satisfy a specific relational expression, and further, the martensite phase is used as a base phase, the ferrite phase is 10 to 60% by volume, or the austenite phase is 30% by volume. A high-strength stainless steel pipe for an oil well, which has a structure containing the following and has excellent corrosion resistance, is described. As a result, it exhibits sufficient corrosion resistance even in a severe corrosion environment at high temperatures up to 230 ° C containing CO ₂ and ^Cl- , and has high strength exceeding yield strength: 654 MPa (95 ksi) and high toughness stainless steel for oil wells. It is said that steel pipes can be manufactured stably.

Patent Document 2 describes a stainless steel pipe for an oil well. In the technique described in Patent Document 2, in mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01 to 0.5%, P: 0.04% or less, S. : 0.01% or less, Cr: 16.0 to 18.0%, Ni: 4.0 to 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3. It contains 0%, Al: 0.001 to 0.10%, N: 0.050% or less, Cr, Cu, Ni and Mo satisfy a specific relationship, and further, (C + N), Mn, Ni, It contains a composition in which Cu and (Cr + Mo) satisfy a specific relationship, a martensite phase and a ferrite phase having a volume ratio of 10 to 40%, has a length of 50 μm in the thickness direction from the surface, and has a pitch of 10 μm. Stainless steel for oil wells, which has a plurality of virtual line segments arranged in a row in a range of 200 μm and a structure in which the ratio of ferrite phases intersecting is more than 85%, and has a 0.2% resistance: 758 MPa or more. It is a steel pipe. As a result, it is said that the stainless steel pipe for oil wells has corrosion resistance in a high temperature environment of 150 to 250 ° C. and has improved sulfide stress corrosion cracking resistance at room temperature.

Patent Document 3 describes a stainless steel pipe for an oil well. In the technique described in Patent Document 3, in mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S. : Less than 0.002%, Cr: 16-18%, Mo: 1.8-3%, Cu: 1.0-3.5%, Ni: 3.0-5.5%, Co: 0.01 It contains ~ 1.0%, Al: 0.001 to 0.1%, O: 0.05% or less, N: 0.05% or less, and Cr, Ni, Mo and Cu satisfy a specific relationship. The composition is preferably a stainless steel tube having a structure containing a ferrite phase of 10% or more and less than 60% by volume, a retained austenite phase of 10% or less, and a martensite phase of 40% or more. As a result, it is said that a stainless steel pipe for oil wells, which has a high yield strength of 758 MPa or more and stable high-temperature corrosion resistance, can be obtained.

Japanese Unexamined Patent Publication No. 2005-336595 International Publication No. 2010/134498 International Publication No. 2013/146406

However, in the above-mentioned techniques shown in Patent Documents 1 to 3, a sufficient rolling reduction cannot be secured in the process of forming a pipe by hot rolling, so that the structure becomes coarse, and as a result, low temperature toughness and sulfide stress crack resistance. There was a problem that it could not be improved sufficiently. Therefore, it is considered that the accessories used for the stainless steel pipes obtained by the techniques of Patent Documents 1 to 3 have not sufficiently improved the low temperature toughness and the sulfide stress cracking resistance as well as the steel pipes.

Therefore, the present invention solves such a problem of the prior art, and particularly includes hydrogen sulfide ₍ H 2S) and a high temperature severe corrosive environment containing carbon dioxide gas (CO ₂ ) and chloride ion (Cl ⁻ ). It is an object of the present invention to provide a stainless steel powder which has high strength, excellent low temperature toughness, and excellent corrosion resistance and is suitable for modeling in an environment or the like. Another object of the present invention is to provide a stainless steel member and a method for manufacturing a stainless steel member, which have high strength, excellent low temperature toughness, and excellent corrosion resistance under the above-mentioned environment.

The "high strength" in the present invention means a case where the yield strength is 758 MPa or more.

“Excellent low temperature toughness” means a V-notch test piece (10 mm thick) so that the longitudinal direction of the test piece is perpendicular to the molding direction and the notch is parallel to the molding direction in accordance with the provisions of JIS Z 2242. ) Is collected and a Charpy impact test is carried out, and the test temperature in the Charpy impact test: -10 ° C., the absorbed energy vE- ₁₀ is 40 J or more.

Further, "excellent corrosion resistance" refers to a case where it has "excellent carbon dioxide gas corrosion resistance", "excellent sulfide stress corrosion cracking resistance" and "excellent sulfide stress cracking resistance".

The above-mentioned "excellent carbon dioxide corrosion resistance" means that the test piece is immersed in a test solution held in an autoclave: a 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., CO ₂ gas atmosphere at 30 atm). When the corrosion rate is 0.125 mm / y or less when the immersion time is 336 hours, and the test piece after the corrosion test is pitted on the surface of the test piece using a loupe with a magnification of 10 times. The presence or absence of corrosion is observed, and the case where there is no pitting corrosion with a diameter of 0.2 mm or more is used.

Further, the above-mentioned "excellent sulfide stress corrosion cracking property" means that the test solution held in the autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 100 ° C., 30 atm CO ₂ gas, 0.1 atm). The test piece was immersed in an aqueous solution adjusted to pH: 3.3 by adding acetic acid + sodium acetate to the _H2S atmosphere), the immersion time was set to 720 hours, and 100% of the breakdown stress was loaded as the load stress. This is the case where the test piece after the test does not crack.

Further, the above-mentioned "excellent sulfide stress cracking resistance" means that the test solution held in the autoclave: 20% mass NaCl aqueous solution (liquid temperature: 25 ° C., 0.9 atm CO ₂ gas, 0.1). The test piece was immersed in an aqueous solution adjusted to pH: 3.5 by adding acetic acid + sodium acetate to the _H2S atmosphere of atmospheric pressure), the immersion time was set to 720 hours, and 90% of the yield stress was used as the load stress. It is a case where the test piece is loaded and the test piece is not cracked after the test.

In order to achieve the above-mentioned object, the present inventors have diligently studied various factors affecting the strength, corrosion resistance and low temperature toughness of the component composition in 17Cr-based stainless steel.

In the present invention, as a method for manufacturing the above-mentioned special members having various shapes, attention is paid to a laminated molding method, that is, a method for manufacturing a three-dimensional shaped object using a so-called 3D printer (3D printing method). Then, various factors affecting the strength, corrosion resistance and low temperature toughness of the component composition in the accessory (member) formed by the laminated molding method were repeatedly examined.

Specifically, the ingot produced in the melting-casting process is used as a master ingot, and then the master ingot is redissolved-stainless steel powder is produced in the gas atomizing process, and the stainless steel powder is used to produce so-called 3D. A modeled object was produced by printing, and the above-mentioned various factors were examined. As a result, in order to satisfy the desired strength, corrosion resistance and low temperature toughness, the components of the stainless steel powder, the particle size (median diameter of the cumulative mass distribution (mass standard)) _D50 and the apparent density should be within the desired range. I found that I needed to.

Further, by subjecting the obtained modeled product to an appropriate heat treatment, a steel structure necessary for satisfying the desired strength, corrosion resistance and low temperature toughness can be obtained. As a result, it has been found that the desired high strength, excellent low temperature toughness, and excellent corrosion resistance (carbon dioxide gas corrosion resistance, sulfide stress corrosion cracking resistance, sulfide stress cracking resistance) can be achieved in any shape. ..

The present invention has been completed with further studies based on the above findings. The gist of the present invention is as follows.
[1] By mass%,
C: 0.001 to 0.06%, Si: 0.01 to 1.0%,
Mn: 0.01-2.0%, P: 0.05% or less,
S: Less than 0.005%, Cr: 15.0% or more and 19.0% or less,
Ni: 2.5-6.0%, V: 0.005-0.5%,
Al: 0.1% or less, N: 0.100% or less,
O: Contains 0.3% or less, and further
Contains one or more selected from Mo: 3.5% or less, Cu: 3.5% or less, W: 3.0% or less, Nb: 0.5% or less.
The balance has a component composition consisting of Fe and unavoidable impurities,
The particle size D ₅₀ is 10 to 200 μm,
A stainless steel powder characterized by an apparent density of 3.5 to 5.0 Mg / m ³ .
[2] In addition to the above-mentioned composition, by mass%,
Ti: 0 to 0.30%, B: 0 to 0.0050%,
Zr: 0 to 0.2%, Co: 0 to 1.0%,
Ta: 0 to 0.1%, Ca: 0 to 0.0050%,
REM: 0 to 0.01%, Mg: 0 to 0.01%,
Sn: 0 to 0.5%, Sb: 0 to 0.5%
The stainless steel powder according to [1], which contains one or more selected from the above.
[3] By mass%,
C: 0.001 to 0.06%, Si: 0.01 to 1.0%,
Mn: 0.01-2.0%, P: 0.05% or less,
S: Less than 0.005%, Cr: 15.0% or more and 19.0% or less,
Ni: 2.5-6.0%, V: 0.005-0.5%,
Al: 0.1% or less, N: 0.100% or less,
O: Contains 0.3% or less, and further
Contains one or more selected from Mo: 3.5% or less, Cu: 3.5% or less, W: 3.0% or less, Nb: 0.5% or less.
The balance has a component composition consisting of Fe and unavoidable impurities,
It has a steel structure consisting of a tempered martensite phase of 45% or more, a ferrite phase of 0 to 40%, and a residual austenite phase of 25% or less by volume.
A stainless steel member having a yield strength of 758 MPa or more and an absorbed energy vE- ₁₀ at a test temperature of −10 ° C. in a Charpy impact test of 40 J or more.
[4] In addition to the above-mentioned composition, by mass%,
Ti: 0 to 0.30%, B: 0 to 0.0050%,
Zr: 0 to 0.2%, Co: 0 to 1.0%,
Ta: 0 to 0.1%, Ca: 0 to 0.0050%,
REM: 0 to 0.01%, Mg: 0 to 0.01%,
Sn: 0 to 0.5%, Sb: 0 to 0.5%
The stainless steel member according to [3], which contains one or more selected from the above.
[5] A modeling step of forming a model using the stainless steel powder according to [1] or [2], and
A quenching process in which the model formed in the molding step is heated to a temperature of 850 to 1150 ° C. and then cooled to a temperature of 50 ° C. or lower at an average cooling rate equal to or higher than air cooling, and a quenching process at 500 to 650 ° C. A heat treatment process that performs a tempering process that heats to the return temperature,
A method for manufacturing a stainless steel member.
[6] The method for manufacturing a stainless steel member according to [5], wherein the modeling step is a step of forming a modeled object by using a laminated modeling method.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a stainless steel powder that is suitably used for manufacturing a stainless steel member, particularly for manufacturing by laminated molding by a laminated molding method.

Further, the stainless steel member manufactured by using the stainless steel powder of the present invention has high strength and excellent low temperature toughness, and has a high temperature of 200 ° C. or higher and a severe corrosive environment containing CO ₂ and Cl ⁻ . It has excellent corrosion resistance even underneath and in an environment containing _H2S .

Hereinafter, the present invention will be described in detail.

The stainless steel powder of the present invention has a component composition described below, and has a particle size D ₅₀ and an apparent density controlled in an appropriate range. Further, the stainless steel member of the present invention is formed from this stainless steel powder, and has the same composition as the stainless steel powder described below and a steel structure controlled within an appropriate range.

First, the component composition of the stainless steel powder and the stainless steel member of the present invention and the reason for the limitation will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as "%".

C: 0.001 to 0.06%
C is an important element that increases the strength of martensitic stainless steel. In the present invention, it is necessary to contain 0.001% or more of C in order to secure the desired high strength. On the other hand, if C is contained in excess of 0.06%, the corrosion resistance is lowered. Therefore, the C content is set to 0.001 to 0.06%. Preferably, the C content is 0.015% or more. Preferably, the C content is 0.04% or less.

Si: 0.01-1.0%
Si is an element that acts as a deoxidizing agent, and in order to obtain such an effect, it is necessary to contain 0.01% or more of Si. On the other hand, if Si is contained in excess of 1.0%, the low temperature toughness is lowered. Therefore, the Si content is set to 0.01 to 1.0%. Preferably, the Si content is 0.1% or more. Preferably, the Si content is 0.6% or less.

Mn: 0.01-2.0%
Mn is an element that increases the strength of martensitic stainless steel, and the content of Mn of 0.01% or more is required to secure the desired strength. On the other hand, if Mn is contained in excess of 2.0%, the low temperature toughness is lowered. Therefore, the Mn content is set to 0.01 to 2.0%. Preferably, the Mn content is 0.5% or less.

P: 0.05% or less P is an element that reduces corrosion resistance such as carbon dioxide gas corrosion resistance and sulfide stress cracking resistance, and is preferably reduced as much as possible in the present invention, but it may be 0.05% or less. Is acceptable. Therefore, the P content is set to 0.05% or less. Preferably, the P content is 0.02% or less. Preferably, the P content is 0.005% or more.

S: Less than 0.005% S is an element that lowers corrosion resistance, and it is preferable to reduce it as much as possible, but if it is less than 0.005%, it is acceptable. Therefore, the S content is set to less than 0.005%. Preferably, the S content is 0.001% or less. Preferably, the S content is 0.0004% or more.

Cr: 15.0% or more and 19.0% or less Cr is an element that forms a protective film on the surface of the steel pipe and contributes to the improvement of corrosion resistance. When the Cr content is 15.0% or less, the desired corrosion resistance is ensured. Can't. Therefore, the content of Cr of more than 15.0% is required. On the other hand, if the content of Cr exceeds 19.0%, the fraction of the ferrite phase becomes too high, and the desired strength cannot be secured. Therefore, the Cr content is set to be more than 15.0% and 19.0% or less. Preferably, the Cr content is 16.0% or more. Preferably, the Cr content is 18.0% or less.

Ni: 2.5-6.0%
Ni is an element that strengthens the protective film on the surface of steel pipes and contributes to improving corrosion resistance. In addition, Ni reduces the ferrite fraction and increases the amount of tempered martensite to improve the low temperature toughness value. Such an effect becomes remarkable when the content of Ni is 2.5% or more. On the other hand, if the content of Ni exceeds 6.0%, the stability of the martensite phase is lowered and the strength is lowered. Therefore, the Ni content is set to 2.5 to 6.0%. Preferably, the Ni content is 3.5% or more. Preferably, the Ni content is 4.5% or less.

V: 0.005 to 0.5%
V is an element that contributes to the improvement of strength as a solid solution, and also combines with C and N to precipitate as V carbonitride (V precipitate), which contributes to the improvement of yield strength. In order to obtain such an effect, the content of V of 0.005% or more is required. On the other hand, the content of V exceeding 0.5% causes a decrease in low temperature toughness and sulfide stress cracking resistance. Therefore, the V content is set to 0.005 to 0.5%. Preferably, the V content is 0.01% or more. More preferably, the V content is 0.02% or more. Preferably, the V content is 0.1% or less.

Al: 0.1% or less Al is an element that acts as a deoxidizing agent. On the other hand, if Al is contained in excess of 0.1%, the amount of oxide increases, the cleanliness decreases, and the low temperature toughness decreases. Therefore, the Al content is set to 0.1% or less. The Al content is preferably 0% or more. Preferably, the Al content is 0.01% or more. More preferably, the Al content is 0.02% or more. Preferably, the Al content is 0.07% or less.

N: 0.100% or less N is an element that improves pitting corrosion resistance. On the other hand, if N is contained in excess of 0.100%, a nitride is formed and the low temperature toughness is lowered. Therefore, the N content is set to 0.100% or less. The N content is preferably 0% or more. Preferably, the N content is 0.02% or more. Preferably, the N content is 0.06% or less.

O: 0.3% or less O (oxygen) exists as an oxide in steel and therefore adversely affects various properties. Therefore, in the present invention, it is desirable to reduce the amount as much as possible. In particular, when O exceeds 0.3%, corrosion resistance and low temperature toughness deteriorate. Therefore, the O content is set to 0.3% or less. Preferably, the O content is 0.2% or less. Preferably, the O content is 0.01% or more.

One or more selected from Mo: 3.5% or less, Cu: 3.5% or less, W: 3.0% or less, Nb: 0.5% or less Mo: 3.5% or less Mo is It is an element that stabilizes the protective film on the surface of steel pipes, increases resistance to pitting corrosion due to Cl ⁻ and low pH, and thereby enhances sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. On the other hand, if the Mo content exceeds 3.5%, the ferrite fraction is increased and the tempered martensite fraction is lowered, which leads to a decrease in low temperature toughness and sulfide stress corrosion cracking resistance. In addition, Mo is an expensive element, which leads to an increase in material cost. Therefore, the Mo content is set to 3.5% or less. Preferably, the Mo content is 0% or more. More preferably, the Mo content is 2.0% or more. Preferably, the Mo content is less than 3.0%. More preferably, the Mo content is 2.7% or less.

Cu: 3.5% or less Cu increases the retained austenite phase and forms precipitates to contribute to the improvement of yield strength (YS), so that high strength can be obtained without lowering the low temperature toughness. can. Therefore, it is a very important element in the present invention. It also has the effect of strengthening the protective film on the surface of the steel pipe, suppressing hydrogen intrusion into the steel, and enhancing sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. On the other hand, if the content of Cu exceeds 3.5%, coarse Cu precipitation is caused and the effect is saturated. Therefore, the Cu content is set to 3.5% or less. Preferably, the Cu content is 0% or more. More preferably, the Cu content is 0.5% or more. More preferably, the Cu content is 1.0% or more. Preferably, the Cu content is 3.0% or less.

W: 3.0% or less W is important because it contributes to improving the strength of steel and stabilizes the protective film on the surface of steel pipes to enhance sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. Element. When W is contained in combination with Mo, the sulfide stress cracking resistance is remarkably improved. On the other hand, the content of W exceeding 3.0% increases the ferrite fraction and lowers the tempered martensite fraction, thereby lowering the low temperature toughness. Therefore, the W content is set to 3.0% or less. Preferably, the W content is greater than 0%. More preferably, the W content is 0.5% or more. More preferably, the W content is 0.8% or more. Preferably, the W content is 2.0% or less.

Nb: 0.5% or less Nb binds to C and N and precipitates as Nb carbonitride (Nb precipitate), which contributes to the improvement of yield strength. On the other hand, the content of Nb exceeding 0.5% causes a decrease in low temperature toughness and sulfide stress cracking resistance. Therefore, the Nb content is set to 0.5% or less. Preferably, the Nb content is 0% or more. More preferably, the Nb content is 0.05% or more. Preferably, the Nb content is 0.2% or less.

The rest other than the above components are Fe and unavoidable impurities.

In the present invention, the above-mentioned components are used as the basic component composition. With the above-mentioned basic composition of ingredients, the properties desired by the present invention can be obtained. In the present invention, the following components may be further added as selective elements, if necessary, for the purpose of further improving the strength and corrosion resistance.

Ti: 0 to 0.30%, B: 0 to 0.0050%, Zr: 0 to 0.2%, Co: 0 to 1.0%, Ta: 0 to 0.1%, Ca: 0 to 0 One or two selected from 0050%, REM: 0 to 0.01%, Mg: 0 to 0.01%, Sn: 0 to 0.5%, Sb: 0 to 0.5%. As described above, Ti, B, Zr, Co and Ta are all elements that increase the strength, and if necessary, one or more of these elements can be selected and contained. Ti, B, Zr, Co and Ta also have the effect of improving the sulfide stress cracking resistance in addition to the above-mentioned effects. In particular, Ta is an element that has the same effect as Nb, and a part of Nb can be replaced with Ta. Since Ti, B, Zr, Co and Ta are selective elements, they do not have to be contained, and the content of each is preferably 0% or more. However, when it is contained to obtain the above-mentioned effects, Ti: 0.01% or more, B: 0.0001% or more, Zr: 0.01% or more, Co: 0.01% or more and Ta. : It is preferable to contain 0.01% or more, respectively. On the other hand, if Ti: 0.30%, B: 0.0050%, Zr: 0.2%, Co: 1.0% and Ta: 0.1% are contained in excess of each, the low temperature toughness is lowered. Therefore, when these elements are contained, Ti: 0.30% or less, B: 0.0050% or less, Zr: 0.2% or less, Co: 1.0% or less, and Ta: 0, respectively. It is preferable to limit it to 1% or less.

Both Ca and REM (rare earth metals) are elements that contribute to the improvement of sulfide stress corrosion cracking resistance through morphological control of sulfides. Since Ca and REM are selective elements, they do not have to be contained, and the content of each is preferably 0% or more. However, when it is contained in order to obtain the above-mentioned effects, it is preferable to contain Ca: 0.0001% or more and REM: 0.001% or more, respectively. On the other hand, even if Ca: 0.0050% and REM: 0.01% are contained in excess of each, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when these elements are contained, it is preferable to limit Ca: 0.0050% or less and REM: 0.01% or less, respectively.

Mg, Sn and Sb are all elements that improve corrosion resistance. Since Mg, Sn and Sb are selective elements, they do not have to be contained, and the content of each is preferably 0% or more. However, when it is contained in order to obtain the above-mentioned effects, it is preferable to contain Mg: 0.002% or more, Sn: 0.01% or more, and Sb: 0.01% or more, respectively. On the other hand, even if Mg: 0.01%, Sn: 0.5% and Sb: 0.5% are contained in excess of each, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when these elements are contained, it is preferable to limit them to Mg: 0.01% or less, Sn: 0.5% or less, and Sb: 0.5% or less, respectively.

Next, the particle size of the stainless steel powder of the present invention will be described.

The stainless steel powder of the present invention has a particle size defined by _D50 of 10 to 200 μm. If the particle size D ₅₀ is excessively fine, uneven filling of the powder occurs due to a decrease in the fluidity of the powder, which causes defects such as voids to be generated during laminated molding. As a result, strength, low temperature toughness, and corrosion resistance are reduced. This appears when the particle size D ₅₀ is less than 10 μm. Therefore, the particle size D ₅₀ is set to 10 μm or more. The particle size D ₅₀ is preferably 20 μm or more, more preferably 30 μm or more. On the other hand, if the particle size D ₅₀ is excessively coarse, it causes defects during laminated molding. As a result, the strength, low temperature toughness, and corrosion resistance due to defects are reduced. This appears when the particle size D ₅₀ exceeds 200 μm. Therefore, the particle size D ₅₀ is set to 200 μm or less. The particle size D ₅₀ is preferably 150 μm or less, more preferably 100 μm or less.

The D ₅₀ indicating the particle size of the stainless steel powder refers to the median diameter of the cumulative mass distribution of the powder. A laser diffraction particle size measuring device can be used for measuring the median diameter. In the present invention, the particle size _D50 of the stainless steel powder is measured by the method described below.

Examples of the laser diffraction particle size measuring device include LA-950V2 manufactured by HORIBA, Ltd. Of course, other devices may be used, but in order to perform accurate measurement, it is preferable to use one having a lower limit of the measurable particle size range of 0.1 μm or less and an upper limit of 45 μm or more. In the above apparatus, a laser beam is irradiated to a solvent in which iron powder is dispersed, and the particle size distribution and the average particle size of the iron powder are measured from the diffraction and scattering intensity of the laser beam. As the solvent for dispersing the iron powder, it is preferable to use ethanol, which has good particle dispersibility and is easy to handle. It is not preferable to use a solvent having a high van der Waals force and low dispersibility, such as water, because the particles aggregate during the measurement and the measurement result is coarser than the original average particle size. Therefore, it is preferable to carry out a dispersion treatment by ultrasonic waves on the ethanol solution containing the iron powder before the measurement.

Since the appropriate dispersion treatment time differs depending on the target powder, the above dispersion treatment time is carried out in 7 steps at intervals of 10 min between 0 and 60 min, and the average particle size of the iron powder is measured after each dispersion treatment. conduct. During each measurement, the measurement is performed while stirring the solvent in order to prevent particle aggregation. Then, among the particle diameters obtained in the seven measurements performed by changing the dispersion treatment time at intervals of 10 min, the smallest value is used as the average particle diameter (particle size D ₅₀ ) of the iron powder.

The stainless steel powder of the present invention in which the particle size D ₅₀ is controlled in this way further increases the apparent density. The above characteristics can be obtained by controlling both the particle size D ₅₀ and the apparent density. The apparent density shall be 3.5 Mg / m ³ or more. If the apparent density is less than 3.5 Mg / m ³ , the decrease in fluidity causes defects during laminated molding. Preferably, the apparent density is 4.0 Mg / m ³ or more. The apparent density shall be 5.0 Mg / m ³ or less. This is because when the apparent density exceeds 5.0 Mg / m ³ , it becomes difficult to stably control the apparent density from an industrial point of view.

For the apparent density, the value measured by the test method specified in JIS Z 2504 shall be used.

Here, as will be described later, the structure of the stainless steel powder of the present invention is a quenching solidification structure from a temperature near the liquid phase, so that the ferrite phase tends to be rich. Due to this, in the molded product laminated and molded using the stainless steel powder of the present invention, the steel structure immediately after molding may not satisfy 40% or less, which is the specified range of the ferrite phase fraction described later. .. However, depending on the application, the model made of this alloy powder is heat-treated (heat-treated) after molding to form a single phase having a tempered martensite phase as the main phase, or ferrite in addition to the tempered martensite phase. When a two-phase or three-phase alloy product (steel member) having one or two types of phase and retained austenite phase is obtained and the steel structure described later is satisfied, the steel structure immediately after molding is the volume of the ferrite phase. Even if the rate is 40% or more, it is acceptable.

Examples of the application destination of the alloy powder having a rich ferrite phase include stainless steel powder for modeling, and for example, it can be provided as stainless steel powder for overlaying, 3D printer, sintering and the like. In particular, it is suitable as an alloy powder for 3D printers.

Next, the steel structure of the stainless steel member of the present invention and the reason for its limitation will be described.

The stainless steel member of the present invention is formed from the above-mentioned stainless steel powder, has the above-mentioned composition, and has a tempered martensite phase of 45% or more in volume ratio and 0 to 40% ferrite. It has a steel structure consisting of a phase and a retained austenite phase of 25% or less.

The stainless steel member of the present invention has a tempered martensite phase as a main phase in order to secure the strength (yield strength) intended in the present invention. Here, the "main phase" refers to a structure that occupies 45% or more of the volume ratio with respect to the steel member. If the tempered martensite phase is less than 45%, the desired strength cannot be obtained. Therefore, the tempered martensite phase should be 45% or more. The tempered martensite phase is preferably 55% or more. From the viewpoint of ensuring the desired low temperature toughness, the tempered martensite phase is preferably 70% or less, more preferably 65% or less.

The rest other than the main phase is a ferrite phase, or a ferrite phase and a retained austenite phase.

In order to secure the strength and sulfide stress cracking resistance desired in the present invention, the ferrite phase has a volume fraction of 40% or less. Therefore, the ferrite phase is set to 0 to 40% by volume. The ferrite phase is preferably 5% or more, preferably 35% or less.

Since the stainless steel member of the present invention can obtain the above-mentioned effect even as a tempered martensite phase single phase, the austenite phase (residual austenite phase) may be 0%. However, it is preferable to precipitate an austenite phase (residual austenite phase) in addition to the ferrite phase. The presence of the retained austenite phase can improve ductility and cold toughness. In order to obtain such an effect of improving ductility and low temperature toughness, it is preferable that the retained austenite phase is precipitated by a volume fraction of more than 10%. On the other hand, precipitation of a large amount of retained austenite phase exceeding 25% by volume causes a decrease in strength. Therefore, the retained austenite phase is preferably 25% or less by volume. More preferably, the retained austenite is more than 10% by volume and 20% or less.

Here, the above-mentioned steel structure of the stainless steel member of the present invention can be measured by the following method. First, a test piece for microstructure observation is collected from a cross section of a material (steel member) molded by a laminated molding method (Additive Manufacturing method) or the like in a direction orthogonal to the molding direction. The tissue observation test piece was corroded with a bilera reagent (a reagent in which picrinic acid, hydrochloric acid and ethanol were mixed at a ratio of 2 g, 10 ml and 100 ml, respectively), and the tissue was imaged with a scanning electron microscope (magnification: 1000 times). Using an image analyzer, the microstructure fraction (% of area ratio) of the ferrite phase is calculated, and this area fraction is defined as the volume fraction (%) of the ferrite phase.

Then, the X-ray diffraction test piece is ground and polished so that the cross section orthogonal to the modeling direction is the measurement surface, and the amount of retained austenite (γ) is measured by using the X-ray diffraction method. The amount of retained austenite is converted into a volume fraction by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α and using the following equation.
γ (volume fraction) = 100 / (1+ (IαRγ / IγRα))
Here, the integrated intensity of Iα: α, the crystallographic theoretically calculated value of Rα: α, the integrated intensity of Iγ: γ, and the crystallographic theoretically calculated value of Rγ: γ.

The microstructure fraction (volume fraction%) of the tempered martensite phase is the balance other than the ferrite phase and the retained austenite phase.

The steel structure of the stainless steel member of the present invention can be adjusted within the range of each of the above-mentioned phases by appropriately controlling the heat treatment steps (quenching treatment and tempering treatment) described later.

As described above, the stainless steel member of the present invention has the above-mentioned specific composition, and the steel structure has a tempered martensite phase of 45% or more and a ferrite phase of 0 to 40% by volume. And by adjusting to consist of a retained austenite phase of 25% or less, the strength and properties (corrosion resistance, low temperature toughness) desired in the present invention can be obtained.

Next, a preferred embodiment of the method for producing stainless steel powder of the present invention will be described.

The stainless steel powder of the present invention is provided as a final material form in the following series of manufacturing steps. For example, the stainless steel powder of the present invention is produced through each step of melting-ingot formation-remelting of the master ingot-preparation of a powder by an atomizing process.

First, in the process of melting-ingot formation, a predetermined amount of the above-mentioned elements is melted as a material in a high-frequency vacuum melting furnace, alloyed, and cast to produce an ingot (master ingot). At this time, it is preferable to dissolve under the conditions of an Ar atmosphere under reduced pressure and a dissolution temperature of 1600 ° C. or higher. The reason for this condition is as follows. If the melting temperature is too low, the molten steel solidifies and the nozzle is closed when the molten steel is dropped from the nozzle. Further, from the viewpoint of preventing oxidation of molten steel, it is desirable to melt in an Ar atmosphere under reduced pressure. The melting furnace used in this step is not limited to the high-frequency vacuum melting furnace, and other melting furnaces (for example, a direct energization heating type melting furnace) may be used in the present invention.

Then, in the process of redissolving the master ingot-atomization process, the cast master ingot is used as a material and redissolved in a melting furnace such as a high-frequency or induction furnace, and the oxygen is low by the gas atomizing method using Ar or He of the inert gas. Obtain an amount of powder.

Then, these powders are classified into a particle size of 10 to 200 μm and provided as the stainless steel powder of the present invention. The classification may be performed using a sieve, or may be performed using another method such as air flow classification. Further, instead of the gas atomizing method, the water atomizing method may be used.

The apparent density is controlled by appropriately adjusting the gas pressure, gas flow rate, gas temperature, and focusing angle in gas atomization.

Next, an embodiment of the method for manufacturing a stainless steel member of the present invention will be described.

The method for manufacturing a stainless steel member of the present invention includes a molding step and a heat treatment step.

First, in the modeling step, a stainless steel laminated model (three-dimensional structure) is modeled using the above-mentioned stainless steel powder, for example, by a layered manufacturing method (metal powder layered manufacturing method).

As the laminated molding method, for example, a 3D printer method can be used. Here, a laser-type Powered fusion 3D printer is used. In particular, the setting conditions for the 3D printer are not specified. From the viewpoint of preventing excessive melting and insufficient melting, for example, the laser output is preferably 150 to 300 W, and the scan speed is preferably 700 to 1100 mm / s.

Then, in the heat treatment step, the three-dimensional structure after molding is subjected to quenching treatment and tempering treatment under predetermined conditions to obtain the stainless steel member of the present invention. In this heat treatment step, after heating to a temperature in the range of 850 to 1150 ° C., a quenching treatment is performed in which the surface temperature is cooled to a temperature of 50 ° C. or lower at an average cooling rate equal to or higher than air cooling. Then, a tempering process is performed in which the temperature is in the range of 500 to 650 ° C. (tempering temperature).

Here, the "average cooling rate of air cooling or higher" is 0.01 ° C./s or higher. In the case of "average cooling rate of water cooling or higher", it is 0.2 ° C./s or higher.

[Quenching]
The heating temperature (temperature of reheating) of the quenching treatment is 850 to 1150 ° C. from the viewpoint of miniaturizing the martensite phase. It is more preferably 900 ° C. or higher, and more preferably 1050 ° C. or lower.

From the viewpoint of soaking property, it is preferable to keep the temperature at the above-mentioned reheating temperature for 10 minutes or more. The holding time at the reheating temperature is preferably 60 minutes or less.

Further, the cooling rate of the quenching treatment is set to be equal to or higher than that of air cooling from the viewpoint of ensuring the desired low temperature toughness. The average cooling rate is preferably 0.01 ° C./s or higher, and preferably 200 ° C./s or lower. Here, the average cooling rate is the average cooling rate from the start of cooling to the stop of cooling. The cooling stop temperature is set to a temperature at which the surface temperature of the three-dimensional structure is 50 ° C. or lower. Above 50 ° C., retained austenite is excessively precipitated, and the desired high strength cannot be obtained.

[Tempering]
When the temperature of the tempering treatment (tempering temperature) exceeds 650 ° C., the austenite phase is excessively precipitated and the desired high strength cannot be obtained. On the other hand, when the tempering temperature is less than 500 ° C., the strength becomes excessively high and the desired low temperature toughness cannot be obtained. Therefore, the tempering temperature is set to 500 to 650 ° C.

From the viewpoint of soaking property, it is preferable to keep the tempering temperature at the above-mentioned tempering temperature for 10 minutes or more. The holding time at the tempering temperature is preferably 60 minutes or less.

Further, the cooling rate of the tempering process is set to be equal to or higher than that of air cooling from the viewpoint of ensuring the desired low temperature toughness. The average cooling rate is preferably 0.01 ° C./s or higher, and preferably 200 ° C./s or lower.

In the method for manufacturing a stainless steel member of the present invention, in order to obtain a desired shape, the three-dimensional structure may be further machined before or after the heat treatment step described above.

As described above, the stainless steel member of the present invention formed from the stainless steel powder of the present invention has high strength, excellent corrosion resistance and low temperature toughness, and therefore is used in a highly corrosive environment such as an oil well. It can be suitably used as a constituent material (for example, a sliding portion) of equipment such as a machine or a pump. It can also be used as an accessory for oil wells. Further, according to the present invention, it is possible to reduce the manufacturing cost of special members having various shapes, and it is also possible to improve the dimensional accuracy.

Hereinafter, the present invention will be described based on examples. The present invention is not limited to the following examples.

The details of the alloy manufacturing method shown in Table 1 will be described. First, a predetermined amount of the component composition shown in Table 1 was melted in a high-frequency vacuum melting furnace (Ar atmosphere under reduced pressure, melting temperature of 1600 ° C. or higher) and cast to produce a master ingot of these alloys. Then, the master ingot of the alloy was redissolved in an Ar atmosphere and pulverized by a gas atomizing method to obtain a powder alloy. Then, each alloy powder (stainless steel powder) having a particle size D ₅₀ different depending on the classification was obtained. The particle size D ₅₀ and the apparent density of the stainless steel powder were measured by the above-mentioned methods, and the values are shown in Table 2.

Then, using a commercially available 3D printer (laser type Powder bed fusion method), a three-dimensional structure having a size of 50 mm × 80 mm × 12 mmt (t: laminated thickness) was produced from these powder alloys. At this time, the laser output was 250 W, the scan speed was 900 mm / s, and the layered thickness of one layer was 40 μm. Then, the produced three-dimensional structure was subjected to a heat treatment step under the conditions shown in Table 2 to obtain a stainless steel member.

Test pieces were collected from the obtained stainless steel members by the methods shown below, and microstructure observation, tensile test, Charpy impact test and corrosion resistance test were carried out. The test method was as follows.

(1) Structure observation From the obtained stainless steel member, a test piece for structure observation was collected so that the cross section in the direction perpendicular to the modeling direction (perpendicular to the laminated surface) was the observation surface. The obtained tissue observation test piece was corroded with a bilera reagent (a reagent in which picrinic acid, hydrochloric acid and ethanol were mixed at a ratio of 2 g, 10 ml and 100 ml, respectively), and the tissue was imaged with a scanning electron microscope (magnification: 1000 times). Then, the microstructure fraction (area ratio) of the ferrite phase was calculated using an image analysis device, and this was taken as the volume fraction (%) of the ferrite phase.

Further, a test piece for X-ray diffraction was collected from the obtained stainless steel member, and ground and polished so that the cross section in the direction orthogonal to the modeling direction became the measurement surface. Then, the amount of retained austenite (γ) was measured by using an X-ray diffraction method. The amount of retained austenite was converted into a volume fraction by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α and using the following equation.
γ (volume fraction) = 100 / (1+ (IαRγ / IγRα))
Here, the integrated intensity of Iα: α, the crystallographic theoretically calculated value of Rα: α, the integrated intensity of Iγ: γ, and the crystallographic theoretically calculated value of Rγ: γ.

Then, the structural fraction (% by volume) of the tempered martensite phase was determined as the balance other than the ferrite phase and the retained austenite phase.

(2) Tensile test From the obtained stainless steel member, a test piece of JIS13B half (GL = 25 mm) was collected so that the direction perpendicular to the modeling direction was the tensile direction. Then, a tensile test was carried out in accordance with JIS standard (Z2241: 2011), and tensile properties (yield strength YS, tensile strength TS) were obtained. Here, those having a yield strength of 758 MPa or more were evaluated as high strength and were accepted. On the other hand, those with a yield strength of less than 758 MPa were rejected.

(3) Charpy impact test A V-notch test piece (10 mm thick) was collected from the obtained stainless steel member so that the longitudinal direction of the test piece was perpendicular to the modeling direction, and conformed to the regulations of JIS Z 2242 (2018). Then, a Charpy impact test was carried out. The test temperature was −10 ° C., the absorbed energy vE-10 at ₋₁₀ ° C. was determined, and the low temperature toughness was evaluated. The number of test pieces was three, and the arithmetic mean of the obtained values was taken as the absorbed energy (J) of the stainless steel member. Here, those having an absorption energy vE-10 at ₋₁₀ ° C. of 40 J or more were evaluated as having high toughness and were accepted. On the other hand, those with vE- ₁₀ less than 40J were rejected.

(4) Corrosion resistance test As a corrosion resistance test, a corrosion test, a sulfide stress cracking resistance test (SSC resistance test), and a sulfide stress corrosion cracking resistance test (SCC resistance test) were performed.

[Corrosion test]
From the obtained stainless steel member, a corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was produced by machining, and a corrosion test was carried out to evaluate the carbon dioxide gas corrosion resistance.

In the corrosion test, the above corrosion test piece was immersed in a test solution held in an autoclave: a 20% by mass NaCl aqueous solution (liquid temperature: 200 ° C., CO ₂ gas atmosphere at 30 atm), and the immersion period was 14 days. It was carried out as (336 hours). The weight of the test piece after the corrosion test was measured, and the corrosion rate was calculated from the weight loss before and after the corrosion test. Those having a corrosion rate of 0.125 mm / y or less were rejected, and those having a corrosion rate of more than 0.125 mm / y were rejected.

In addition, the presence or absence of pitting corrosion on the surface of the test piece was observed using a loupe with a magnification of 10 times for the test piece after the corrosion test. In addition, the presence of pitting corrosion means the case where there is pitting corrosion having a diameter of 0.2 mm or more. Here, those without pitting corrosion were regarded as acceptable, and those with pitting corrosion were rejected.

In this embodiment, those having a corrosion rate of 0.125 mm / y or less and no pitting corrosion are evaluated as having excellent carbon dioxide corrosion resistance.

[SSC resistance test]
From the obtained stainless steel member, a round bar-shaped test piece (diameter: 6.4 mmφ) was machined so that the longitudinal direction of the test piece was perpendicular to the modeling direction, and NACE (National Association of Corrosion and Engineerings) TM0177. A sulfide stress cracking resistance test (SSC (Sulfide Stress Cracking) test) was carried out in accordance with Method A.

In the SSC resistance test, acetic acid + acetic acid was added to the test solution held in the autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 25 ° C., _H2S : 0.1 atm, CO ₂ : 0.9 atm atmosphere). Using an aqueous solution adjusted to pH: 3.5 by adding Na, the test piece was immersed in this aqueous solution, the immersion time was set to 720 hours, and 90% of the yield stress was loaded as a load stress. Then, the presence or absence of cracks was observed in the test piece after the test. Here, those without cracks were regarded as acceptable, and those with cracks were regarded as rejected.

[SCC resistance test]
From the obtained stainless steel member, a 4-point bending test piece having a thickness of 3 mm, a width of 10 mm, and a length of 65 mm was collected by machining, and sulfide stress corrosion resistance resistance corrosion resistance was obtained in accordance with EFC (European Federation of Corrosion) 17. A cracking test (SCC (Sulfide Stress Corrosion Cracking) test) was carried out.

In the SCC resistance test, acetic acid + sodium acetate was added to a test solution held in an autoclave: a 20% by mass NaCl aqueous solution (liquid temperature: 100 ° C., _H2S : 0.1 atm, CO ₂ : 30 atm atmosphere). In addition, using an aqueous solution adjusted to pH: 3.3, the test piece was immersed in this aqueous solution, the immersion time was set to 720 hours, and 100% of the yield stress was loaded as a load stress. The presence or absence of cracks was observed in the test piece after the test. Here, those without cracks were regarded as acceptable, and those with cracks were regarded as rejected.

The obtained results are shown in Table 3.

All of the examples of the present invention were stainless steel powders in which the above-mentioned composition, particle size _D50 and apparent density were controlled within appropriate ranges. The stainless steel member formed from this steel powder has high strength and excellent low-temperature toughness in a low-temperature environment, and also in a high-temperature severe corrosion environment containing CO ₂ , ^Cl- and an environment containing H _2S . It had excellent corrosion resistance underneath.

On the other hand, in the comparative example outside the scope of the present invention, the stainless steel powder was not controlled within an appropriate range. The stainless steel member formed from this steel powder does not have at least one of yield strength, corrosion resistance, and low temperature toughness, which is the characteristic value desired in the present invention.

Claims (6)

By mass%,
C: 0.001 to 0.06%, Si: 0.01 to 1.0%,
Mn: 0.01-2.0%, P: 0.05% or less,
S: Less than 0.005%, Cr: 15.0% or more and 19.0% or less,
Ni: 2.5-6.0%, V: 0.005-0.5%,
Al: 0.1% or less, N: 0.100% or less,
O: Contains 0.3% or less, and further
Contains one or more selected from Mo: 3.5% or less, Cu: 3.5% or less, W: 3.0% or less, Nb: 0.5% or less.
The balance has a component composition consisting of Fe and unavoidable impurities,
The particle size D ₅₀ is 10 to 200 μm,
A stainless steel powder characterized by an apparent density of 3.5 to 5.0 Mg / m ³ . In addition to the composition of the above components, by mass%,
Ti: 0 to 0.30%, B: 0 to 0.0050%,
Zr: 0 to 0.2%, Co: 0 to 1.0%,
Ta: 0 to 0.1%, Ca: 0 to 0.0050%,
REM: 0 to 0.01%, Mg: 0 to 0.01%,
Sn: 0 to 0.5%, Sb: 0 to 0.5%
The stainless steel powder according to claim 1, wherein the stainless steel powder is contained in one or more selected from the above. By mass%,
C: 0.001 to 0.06%, Si: 0.01 to 1.0%,
Mn: 0.01-2.0%, P: 0.05% or less,
S: Less than 0.005%, Cr: 15.0% or more and 19.0% or less,
Ni: 2.5-6.0%, V: 0.005-0.5%,
Al: 0.1% or less, N: 0.100% or less,
O: Contains 0.3% or less, and further
Contains one or more selected from Mo: 3.5% or less, Cu: 3.5% or less, W: 3.0% or less, Nb: 0.5% or less.
The balance has a component composition consisting of Fe and unavoidable impurities,
It has a steel structure consisting of a tempered martensite phase of 45% or more, a ferrite phase of 0 to 40%, and a residual austenite phase of 25% or less by volume.
A stainless steel member having a yield strength of 758 MPa or more and an absorbed energy vE- ₁₀ at a test temperature of −10 ° C. in a Charpy impact test of 40 J or more. In addition to the composition of the above components, by mass%,
Ti: 0 to 0.30%, B: 0 to 0.0050%,
Zr: 0 to 0.2%, Co: 0 to 1.0%,
Ta: 0 to 0.1%, Ca: 0 to 0.0050%,
REM: 0 to 0.01%, Mg: 0 to 0.01%,
Sn: 0 to 0.5%, Sb: 0 to 0.5%
The stainless steel member according to claim 3, wherein the stainless steel member contains one or more selected from the above. A modeling step of forming a model using the stainless steel powder according to claim 1 or 2.
A quenching process in which the model formed in the molding step is heated to a temperature of 850 to 1150 ° C. and then cooled to a temperature of 50 ° C. or lower at an average cooling rate equal to or higher than air cooling, and a quenching process at 500 to 650 ° C. A heat treatment process that performs a tempering process that heats to the return temperature,
A method for manufacturing a stainless steel member. The method for manufacturing a stainless steel member according to claim 5, wherein the modeling step is a step of forming a modeled object by using a laminated modeling method.