WO2017168806A1

WO2017168806A1 - Chromium-based two-phase alloy product and production method therefor

Info

Publication number: WO2017168806A1
Application number: PCT/JP2016/081573
Authority: WO
Inventors: 雅史能島; 友則木村; 近藤　保夫; 青野　泰久
Original assignee: 株式会社日立製作所
Priority date: 2016-03-30
Filing date: 2016-10-25
Publication date: 2017-10-05
Also published as: JP2019131833A

Abstract

A Cr-based two-phase alloy product according to the present invention is a product in which a Cr-based two-phase alloy in which two phases including a ferrite phase and an austenite phase are mixed is used, the Cr-based two-phase alloy product being characterized in that: the chemical composition of the Cr-based two-phase alloy includes main components, accessory components, and impurities; the main components comprise 34-70 mass% Cr, 17-45 mass% Ni, and 10-35 mass% Fe; the mass content of said Cr is the highest; the total content of said Ni and said Fe is 30-65 mass%; the accessory components comprise 0.1-2 mass% Mn and 0.1-1 mass% Si; the impurities contain up to 0.04 mass% P, up to 0.01 mass% S, up to 0.03 mass% C, up to 0.02 mass% N, and up to 0.03 mass% O; the product has, as a microstructure, a forged structure, and grains of the austenite phase are dispersedly precipitated in the crystal grains of the ferrite phase; the occupancy of the ferrite phase is 15-85%; and in a room-temperature tensile test, the 0.2% proof stress is at least 600 MPa, and the breaking elongation is at least 5%.

Description

Chromium-based two-phase alloy product and method for producing the same

The present invention relates to a technology for a high corrosion resistance and high strength alloy, and particularly to a product using a chromium-based two-phase alloy in which two phases of an austenite phase and a ferrite phase are mixed, and a method for manufacturing the same.

In the past, carbon steel and corrosion inhibitors (inhibitors) were commonly used as materials for oil well equipment used for drilling crude oil and natural gas. In recent years, due to changes in the drilling environment accompanying the progress of deepening in oil well drilling, higher corrosion resistance and mechanical properties (for example, hardness) have been required for oil well equipment materials, and excellent in corrosion resistance. Steel (alloy steel) has come to be used. For example, the addition of chromium (Cr) remarkably improves the corrosion resistance of iron (Fe). For oil wells containing metal corrosion components, martensitic stainless steel containing 13% by mass of Cr (for example, SUS420) has been widely used. It was.

However, in an environment containing chloride and acid gas (for example, carbon dioxide gas or hydrogen sulfide), SUS420 has a weak point that it easily causes stress corrosion cracking (SCC). For this reason, when drilling oil wells in such a severe corrosive environment, expensive nickel (Ni) -based alloys (for example, alloys containing 40% by mass or more of Ni) are often used in the past, and material costs (and therefore drilling costs) ) Would rise significantly.

On the other hand, there are Cr-based alloys as corrosion-resistant and heat-resistant alloys that are cheaper than Ni-based alloys, and various Cr-based alloys have been proposed. For example, Patent Document 1 (Japanese Patent Laid-Open No. 04-301048) has a chemical composition comprising Cr: 65 to 80%, Co: 10 to 15%, the balance Fe and impurities, and optionally containing N: 0.1 to 1.5%. A Cr-Fe heat-resistant alloy is disclosed, and Patent Document 2 (Japanese Patent Laid-Open No. 04-301049) discloses a heat-resistant alloy having a chemical composition consisting of Cr: 70 to 95%, N: 0.1 to 1.5%, the balance Fe and an impurity. It is disclosed. According to Patent Documents 1 and 2, it has excellent compression deformation resistance, oxidation resistance, etc. in a high-temperature atmosphere furnace, improved durability as a heated steel material support surface member, reduced maintenance, and associated furnace operation efficiency. It is said to contribute greatly to improvement.

Patent Document 3 (Japanese Patent Application Laid-Open No. 08-291355) contains, in mass%, Cr: more than 95%, N: 0.1-2.0%, the balance of one or more of Fe, Ni and Co and unavoidable impurities. Thus, a Cr-based heat-resistant alloy further containing 0.3% or more in total of one or more of Ti, Al, Zr, Nb, B, and V as desired is disclosed. According to Patent Document 3, it is said that a Cr-based heat-resistant alloy excellent in high-temperature strength used for a member that requires strength, ductility, and corrosion resistance at an ultra-high temperature (for example, a heated steel material support member in a heating furnace) can be provided. ing.

Patent Document 4 (Japanese Patent Laid-Open No. 07-258801) discloses that Cr: 15-50%, Ni: 6.1-50%, O + P + S: 200 ppm or less, the balance being Fe and unavoidable impurities, crystal grain size number: 8 As described above, an Fe—Cr—Ni alloy excellent in corrosion resistance of a processed portion, characterized by C + N: 400 to 1200 ppm, if desired, is disclosed. According to Patent Document 4, it is said that an Fe—Cr—Ni alloy can be provided which improves the corrosion resistance without reducing the workability and does not decrease the corrosion resistance even if processed.

Japanese Patent Laid-Open No. 04-301048 JP 04-301049 A Japanese Patent Application Laid-Open No. 08-291355 Japanese Patent Application Laid-Open No. 07-258801

High Cr-based alloys (alloys with a high Cr content) as described in Patent Documents 1 to 3 are intended for use in a high temperature environment of 1300 ° C. or higher, and are excellent even in the high temperature environment. Corrosion resistance and mechanical properties. However, such a high Cr-based alloy exhibits brittleness (insufficient toughness) in the temperature range of the oil well environment (room temperature to about 350 ° C.), and is therefore not considered suitable as an oil well equipment material.

The Fe—Cr—Ni alloy described in Patent Document 4 is intended for austenitic stainless steel, but austenitic stainless steel is stress-corrosion caused by hydrogen embrittlement in a high temperature and high pressure environment containing chloride. It is known that cracking (SCC) is likely to occur, and it is considered that it is not suitable as an oil well equipment material, as is the case with high Cr-based alloys.

As described above, with the progress of deepening in oil well drilling, there is a strong demand for a metal material that has a corrosion resistance and mechanical properties that are as high as or better than conventional ones and that is less expensive than a Ni-based alloy. In addition to the hardness and mechanical strength, it is very important to secure ductility and toughness from the viewpoint of durability as mechanical properties of oil well equipment materials.

Therefore, an object of the present invention is a metal material that can be suitably used even in a temperature range and highly corrosive environment such as an oil well, and has high corrosion resistance equal to or higher than conventional ones and good mechanical properties and low cost. An object of the present invention is to provide a product using a Cr-based two-phase alloy and a method for producing the product.

(I) One aspect of the present invention is a product using a Cr-based two-phase alloy in which two phases of a ferrite phase and an austenite phase are mixed,
The chemical composition of the Cr-based two-phase alloy consists of a main component, subcomponents, impurities and optional subcomponents,
The main component consists of 34 mass% to 70 mass% Cr, 17 mass% to 45 mass% Ni (nickel), and 10 mass% to 35 mass% Fe (iron), The mass content of Cr is the largest, and the total content of Ni and Fe is 30% by mass to 65% by mass,
The subcomponent consists of 0.1% by mass or more and 2% by mass or less of Mn (manganese) and 0.1% by mass or more and 1% by mass or less of Si (silicon),
The impurities include P (phosphorus) of more than 0% by mass and 0.04% by mass, S (sulfur) of more than 0% by mass and 0.01% by mass, C (carbon) of more than 0% by mass and 0.03% by mass, Including N (nitrogen) of more than 0.02% by mass and O (oxygen) of more than 0% by mass and 0.03% by mass,
The product has a forged structure as a fine structure and the austenite phase grains are dispersed and precipitated in the ferrite phase crystal grains, and the ferrite phase occupation ratio is 15% or more and 85% or less,
The present invention provides a Cr-based two-phase alloy product characterized by a 0.2% proof stress by a room temperature tensile test of 600 MPa or more and a breaking elongation of 5% or more.
In the present invention, the optional subcomponent means a component that may or may not be added.

The present invention can add the following improvements and changes to the Cr-based two-phase alloy product (I) according to the present invention.
(I) The optional subcomponent comprises at least one of V (vanadium), Nb (niobium), Ta (tantalum), and Ti (titanium),
When the Cr-based two-phase alloy contains the optional subcomponent, the total atomic content of the V, Nb, Ta and Ti is 0.8 to 2 times the total atomic content of the C, N and O It is a range.
(Ii) The austenite phase grains dispersed and precipitated (austenite phase precipitated grains) have a needle-like and / or scale-like shape.
(Iii) The austenite phase layer is interposed between adjacent crystal grains of the ferrite phase.
(Iv) The product is a rotating machine shaft or bearing.

(II) Another aspect of the present invention is a method for producing the above-described Cr-based two-phase alloy product,
A raw material mixing and melting step in which the raw material of the Cr-based two-phase alloy is mixed and melted to form a molten metal and then solidified to form a raw material alloy lump,
A remelting step of remelting the raw material alloy lump to prepare a purified molten metal;
A casting step of casting the cleaned molten metal to form an ingot;
A ferrite rate adjusting heat treatment step of forming a ferrite rate adjusting ingot by adjusting the occupation ratio of the ferrite phase by performing a heat treatment at a temperature of 1050 ° C. or higher and 1300 ° C. or lower with respect to the ingot,
A hot forging molding step of forming a compact by hot forging within a temperature range of 900 ° C. or higher and 1300 ° C. or lower with respect to the ferrite ratio adjusting ingot,
A solution heat treatment step of performing a solution treatment at a temperature of 1050 ° C. or higher and 1250 ° C. or lower on the molded body;
The present invention provides a method for producing a Cr-based two-phase alloy product, comprising an aging heat treatment step of performing an aging treatment at a temperature of 800 ° C. or higher and 1000 ° C. or lower on the solution-treated compact. .

According to the present invention, as a metal material that can be suitably used even in a temperature range and highly corrosive environment such as an oil well, the metal material has high corrosion resistance equal to or higher than that of conventional ones and good mechanical properties, and is low in cost. A product using a Cr-based two-phase alloy and a method for producing the product can be provided.

3 is an optical micrograph showing an example of a metal structure of a Cr-based two-phase alloy product according to the present invention. It is process drawing which shows an example of the manufacturing method of the Cr-based two-phase alloy product which concerns on this invention.

The inventors of the present invention used a chemical composition, a metal, and a metal composition in a product using a Cr—Ni—Fe alloy containing Cr, Ni and Fe as main components, particularly a Cr—Ni—Fe alloy containing 34 mass% or more of Cr. The present invention was completed by intensive investigations and investigations on the relationship between the morphology, mechanical properties, and corrosion resistance.

Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. However, the present invention is not limited to the embodiments described here, and can be appropriately combined and improved without departing from the technical idea of the present invention.

(Metal structure of the Cr-based two-phase alloy product of the present invention)
First, the metal structure (microstructure) of the Cr-based two-phase alloy product according to the present invention will be described.

The alloy of the present invention is a Cr—Ni—Fe alloy containing Cr, Ni and Fe as main components. The metal structure of an alloy containing Fe as a main component is usually a ferrite structure having a body-centered cubic lattice crystal structure (also referred to as a ferrite phase or α phase) and an austenite structure having a face-centered cubic lattice crystal structure (austenite phase). , Also referred to as a γ phase), and a martensite structure having a distorted body-centered cubic lattice crystal structure (also referred to as a martensite phase or an α ′ phase).

Generally, the ferrite phase has excellent corrosion resistance (for example, SCC resistance) and high mechanical strength (for example, 0.2% proof stress), but it is said that the ductility and toughness are relatively low compared to the austenite phase. ing. The austenite phase has relatively high ductility and toughness compared to the ferrite phase, but is considered to have relatively low mechanical strength. Moreover, although high corrosion resistance is shown in a normal environment, when the corrosive environment becomes severe, it is said that SCC resistance will fall rapidly. The martensite phase has high mechanical strength (for example, hardness), but is considered to have relatively low corrosion resistance.

On the other hand, the two-phase alloy according to the present invention is an alloy in which two phases of an austenite phase and a ferrite phase are mixed as a metal structure (fine structure). Two-phase alloys are characterized by combining the advantages of an austenite phase (excellent ductility and toughness) with the advantages of a ferrite phase (high mechanical strength and excellent corrosion resistance including SCC resistance). Further, since Cr, which is cheaper than Ni, is used as a main component, there is an advantage that material costs can be reduced as compared with a Ni-based alloy having Ni as the maximum component.

The two-phase alloy of the present invention has a ferrite phase occupancy (hereinafter sometimes simply referred to as “ferrite ratio”) of 15% or more and 85% or less, and the balance (ie, 85% or less and 15% or more) is austenite. A phase is preferred. The phase occupancy in the present invention is defined as the content (unit:%) of the phase when backscattered electron diffraction image (EBSP) analysis is performed on the polished surface of the alloy bulk sample.

When the ferrite ratio exceeds 85%, the ductility of the two-phase alloy is too low, and it is difficult to ensure the expected durability. On the other hand, when the ferrite ratio is less than 15%, it is difficult to ensure expected mechanical strength (for example, 0.2% proof stress, tensile strength). The ferrite ratio is more preferably 20% or more and 70% or less, and further preferably 25% or more and 60% or less.

In addition, the Cr-based two-phase alloy product of the present invention preferably has a metal structure (for example, a forged structure) having a small crystal grain size from the viewpoint of mechanical properties and corrosion resistance. In other words, the product of the present invention is preferably formed by forging. By having a metal structure having a small crystal grain size, better mechanical properties and corrosion resistance can be obtained than a cast solidified structure having coarse crystal grains. Specifically, the average crystal grain size is preferably 30 μm or less, and more preferably 20 μm or less.

The average crystal grain size in the present invention was binarized after reading an optical microscope observation image or an electron microscope observation image of the polished surface of the alloy bulk sample with image analysis software (NIH Image, public domain software), and then binarizing. It is defined as an average value calculated from the minor axis and major axis of the crystal grains. Further, it may be a metal structure subjected to solution heat treatment after forging, or may be a metal structure subjected to aging heat treatment after solution heat treatment. The average crystal grain size in the present invention does not consider the austenite phase precipitated grains dispersed and precipitated in the ferrite phase crystal grains described later and the austenite phase layer interposed between the ferrite phase crystal grains.

FIG. 1 is an optical micrograph showing an example of a metal structure of a Cr-based two-phase alloy product according to the present invention. As shown in FIG. 1, it is confirmed that the Cr-based two-phase alloy according to the present invention has a metal structure in which a dark ferrite phase P1 and a light austenite phase P2 are dispersed and mixed with each other. Since hot forging is performed, a structure (so-called forged structure) in which a cast solidified structure (for example, a dendritic crystal peculiar to the cast solidified structure) is destroyed and an equiaxed crystal grain is seen at least partially. It is confirmed that the average crystal grain size is 30 μm or less.

Further, when the crystal grains of the ferrite phase P1 are observed in detail, the austenite phase P2 grains are dispersed and precipitated in each crystal grain, and the precipitated grains have a needle-like and / or scale-like shape. Is confirmed. Further, it is confirmed that an austenite phase P2 layer is interposed between adjacent crystal grains of the ferrite phase P1.

(Chemical composition of Cr-based two-phase alloy of the present invention)
As described above, the two-phase alloy according to the present invention is a Cr—Ni—Fe-based alloy containing Cr, Ni, and Fe as main components, contains at least Mn and Si as subcomponents, and contains impurities. More preferably, it further optionally contains at least one of V, Nb, Ta and Ti. Hereinafter, the composition (each component) of the two-phase alloy according to the present invention will be described.

Cr: 34% by mass or more and 70% by mass or less Cr component is one of the main components of this Cr-based two-phase alloy. It forms a high-strength ferrite phase and improves the corrosion resistance by forming a solid solution in the austenite phase. It is a contributing component. The Cr component content is preferably 34% by mass to 70% by mass, more preferably 34% by mass to 65% by mass, and still more preferably 40% by mass to 60% by mass. When the Cr content is less than 34% by mass, the mechanical strength of the Cr-based two-phase alloy decreases. On the other hand, when the Cr content exceeds 70% by mass, the ductility and toughness of the Cr-based two-phase alloy deteriorates. Further, from the viewpoint of corrosion resistance and material cost, it is preferable that the Cr content is the maximum content among the three main components (Cr, Ni, Fe).

Ni: 17% by mass or more and 45% by mass or less Ni component is one of the main components of this Cr-based two-phase alloy, and stabilizes the austenite phase and contributes to maintaining the two-phase state of the alloy (for example, ferrite It is a component that imparts ductility and toughness to a Cr-based two-phase alloy, as well as maintaining a two-phase state even when a rate adjusting heat treatment or solution treatment is performed. The content of the Ni component is preferably 17% by mass or more and 45% by mass or less, and more preferably 20% by mass or more and 40% by mass or less. When the Ni content is less than 17% by mass, the ductility and toughness of the Cr-based two-phase alloy deteriorates. On the other hand, when the Ni content exceeds 45% by mass, the mechanical strength of the Cr-based two-phase alloy decreases.

Fe: 10% by mass or more and 35% by mass or less The Fe component is also one of the main components of the present Cr-based two-phase alloy and is a basic component for ensuring mechanical strength. The content of the Fe component is preferably 10% by mass to 35% by mass, and more preferably 10% by mass to 32% by mass. When the Fe content is less than 10% by mass, the ductility and toughness of the Cr-based two-phase alloy is lowered. On the other hand, when the Fe content exceeds 35% by mass, the σ phase of the intermetallic compound is likely to be generated in the temperature range near 800 ° C., and the ductility and toughness of the Cr-based two-phase alloy are significantly reduced (so-called σ phase). Embrittlement). In other words, by controlling the Fe content in the range of 10 to 35% by mass, the formation of σ phase is suppressed and the deterioration of ductility and toughness is suppressed while ensuring the mechanical strength of the Cr-based two-phase alloy. be able to.

Ni + Fe: 30% by mass to 65% by mass The total content of Ni component and Fe component is preferably 30% by mass to 65% by mass, more preferably 40% by mass to 62% by mass, and more preferably 45% by mass to 55% by mass. A mass% or less is more preferable. When the total content is less than 30% by mass, the ductility and toughness of the Cr-based two-phase alloy becomes insufficient. On the other hand, when the total content exceeds 65% by mass, the mechanical strength is greatly reduced.

Mn: 0.1% by mass or more and 2% by mass or less Mn component plays a role of desulfurization and deoxidation in this Cr-based two-phase alloy, and contributes to improvement of mechanical strength and toughness and carbon dioxide corrosion resistance It is. The content of the Mn component is preferably 0.1% by mass or more and 2% by mass or less, and more preferably 0.3% by mass or more and 1.8% by mass or less. When the Mn content is less than 0.1% by mass, the effect of the Mn component cannot be sufficiently obtained. On the other hand, when the Mn content exceeds 2 mass%, coarse particles of sulfide (for example, MnS) are formed, which causes deterioration of corrosion resistance and mechanical strength.

Si: 0.1 mass% or more and 1 mass% or less Si component plays a role of deoxidation in the present Cr-based two-phase alloy and is a secondary component contributing to improvement of mechanical strength and toughness. The content of the Si component is preferably 0.1% by mass or more and 1% by mass or less, and more preferably 0.3% by mass or more and 0.8% by mass or less. When the Si content is less than 0.1% by mass, the effect of the Si component cannot be sufficiently obtained. On the other hand, when the Si content exceeds 1% by mass, coarse particles of oxide (for example, SiO ₂ ) are formed, which causes a decrease in toughness.

Impurities Examples of impurities in the two-phase alloy include P, S, C, N, and O. Hereinafter, these impurities will be described.

P: more than 0% by mass and 0.04% by mass or less The P component is an impurity component that easily segregates at the crystal grain boundaries of the two-phase alloy and lowers the toughness of the alloy and the corrosion resistance of the grain boundaries. By controlling the content of the P component to 0.04% by mass or less, it is possible to suppress those negative effects. The P content is more preferably 0.03% by mass or less.

S: more than 0% by mass and 0.01% by mass or less S component is easy to form a relatively low melting point sulfide (for example, Fe sulfide) by combining with the components of this two-phase alloy, and the toughness and pore resistance of the alloy It is an impurity component that lowers food habits. By controlling the content of the S component to 0.01% by mass or less, the negative influence can be suppressed. The S content is more preferably 0.003% by mass or less.

C: More than 0% by mass and 0.03% by mass or less C component has the effect of hardening the alloy by solid solution, but combines with the components of this two-phase alloy to produce carbide (for example, Cr carbide). It is also an impurity component that easily precipitates at grain boundaries and lowers the corrosion resistance and toughness of the alloy. By controlling the content of the C component to 0.03% by mass or less, it is possible to suppress those negative effects. The C content is more preferably 0.02% by mass or less.

N: more than 0% by mass and 0.02% by mass or less The N component has the effect of improving mechanical properties (for example, hardness) by being dissolved in the present Cr-based two-phase alloy. The content of the N component is preferably more than 0% by mass and 0.02% by mass or less, more preferably more than 0% by mass and 0.015% by mass or less. The effect cannot be obtained unless the N component is added. When the N content exceeds 0.02% by mass, it combines with the constituent components of the Cr-based two-phase alloy to form and precipitate nitrides (for example, Cr nitride), and the ductility and toughness of the Cr-based two-phase alloy are reduced. descend.

O: more than 0% by mass and 0.03% by mass or less O component is an impurity component that easily forms and precipitates an oxide (for example, Fe oxide) by combining with the constituent components of this two-phase alloy and lowers the toughness of the alloy. is there. By controlling the content of the O component to 0.03% by mass or less, the negative influence can be suppressed. The O content is more preferably 0.02% by mass or less.

Optional Subcomponent The two-phase alloy preferably further includes at least one of V, Nb, Ta and Ti as an optional subcomponent. Hereinafter, these optional subcomponents will be described. As described above, the optional subcomponent means a component that may or may not be added.

The V component, the Nb component, the Ta component, and the Ti component are components that play a role of decarburization, denitrification, and deoxidation in the two-phase alloy, respectively. By forming a compound with impurity components of C, N, and O, and assembling and stabilizing the impurity components, the toughness of the alloy can be improved (decrease in toughness can be suppressed).

Also, the addition of a small amount of the V component has a secondary effect of improving the mechanical properties (for example, hardness) of the alloy. The addition of a small amount of the Nb component also has a secondary effect of improving the mechanical properties (for example, toughness) of the alloy. Addition of a small amount of Ta component or Ti component has a secondary effect of improving the corrosion resistance of the alloy.

The total atomic content (atomic%) of the above-mentioned optional subcomponents can be controlled to be in the range of 0.8 to 2 times the total atomic content (atomic%) of C, N and O of the impurity components. The range of 0.8 times to 1.5 times is more preferable. When the total content of optional subcomponents is less than 0.8 times the total atomic content of C, N and O, the above-described effects cannot be obtained sufficiently. On the other hand, when the total atomic content of optional subcomponents exceeds twice the total atomic content of C, N, and O, the ductility and toughness of the alloy decrease.

(Method for producing a Cr-based two-phase alloy product of the present invention)
Next, a method for producing the above Cr-based two-phase alloy product will be described. FIG. 2 is a process diagram showing an example of a method for producing a Cr-based two-phase alloy product according to the present invention.

As shown in FIG. 2, in this manufacturing method, first, the raw material of the two-phase alloy is mixed and melted so as to have a desired composition (main component + subcomponent + optional subcomponent if necessary). The raw material mixing and dissolving step (step 1: S1) is performed. There are no particular limitations on the method of mixing and melting the raw materials, and conventional methods in the production of high corrosion resistance and high strength alloys can be used. For example, vacuum melting can be suitably used as a melting method. Further, it is preferable to refine the molten metal 10 together with a vacuum carbon deoxidation method or the like. In the raw material mixing and melting step S1, at the end of the step, the molten metal 10 is once solidified to form a raw material alloy lump.

Next, a remelting step (step 2: S2) is performed to control the content of impurity components (P, S, C, N, and O) in the alloy (to increase the cleanliness of the alloy). The remelting method is not particularly limited as long as the cleanliness of the alloy can be increased. For example, vacuum arc remelting (VAR) or electroslag remelting (ESR) can be preferably used. The cleaning molten metal 11 is prepared by this process.

Next, a casting process is performed in which the ingot 20 is formed by casting using a predetermined mold (step 3: S3).

Next, the ingot 20 is subjected to a ferrite rate adjustment heat treatment step (step 4: S4) in which heat treatment is performed to adjust the phase ratio of the two phases of the ferrite phase and the austenite phase (ferrite rate adjustment). The temperature of the ferrite ratio adjusting heat treatment is preferably in the range of 1050 to 1300 ° C, more preferably in the range of 1100 to 1250 ° C. The heat treatment time may be appropriately adjusted in the range of holding for 0.5 to 6 hours so that the ferrite rate is 15% to 85%.

The significance of performing the ferrite ratio adjusting heat treatment step S4 is as follows.

As described above, in order to ensure sufficient mechanical properties in the Cr-based two-phase alloy product, it is preferable to control the average crystal grain size to 30 μm or less. In order to obtain such a metal structure, it is necessary to increase the forging deformation amount in hot forging. In other words, the ingot 20 to be hot forged is required to have sufficient ductility and toughness to withstand a large amount of forging deformation.

On the other hand, in the ingot 20, the ferrite ratio is often larger than the expected ferrite ratio from the alloy composition. In particular, when the ferrite ratio exceeds 85%, the ductility and toughness of the Cr-based two-phase alloy deteriorates too much and cannot withstand the strain caused by hot forging, and cracking is likely to occur, and the yield of product forming by hot forging decreases. To do. By performing the ferrite ratio adjusting heat treatment, it is possible to transform the excess ferrite phase into the austenite phase and to secure the ductility and toughness required for the ingot 20.

In addition, when the ingot 20 is less than the expected ferrite rate from the alloy composition (the austenite phase is excessive), this ferrite rate adjusting heat treatment is performed to transform the excess austenite phase into the ferrite phase. The mechanical strength of the product can be adjusted.

Next, a hot forging process (step 5: S5) is performed in which the ingot with the ferrite ratio adjusted is subjected to hot forging to form a substantially final shape. There is no particular limitation on the hot forging / forming method, and a conventional method can be used. However, the hot forging forming step is preferably performed within a temperature range of 900 to 1300 ° C. By performing hot forging within the temperature range (the temperature range cannot be removed during hot forging), the casting defects of the ingot are eliminated, and the coarse cast solidification structure is broken while maintaining the desired ferrite ratio. And a compact 30 of a two-phase alloy having a forged structure having an average crystal grain size of 30 μm or less can be obtained.

After the hot forging and forming step S5, a solution heat treatment step (step 6: S6) for subjecting the forged formed body 30 to a solution treatment is performed. The temperature of the solution heat treatment is preferably in the range of 1050 to 1250 ° C, more preferably around 1100 ° C. By applying the solution treatment, the chemical composition can be homogenized in each phase of the austenite phase and the ferrite phase.

In addition, it is preferable to perform an aging heat treatment step (step 7: S7) after the solution heat treatment step S5. The temperature of the aging heat treatment is preferably in the range of 800 to 1000 ° C, more preferably around 900 ° C. The heat treatment time may be appropriately adjusted within a range of 0.5 to 6 hours.

By performing this aging heat treatment, compound formation of optional subcomponents and impurity components (C, N, and O) is promoted, and the impurity components can be further aggregated and stabilized. As a result, the toughness of the product can be further improved (a decrease in toughness can be further suppressed).

The product manufactured as described above is made of a two-phase alloy having Cr as a maximum component, which is cheaper than Ni, and thus has a Ni-based alloy while having high corrosion resistance and mechanical properties equal to or higher than those of conventional products. Cost reduction can be achieved compared to a product made of an alloy. As a result, the two-phase alloy product according to the present invention is an oil well equipment member (for example, a rotating machine (compressor, pump, etc.) member (shaft, bearing, etc.)) or seawater environment equipment used in a severe corrosive environment. It can be suitably used as a member (eg, seawater desalination plant equipment member, umbilical cable) or a chemical plant equipment member (eg, liquefied natural gas vaporizer member).

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. The present invention is not limited to these examples.

(Production of alloy products of Examples 1 to 22 and Comparative Examples 1 to 6)
Alloy products (Examples 1 to 22 and comparative examples) using alloys A1 to A9 (alloys containing no optional subcomponents) and alloys B1 to B9 (alloys containing optional subcomponents) having the chemical composition shown in Table 1 1 to 6) were produced. The content (unit: mass%) of each component is converted so that the total of the chemical compositions shown in Table 1 is 100 mass%. Alloy A9 is a commercially available duplex stainless steel called super duplex stainless steel.

Each alloy product was produced in accordance with the production method shown in FIG. First, the raw materials of each alloy were mixed, vacuum-melted (2 × 10 ⁻³ Pa or lower, 1700 ° C. or higher) using a high-frequency vacuum melting furnace, and then solidified to form a raw material alloy lump. Next, a remelting step of the raw material alloy lump was performed using a vacuum arc remelting furnace to prepare a cleaned molten metal. Next, the purified molten metal was cast using a predetermined mold to produce ingots of the respective alloys.

Next, the ingots used in Examples 1 to 22 and Comparative Examples 2 to 5 were subjected to a heat treatment for adjusting the ferrite ratio (held at 1150 to 1250 ° C. for 1 to 3 hours and then cooled with water). On the other hand, the ferrite ratio adjusting heat treatment was not performed on the ingots to be Comparative Examples 1 and 6.

Next, each of the ingots was molded by hot forging so as to have a predetermined shape. The hot forging conditions were as follows: forging temperature: 1050 to 1300 ° C., strain rate: 8 mm / s or less, amount of reduction per forging: 10 mm or less, number of forgings: 6 times or more.

Note that the range of the forging temperature is determined as follows. A test piece for a tensile test was cut out separately from the ingot of each example subjected to heat treatment for adjusting the ferrite ratio, and a high temperature tensile test (test temperature: 800 to 1350 ° C.) was performed on the test piece using a greeble tester. , Tensile speed: 10 mm / s). As a result of the high-temperature tensile test, the temperature range where the drawing is 60% or more was defined as the forging temperature range.

Next, each alloy sample subjected to hot forging was subjected to solution heat treatment (holding at 1050 to 1150 ° C for 0.5 to 1.5 hours, then water cooling), and then aging heat treatment (from 800 to 1000 ° C for 1 to After holding for 3 hours, water cooling was performed. Through the above steps, test and evaluation alloy products (Examples 1 to 22 and Comparative Examples 1 to 6) were produced.

(Test and evaluation for alloy products of Examples 1 to 22 and Comparative Examples 1 to 6)
(1) Microstructure evaluation After specimens for structure observation were collected from each alloy product, the surfaces of the specimens were mirror-polished and subjected to electric field etching in an oxalic acid aqueous solution. The polished surface was observed with an optical microscope. FIG. 1 shown above is an optical micrograph of the metal structure of Example 3.

As described above, the Cr-based two-phase alloy product of the present invention has a metal structure in which a dark ferrite phase P1 and a light austenite phase P2 are dispersed and mixed with each other. Since hot forging is performed, the cast solidification structure (for example, dendritic crystals peculiar to the cast solidification structure) is destroyed, and at least part of the structure is an equiaxed crystal grain (so-called forging structure) It is confirmed that

Also, it is confirmed that the grains of the austenite phase P2 are dispersed and precipitated in each crystal grain of the ferrite phase P1, and the precipitated grains have a needle-like shape and / or a scale-like shape. Further, it is confirmed that an austenite phase P2 layer is interposed between adjacent crystal grains of the ferrite phase P1. The other examples were the same.

Next, as another microstructural evaluation, the ferrite ratio was measured. Backscattered electron diffraction image (EBSP) analysis was performed on the polished surface of the above-mentioned specimen for observation of structure, and the occupancy of the ferrite phase (ferrite ratio, unit:%) was measured. For the measurement, a device in which a crystal orientation measuring device manufactured by TSL Solutions Inc. was added to a scanning electron microscope (S-4300SE) manufactured by Hitachi High-Technologies Corp. was used. The results are shown in Tables 2 to 3 described later.

Next, as another one of the fine structure evaluation, the average crystal grain size was measured. For each optical microscope observation image of each example, each crystal grain was binarized using image analysis software (NIH Image, public domain software), and the average of the minor axis and major axis of each binarized crystal grain From this, the average crystal grain size was calculated. As a result, it was confirmed that all examples had an average crystal grain size of 30 μm or less.

(2) Mechanical property evaluation As one of the mechanical property evaluations, a Vickers hardness test (load: 500 g, load application time: 20 s) was performed on the previous specimen for tissue observation using a Vickers hardness tester. went. The Vickers hardness was obtained as an average value of 5 measurements. The results are also shown in Tables 2-3.

Next, a specimen for a tensile test (diameter: 4 mm, parallel part length: 20 mm) was taken from each prepared alloy product. As another mechanical property evaluation, a room temperature tensile test (strain rate: 5 × 10 ^-4 s ^-1 ) was performed on each specimen using a tensile tester, and 0.2% proof stress, tensile strength, and elongation at break were measured. It was measured. In addition, when the test piece broke before the clear tensile strength was measured, the breaking stress was measured. The results of these tensile tests were determined as the average of 3 measurements.

As a result of measuring elongation at break, 15% or more is rated as A rank, 5% or more and less than 15% is evaluated as B rank, 2% or more and less than 5% is evaluated as C rank, and less than 2% is rated as D rank. evaluated. B rank or higher was determined to be acceptable, and C rank or lower was determined to be unacceptable. The results of the room temperature tensile test are also shown in Tables 2-3.

(3) Corrosion resistance evaluation A pitting corrosion test was conducted as one type of corrosion resistance evaluation. Polarized specimens for pitting corrosion tests were collected from each prepared alloy product. The pitting corrosion test was performed on each polarization test piece in accordance with JIS G0577 (2005). Specifically, a crevice corrosion prevention electrode is attached to a polarization test piece, a saturated calomel electrode is used as a reference electrode, and the anodic polarization curve of the polarization test piece is measured to generate pitting corrosion corresponding to a current density of 100 μA / cm ^2. The potential was determined. The results of the pitting corrosion test are also shown in Tables 2-3. In addition, after measurement of the anodic polarization curve, the occurrence of pitting corrosion was investigated using an optical microscope.

As another type of corrosion resistance evaluation, a sulfuric acid resistance test was conducted. Similar to the pitting corrosion test, a polarization test piece for the sulfuric acid resistance test was collected from each alloy product. Specifically, in the sulfuric acid resistance test, a crevice corrosion prevention electrode is attached to the polarization test piece, and the anodic polarization curve of the polarization test piece in a sulfuric acid aqueous solution (pH = 2.0, 30 ° C.) (from the natural immersion potential to the sweep rate of 200 μA). (Measured until the potential reached 1300 mV (vs. SHE)) using the / s dynamic potential method. Corrosion current density corresponding to a potential of 400 mV (vs. SHE) was determined from the obtained polarization curve.

As a result of the measurement, the corrosion current density corresponding to a potential of 400 mV (vs. SHE) in Comparative Example 6 (commercially available duplex stainless steel) was 1.32 μA / cm ² . Taking this as 100%, the ratio of the corrosion current density of each alloy product was calculated. The results of the sulfuric acid resistance test are shown in Tables 2-3.

As shown in the table, in Comparative Example 1 in which the ferrite ratio adjusting heat treatment was not performed, the ferrite ratio deviated from the provisions of the present invention, and although it exhibited good corrosion resistance, the ductility was insufficient. In Comparative Examples 2 to 6, the chemical composition of the alloy was not within the scope of the present invention, and there were difficulties in mechanical properties (ductility and mechanical strength) or corrosion resistance. In addition, the ferrite rate of the comparative example 6 which consists of a commercially available duplex stainless steel (A9) was 45%.

In contrast to these comparative examples, each of the examples according to the present invention is a two-phase alloy in which the ferrite ratio in which the austenite phase and the ferrite phase are mixed is in the range of 15% to 85%, and the average grain size is 30 μm or less. It had a forged structure.

Also, the examples according to the present invention all have good mechanical properties (for example, Vickers hardness of 200 Hv or more, 0.2% proof stress of 600 MPa or more, tensile strength of 850 MPa or more, elongation at break of 5% or more. ) Was confirmed.

As the corrosion resistance, in the examples where the pitting corrosion test was performed, the pitting corrosion occurrence potential corresponding to a current density of 100 μA / cm ² was 1.1 V or more. It became outbreak. In all these samples, no pitting corrosion was observed. In addition, the examples subjected to the sulfuric acid resistance test showed a corrosion current density of 0.8 to 16% as compared with Comparative Example 6. That is, it was confirmed that the examples according to the present invention have excellent corrosion resistance.

(4) Structure stability evaluation Next, from the viewpoint of long-term reliability of the alloy product, a structure stability test was performed. A specimen for a structural stability test was collected from the alloy product of each example, and then subjected to a heat treatment that was held at 800 ° C. for 60 minutes. X-ray diffraction measurement was performed on the surface of each test piece, and the presence or absence of the generation of σ phase of the intermetallic compound was investigated. As a result of the investigation, it was confirmed that in all examples according to the present invention, no σ phase was detected, and it was difficult to generate the σ phase.

From the test and evaluation results as described above, it was confirmed that the examples according to the present invention had good mechanical properties equivalent to or better than conventional materials and excellent corrosion resistance. Furthermore, since the content of Cr component is high, it can be said that the cost can be reduced as compared with the conventional Ni-based alloy material.

The above-described embodiments and examples are described for the purpose of helping understanding of the present invention, and the present invention is not limited to the specific configurations described. For example, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. That is, according to the present invention, a part of the configurations of the embodiments and examples of the present specification can be deleted, replaced with other configurations, and added with other configurations.

P1 ... ferrite phase, P2 ... austenite phase, 10 ... molten metal, 11 ... cleaned molten metal, 20 ... ingot, 30 ... molded body.

Claims

A product using a Cr-based two-phase alloy in which two phases of a ferrite phase and an austenite phase are mixed,
The chemical composition of the Cr-based two-phase alloy consists of a main component, subcomponents, impurities and optional subcomponents,
The main component is composed of 34 mass% or more and 70 mass% or less of Cr, 17 mass% or more and 45 mass% or less of Ni, and 10 mass% or more and 35 mass% or less of Fe. The largest, the total content of Ni and Fe is 30% by mass or more and 65% by mass or less,
The subcomponent consists of Mn of 0.1% by mass to 2% by mass and Si of 0.1% by mass to 1% by mass,
The impurities include more than 0% by mass 0.04% by mass P, more than 0% by mass 0.01% by mass S, more than 0% by mass 0.03% by mass C, and more than 0% by mass 0.02% by mass N. And O in excess of 0% by mass and 0.03% by mass or less,
The product has a forged structure as a fine structure and the austenite phase grains are dispersed and precipitated in the ferrite phase crystal grains, and the ferrite phase occupation ratio is 15% or more and 85% or less,
A Cr-based two-phase alloy product characterized by a 0.2% proof stress by a room temperature tensile test of 600 MPa or more and a breaking elongation of 5% or more.
In the Cr-based two-phase alloy product according to claim 1,
The optional subcomponent consists of at least one of V, Nb, Ta and Ti,
When the Cr-based two-phase alloy contains the optional subcomponent, the total atomic content of the V, Nb, Ta and Ti is 0.8 to 2 times the total atomic content of the C, N and O A Cr-based two-phase alloy product characterized by being in the range.
In the Cr-based two-phase alloy product according to claim 1 or 2,
The Cr-based two-phase alloy product, wherein the dispersed and precipitated austenite phase grains have a needle-like and / or scale-like shape.
In the Cr-based two-phase alloy product according to any one of claims 1 to 3,
A Cr-based two-phase alloy product characterized in that the austenite phase layer is interposed between adjacent crystal grains of the ferrite phase.
In the Cr-based two-phase alloy product according to any one of claims 1 to 4,
A Cr-based two-phase alloy product characterized in that the product is a shaft or a bearing of a rotary machine.
A method for producing a Cr-based two-phase alloy product according to any one of claims 1 to 5,
A raw material mixing and melting step in which the raw material of the Cr-based two-phase alloy is mixed and melted to form a molten metal and then solidified to form a raw material alloy lump,
A remelting step of remelting the raw material alloy lump to prepare a purified molten metal;
A casting step of casting the cleaned molten metal to form an ingot;
A ferrite rate adjusting heat treatment step of forming a ferrite rate adjusting ingot by adjusting the occupation ratio of the ferrite phase by performing a heat treatment at a temperature of 1050 ° C. or higher and 1300 ° C. or lower with respect to the ingot,
A hot forging molding step of forming a compact by hot forging within a temperature range of 900 ° C. or higher and 1300 ° C. or lower with respect to the ferrite ratio adjusting ingot,
A solution heat treatment step of performing a solution treatment at a temperature of 1050 ° C. or higher and 1250 ° C. or lower on the molded body;
A method for producing a Cr-based two-phase alloy product comprising: an aging heat treatment step of performing an aging treatment at a temperature of 800 ° C. or higher and 1000 ° C. or lower on the solution-treated compact.