WO2015029167A1 - Duplex stainless steel, and duplex stainless steel structure, marine structure, petroleum/gas environment structure, pump impeller, pump casing, and flow adjustment valve body using same - Google Patents

Abstract

This duplex stainless steel is characterized in comprising, in mass%, N: 0.3% or less, C: 0.1% or less, P: 0.1% or less, Si: 3.0% or less, Mn: 8.0% or less, Ni: 3.0-12.0%, Cr: 20.0-40.0%, Mo: 7.0% or less, W: 6.5% or less, and Ta: 0.05-1.0%, with the remainder being Fe and unavoidable impurities. The formation of tantalum nitride can thereby be limited.

Description

Duplex stainless steel and duplex stainless steel structure using the same, marine structure, oil and gas environment structure, pump impeller, pump casing and valve body of flow control valve

The present invention relates to duplex stainless steel and a structure using the same.

A duplex stainless steel mainly composed of a two-phase metal structure of a ferrite phase (α phase) and an austenite phase (γ phase) has high strength and is resistant to pitting corrosion in a chloride / sulfide environment, And excellent in crevice corrosion resistance characteristics. Utilizing this property, it is widely used as a material for marine structures and petrochemical industries. However, when exposed to high temperatures depending on the manufacturing conditions and conditions of use, hard and brittle intermetallic compounds (σ phase, χ phase, Laves phase) and nitride / carbide embrittled phases mainly composed of Cr, Mo, etc. Is formed and the toughness is known to decrease.

In duplex stainless steel, the higher the pitting resistance index (PREW) represented by the following formula, the higher the corrosion resistance.

(PREW) =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 30 ×% N
(In the formula,% Cr,% Mo,% W and% N are values of each composition expressed by mass%.)
However, the higher the content of Cr, Mo and W, the more easily the intermetallic compound precipitates. In addition, as N is higher, nitrides are more likely to precipitate, and when the addition amount is excessive, defects are generated due to the generation of blow holes during production.

In the production process of duplex stainless steel, solution heat treatment is performed at 950 ° C to 1200 ° C in order to optimize the phase ratio between ferrite phase and austenite phase, and the precipitation of the above-mentioned embrittled phase and 475 ° C embrittlement are avoided after solution heat treatment In order to achieve this, a quenching treatment such as water cooling is performed from the solution heat treatment temperature to room temperature. At that time, thin-walled materials such as thin plates and pipes do not pose a major problem, but in large-sized structures, particularly in thick-walled structures produced by casting or forging, due to the difference between the surface and the internal cooling rate Since the embrittled phase precipitates inside the material, there is a problem that stable production is difficult.

In addition, even in a place affected by heat due to welding or annealing for the purpose of removing residual stress, there has been a problem of the above-mentioned decrease in toughness due to the precipitation of the embrittled phase.

Heretofore, a method of suppressing the embrittlement phase at the time of production or use has been proposed, focusing on the material composition.

In Patent Document 1, Cr: 21.0% by weight for the purpose of suppressing the formation of intermetallic compounds that degrade corrosion resistance and mechanical properties, such as sigma (σ) phase and chi (χ) phase. ~ 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5%, Si: 3.0% or less, Al: 1 .0% or less, Mn: 8.0% or less, N: 0.2% to 0.7%, C: 0.1% or less; and B: 0.1% or less, Cu: 3.0% or less, A super duplex stainless steel containing at least one of Co: 3.0% or less is disclosed. This super duplex stainless steel further contains Ca: 0.5% or less, Mg: 0.5% or less, Ta: 0.5% or less, Nb: 0.5% or less, Ti: 1.5% or less, Zr It is also described that it is desirable to contain one or more elements selected from the group consisting of: 1.0% or less, Sn: 1.0% or less, and In: 1.0% or less.

JP, 2011-174183, A

In the case of Patent Document 1, since the content of nitrogen is large, a nitride is easily formed, the additive element is not appropriately dissolved in the alloy, and the embrittlement may proceed.

The present invention suppresses formation of intermetallic compounds (σ phase, χ phase, Laves phase) and nitride in duplex stainless steel, and improves corrosion resistance, embrittlement resistance, manufacturability, weldability and heat treatment property. With the goal.

The duplex stainless steel of the present invention is, by mass%, N: 0.3% or less, C: 0.1% or less, P: 0.1% or less, Si: 3.0% or less, Mn: 8.0 % Or less, Ni: 3.0 to 12.0%, Cr: 20.0 to 40.0%, Mo: 7.0% or less, W: 6.5% or less, Ta: 0.05 to 1.0 %, And the balance is Fe and unavoidable impurities.

According to the present invention, since the amount of nitrogen contained in the duplex stainless steel containing tantalum is small, the formation of nitride can be suppressed. Furthermore, since metal tantalum which does not become nitride inhibits the diffusion of the intermetallic compound-forming element, the corrosion resistance, embrittlement resistance, manufacturability, weldability and heat treatment property of duplex stainless steel can be improved. .

It is a conceptual diagram which shows the embrittlement phase formation mechanism in the conventional duplex stainless steel. It is a conceptual diagram which shows the embrittlement phase formation suppression mechanism in the duplex stainless steel of this invention. It is an external appearance photograph of the produced preparation material A. It is an external appearance photograph of the produced preparation material B. It is an external appearance photograph of the produced preparation material C. It is a metal phase observation result of the preparation material A. It is a metal phase observation result of the preparation material B. It is a metal phase observation result of the preparation material C. It is a graph which shows the relationship between the heat processing time in 800 degreeC, and the amount of residual ferrites. It is a metal phase observation photograph of preparation material A after giving heat processing for 800 ° C and 30 minutes. It is a metal phase observation photograph of preparation material B after performing heat treatment for 30 minutes at 800 ° C. It is a metal phase observation photograph of preparation material C after giving heat processing for 800 ° C and 30 minutes. It is a graph which shows the measurement result of the Charpy impact value as it is in solution and after heat treatment at 800 ° C. for 5 minutes. It is an electron micrograph of the preparation materials A which are comparative materials. It is a graph which shows the result of having measured concentration distribution of each element in the arrow direction in an analysis position (line segment) of Drawing 7A. It is an electron micrograph of the preparation material C which is an invention material. It is a graph which shows the result of having measured concentration distribution of each element of the arrow direction in an analysis position (line segment) of Drawing 7C. It is a graph which shows the result of having compared the residual stress before and behind heat processing. It is a graph which shows the result of having compared the result of the Charpy impact test before and behind heat treatment. It is a graph which compares and shows the pitting corrosion electrical potential of an invention material and a comparison material. It is sectional drawing which shows the vertical axis mixed flow seawater pump produced using the invention material. It is sectional drawing which shows the flow control valve produced using the invention material.

The present invention relates to a duplex stainless steel and a structure using the same, and more particularly, it is formed at the time of manufacturing of a high corrosion resistant duplex stainless steel (in casting, forging, hot rolling or welding), welding and heat treatment. Resistance to corrosion while maintaining high corrosion resistance by suppressing the formation of a brittle phase (such as nitrides, precipitates such as carbides, and intermetallic compounds such as sigma (.sigma.) Phases and chi (.zeta.) Phases). The present invention relates to duplex stainless steel which realizes embrittlement and manufacturability and a product using the same.

Although the present application contains a plurality of means for solving the above problems, as one example, in order to suppress the formation of the intermetallic compound in the duplex stainless steel, positively adding Ta which inhibits the diffusion of the intermetallic compound forming element It is characterized by

That is, N: 0.7% or less, C: 0.1% or less, P: 0.1% or less, Si: 3.0% or less, Mn: 8.0% or less, Ni: 3.% by mass. 0 to 12.0%, Cr: 20.0 to 40.0%, Mo: not more than 7%, W: not more than 6.5%, Ta: 0.05 to 1.0% added Stainless steel. The N content is more preferably 0.3% or less.

More preferably, in order not to impair the effect by the addition of Ta, it is preferable to suppress the formation of nitride and carbide with N: 0.05 to 0.25%, C: 0.02% or less. N is particularly preferably 0.05 to 0.19%.

Further, Si that promotes the formation of the intermetallic compound is preferably reduced to 0.5% or less because diffusion inhibition by Ta can not be expected. Moreover, it is preferable to limit the range of the element resulting from corrosion resistance from a viewpoint of corrosion resistance, and to satisfy the pitting resistance index (PREW) defined by the following formula with 40 or more.

(PREW) =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 30 ×% N
(In the formula,% Cr,% Mo,% W and% N are values of each composition expressed by mass%.)
That is, by mass%, N: 0.05 to 0.25%, C: 0.02% or less, P: 0.02% or less, Si: 0.5% or less Mn: 1.2% or less, Ni: 6.0 to 8.0%, Cr: 24.0 to 26.0%, Mo: 3.0 to 5.0%, W: 4.0% or less, Ta: 0.2 to 0.5% The super duplex stainless steel has a pitting resistance index (PREW) of 40 or more.

An alloy of the above composition is prepared by forging or casting and then subjected to solution heat treatment at a temperature of 950 ° C. to 1200 ° C. for 30 minutes to 2 hours to set the austenite / ferrite phase ratio to 0.2 to 0.8. The duplex stainless steel structure can suppress the formation of an embrittled phase particularly in the inside of the structure, and can provide a product with good toughness.

Particularly useful as structures of alloys of the above components are pump impellers, pump casings and flow control valves used in marine structures, oil and gas environment structures, and chemical plant structures.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The inventors have found that embrittlement phase precipitation by intermetallic compounds and carbonitrides in order to improve the manufacturability and resistance to embrittlement of thick-walled cast products, forged products and hot-worked products while maintaining high corrosion resistance. As a result of researching suppression technology, the following facts were found.

First, the embrittlement phase formation mechanism of the conventional example not containing Ta will be described.

FIG. 1A is a conceptual view showing an embrittlement phase formation mechanism in a conventional duplex stainless steel.

In the figure, the duplex stainless steel includes a ferrite phase 1, an austenite phase 2, and grain boundaries 3 formed therebetween. In the ferrite phase 1, Cr, Mo, W, etc., which are elements forming the intermetallic compound (intermetallic compound forming element 5), diffuse through the holes 4 and move toward the grain boundary 3.

Intermetallic compounds 6 and carbo-nitrides 7 (carbides and nitrides) are generated in the grain boundary regions including the grain boundaries 3. These are also called embrittled phases. When the amount of this embrittlement phase is large, the material becomes brittle, and the corrosion resistance, the embrittlement resistance, the manufacturability, the weldability and the heat treatment property tend to be lowered.

Below, the embrittlement phase formation suppression mechanism in stainless steel of this invention containing Ta is demonstrated.

FIG. 1B is a conceptual view showing an embrittlement phase formation suppression mechanism in the duplex stainless steel of the present invention.

In the case of this figure, since the tantalum atom 11 tends to occupy the holes 4 more easily than the intermetallic compound forming element 5, the diffusion of the intermetallic compound forming element 5 is inhibited. Thereby, it is possible to prevent the formation of nitrides or the like of intermetallic compound formation element 5 in grain boundary region 12.

The intermetallic compound 6 is composed of a sigma phase, χ phase, etc., and is known to be easily precipitated on the に phase side from the 相 phase // phase interface. The elements constituting the intermetallic compound 6 (intermetallic compound forming element 5), Cr, Mo, Si and W are concentrated in the metal matrix at the grain boundaries of the α phase / γ phase interface, and the intermetallic compound 6 It precipitates as. Therefore, if the diffusion rate of these intermetallic compound forming elements 5 can be reduced, it is considered that the precipitation of the intermetallic compound 6 can be delayed. Among these elements, Cr, Mo and W are oversize elements having an atomic radius larger than the average atomic diameter of the elements constituting the stainless steel, and atomic vacancies in the metal matrix (vacancy 4) And the air holes 4 are considered to move as a preferential diffusion path.

Because of such a phenomenon, an atomic radius is larger than these elements, and an element that interacts more strongly with the holes 4 is added to trap the holes 4 in the additive element, and the diffusion of the intermetallic compound forming element 5 It is important to inhibit Thereby, the diffusion rate of the intermetallic compound-forming element 5 can be reduced particularly in the temperature range of 650 ° C. to 950 ° C. where the precipitation of the embrittled phase is a problem. The embrittlement in this temperature range can be avoided by quenching in the case of a stainless steel (steel material) having a small size, but becomes a problem when it is difficult to quench the inside of the steel material because the size is large. The present invention solves this problem by adjusting the composition of the steel material.

Although there are several possible candidates for the additive element having a large atomic radius, in general, a metal element having a large atomic radius has extremely low free energy to form nitrides and carbides.

As a result of thermal equilibrium calculation, in the case of super duplex stainless steel in which the corrosion resistance is improved by adding nitrogen, especially when an element having high ability to form nitrogen and carbide, such as Zr, Ti, Hf, etc., is added, At the time of production, a nitride is formed at the liquid phase stage, and it is difficult to form a solid solution in the matrix. Further, among elements having a relatively low ability to form nitride, Nb itself is considered to be an element that is easily taken into the σ phase, which is an intermetallic compound.

From the above, it was relatively easy to form a solid solution in the metal matrix at the time of production, and Ta was selected as an additive element, which is difficult to precipitate as an intermetallic compound.

Hereinafter, the role of the alloying elements added to the duplex stainless steel according to the present invention and the reason for limiting the range of the chemical composition will be described.

Chromium (Cr): 20.0 to 40.0%
Chromium is the most important basic element in maintaining the corrosion resistance of stainless steel. In the case of duplex stainless steel, it is necessary to obtain a two-phase structure of austenite and ferrite, so the chromium equivalent (Cr _eq ) and nickel equivalent (Ni _eq ) defined by the following formulas and the ferrite phase determined thereby The amount of chromium was 20% or more in consideration of the ratio (fraction) of In addition, since it is necessary to increase Ni _eq if Cr _eq is increased, the upper limit is set to 40% in consideration of economics. A more preferable range is 24% to 26%.

Cr _eq =% Cr + 2% Si + 1.5% Mo + 0.75% W + 5% V + 5.5% Al + 1.75% Nb + 1.5% Ti
Ni _eq =% Ni + 0.5% Mn + 30% C + 0.3% Cu + 25% N +% Co
(Wherein,% Cr,% Si,% Mo,% W,% V,% Al,% Nb,% Ti,% Ni,% Mn,% C,% Cu,% N and% Co are% by mass) It is the value of each composition expressed.)
Fraction of ferrite phase (volume%) = 55 × (Cr _eq / Ni _eq ) -66.1
Nickel (Ni): 3.0% to 12.0%
Nickel is a useful element that increases overall corrosion resistance in connection with corrosion resistance as an austenite stabilizing element, so it is at least 3% or more. The upper limit is set to 12% or less in consideration of the relationship between the chromium equivalent and the nickel equivalent, the phase ratio, and the economy. A more preferable range is 6% to 8%.

Molybdenum (Mo): 7.0% or less Molybdenum, together with chromium, is an element important for maintaining corrosion resistance, and acts to stabilize the ferrite phase, but promotes the formation of intermetallic compounds by addition. For this reason, the amount is limited to 7.0% or less. A more preferable range is 3.0% to 5.0%.

Tungsten (W): 6.5% or less Tungsten is an element that improves the corrosion resistance, retards the deposition rate of intermetallic compounds by replacing it with Mo in one-half amount, and improves the corrosion resistance and mechanical properties. is there. However, tungsten is an expensive alloying element, and addition of a large amount thereof promotes the formation of intermetallic compounds and reduces the corrosion resistance of the weld, so the content is limited to 6.5% or less. A more preferable range is 4.0% or less.

Silicon (Si): 3.0% or less Silicon is an element that stabilizes the ferrite structure, and is an element effective for deoxidation during production. In addition, it is an element that reduces the surface defects by increasing the fluidity of the molten steel at the time of production or welding. However, 3.0% or less is preferable because it is an element that increases the deposition rate of intermetallic compounds and reduces the ductility of the steel. More preferably, it is 0.5% or less.

Manganese (Mn): 8.0% or less Manganese is an austenite stabilizing element that can replace expensive nickel, and is an element that increases the solid solution degree of nitrogen and increases the deformation resistance at high temperature. In particular, when nitrogen is positively added to improve corrosion resistance, addition of an appropriate amount of manganese is essential. It has a deoxidizing effect at the time of smelting and refining, but when added in large amounts, the corrosion resistance decreases and promotes the formation of intermetallic compounds. For this reason, the upper limit was limited to 8% or less. A more preferable range is 1.2% or less.

Nitrogen (N): 0.7% or less Nitrogen is a useful element that improves resistance to pitting corrosion. The effect is one of the most important elements associated with corrosion resistance reaching about 30 times that of chromium. In addition, when the carbon content is lowered for the purpose of preventing grain boundary sensitization, nitrogen is added to compensate for the strength. However, if it is added in excess of 0.7%, cracks may occur due to blow holes during production. Therefore, it is preferable to make it 0.7% or less. In particular, when Ta is added, a nitride containing Ta is formed to inhibit the effect. Therefore, in order to form a solid solution in a balanced manner in the α phase and the γ phase and to prevent the corrosion resistance from being impaired, the content is more preferably 0.3% or less, preferably 0.05% to 0.25%. Is even more preferred. Furthermore, a range of 0.05 to 0.19% is particularly desirable.

Carbon (C): 0.1% or less Carbon is an element that forms carbides and induces grain boundary sensitization during welding. In particular, when Ta is added, a carbide containing Ta is formed, and the effect of Ta addition is inhibited. However, since the reduction of C causes an increase in manufacturing cost, it is 0.1% or less. A more preferable range is 0.02% or less.

Tantalum (Ta): 0.05% to 1.0%
Tantalum is one of the elements that characterize the present invention. As described above, since the atomic radius is large compared to the average atomic radius of the elements constituting the duplex stainless steel, the effect of preventing the diffusion of the main intermetallic compound-forming elements and reducing the precipitation rate of the intermetallic compounds There is. However, if the addition amount is too large, not only it is not economical but also the balance of the ferrite / austenite phase ratio is broken, so the upper limit is limited to 1.0%. On the other hand, if it is less than 0.05%, the addition effect can not be expected. Further, in view of the balance of the solid solution amount in the nitride phase and the ferrite phase, it is more preferably in the range of 0.2 to 0.5%.

Phosphorus (P): 0.1% or less Phosphorus is an impurity inevitably mixed in steel and not only degrades corrosion resistance but also segregates in grain boundaries and promotes precipitation of the embrittled phase, so the less it is desirable. Therefore, 0.1% or less is desirable, 0.02% or less is more desirable, and 0.005% or less is particularly desirable. However, excessive reduction of P causes an increase in manufacturing cost. Therefore, the addition amount of P is determined in consideration of this point as well.

Examples will be described below.

Table 1 shows the chemical composition (unit: mass%) of the duplex stainless steel of Example 1 (inventive material (preparation material C)) and comparative examples 1 and 2 (comparative materials (preparation materials A and B)) It is a thing.

These materials were subjected to heat treatment simulating cooling during production and reheating by welding and compared.

20 kg of each of the duplex stainless steels of the chemical composition shown in Table 1 was melted in a vacuum melting furnace to obtain an ingot. The manufacturing material A has a component equivalent to the standard material S32750. The production material B has a reduced content of N, C and Si. The production material C is a alloy in which a small amount of Ta is added to an alloy having the same component as the production material B.

The above ingot was heated to 1250 ° C. and forged to obtain a plate of 20 × 50 × 150 (mm). The forged plate material was subjected to solution heat treatment at 1100 ° C. for 1 hour in order to obtain an appropriate phase ratio of ferrite phase / austenite phase, and then quenched by water cooling to avoid precipitation of the embrittled phase.

FIGS. 2A, 2B and 2C respectively show appearance photographs of forged preparation materials A, B and C. FIG.

From these photographs, it can be seen that production is possible without causing cracks or defects due to forging.

FIGS. 3A, 3B and 3C show the results of observation of the gold phase of the manufactured materials A, B and C, respectively.

In the metal phase observation, after polishing to No. 2000 with SiC abrasive paper and finish polishing using 1 μm diamond abrasive grains, electrolytic etching was performed using a 10% aqueous NaOH solution. As a result, the ferrite phase 31 is colored brown, and the intermetallic compounds, carbides and nitrides are colored black. Moreover, the austenite phase 32 is white.

The test pieces were subjected to ultrasonic cleaning with acetone and distilled water and then observed with an optical microscope. The subsequent gold phase observation is also carried out in the same manner. As a result of the metal phase observation shown in these figures, it can be seen that any of the prepared materials has a distinct ferrite / austenite two-phase structure.

Heat treatment at 800 ° C., which is a temperature range in which the embrittled phase tends to precipitate, was performed on the above-described materials in order to evaluate embrittled phase precipitation under reheating conditions by cooling during production and welding.

FIG. 4 is a graph showing the relationship between the heat treatment time at 800 ° C. and the amount of residual ferrite. The heat treatment time is taken on the horizontal axis, and the amount of ferrite is taken on the vertical axis. The amount of ferrite was measured by a ferrite scope using a magnetic induction method. Among the precipitation of the embrittlement phase, the intermetallic compound precipitation proceeds by the decomposition of the ferrite phase into the intermetallic compound phase and the austenite phase under the precipitation temperature conditions, so the precipitation tendency of the intermetallic compound is evaluated by evaluating the amount of residual ferrite. It can be evaluated.

From this figure, it can be seen that in Example 1, the reduction rate of the ferrite phase is slower than in Comparative Examples 1 and 2, and the decomposition of the ferrite phase is suppressed.

FIGS. 5A, 5B, and 5C are respectively metal phase observation photographs of fabrication materials A, B, and C after heat treatment at 800 ° C. for 30 minutes.

In this figure, it can be seen that the embrittlement phase 53 is increased in addition to the ferrite phase 51 and the austenite phase 52.

It can be seen that the amount of precipitation of the embrittlement phase 53 is smaller in the preparation material C of the invention material than in the preparation materials A and B, which are comparison materials, and the precipitation of the embrittlement phase 53 is suppressed. In the production material B, the embrittled phase 53 is particularly large.

FIG. 6 shows the measurement results of Charpy impact value after heat treatment at 800 ° C. for 5 minutes.

The Charpy impact value was measured in accordance with JIS Z2242 (2005). The outline of the measurement procedure is as follows.

For the plate before and after heat treatment, a 10 mm × 10 mm × 55 mm 2 mm V-notch Charpy test specimen was sampled from the longitudinal direction of the plate so that the central portion was a notch portion, and the impact value was measured.

From this figure, compared with the preparation materials A and B which are comparative materials, in the preparation material C which is an invention material, the Charpy impact value after heat processing is high. From this, it is understood that the toughness is improved by suppressing the formation of the intermetallic compound.

FIGS. 7A to 7D show the results of EDX measurement at grain boundaries (α / γ boundaries) after heat treatment at 800 ° C. for 1 minute.

FIG. 7A is an electron micrograph of fabrication material A which is a comparative material, and FIG. 7B shows the result of measuring the concentration distribution of each element in the arrow direction at the analysis position (line segment) in FIG. 7A. .

Moreover, FIG. 7C is an electron micrograph of the manufacturing material C which is an invention material, FIG. 7D shows the result of having measured the concentration distribution of each element in the arrow direction in the analysis position (line segment) of FIG. 7C. It is.

In FIGS. 7A and 7C, the microstructures of the ferrite phase 71 and the austenite phase 72 are clearly represented. Grain boundaries are indicated by broken lines. At the analysis position 73 represented by the line segment, the concentration of each element was measured in the direction of the arrow (from the austenite phase 72 to the ferrite phase 71).

In FIGS. 7B and 7D, the horizontal axis is a distance, and the vertical axis is a concentration.

In the comparative material shown in FIG. 7B, the concentrations of Mo and Cr in the vicinity of the grain boundary on the ferrite phase side are high.

On the other hand, in the invention material shown in FIG. 7D, a peak of concentration of Ta occurs near the grain boundary on the ferrite phase side, and the concentrations of Mo and Cr are lower than those of FIG. 7B.

In other words, it is understood that Ta preferentially diffuses to the α / γ grain boundaries, and inhibits the diffusion of Mo and Cr, which are intermetallic compound forming elements.

From the above, it was found that when Ta is added, the diffusion of Ta itself to the grain boundaries inhibits the diffusion of the intermetallic compound-forming element such as Mo and Cr, and delays the formation of the intermetallic compound.

(Effect of heat treatment on residual stress and impact value)
The post-welding heat treatment (PWHT) for the purpose of residual stress relaxation was assumed, the heat treatment was performed on the invention material and the comparative material, and the influence of the heat treatment on the residual stress and the impact value was evaluated.

With respect to the preparation materials A, B and C, tensile residual stress was applied by using a grindstone of particle size # 46 and applying a strongly processed layer by surface grinding to the plate surface at a rotational speed of 1440 rpm and a cutting amount of 0.01 mm. A heat treatment was performed on a test material having residual stress applied to the surface by surface grinding, assuming PWHT at 650 ° C. for 30 minutes to evaluate the influence of heat treatment conditions on residual stress and mechanical properties.

FIG. 8 shows the results of comparison of residual stress before and after heat treatment. The residual stress was measured in each of the ferrite phase and the austenite phase, and the average value obtained by multiplying the volume ratio was evaluated as macro stress.

Surface tension applied a tensile stress of about 900 to 1100 MPa, but heat treatment at 650 ° C for 30 minutes reduced the tensile stress to about 200 MPa and a stress relaxation effect of about 80% was obtained. .

FIG. 9 shows the results of comparison of the results of Charpy impact test before and after heat treatment.

From this figure, compared with the comparison material, the invention material improves the impact value, leaving an impact value of about 100 J / cm ² after heat treatment, and relieves 80% of the residual stress by heat treatment at 650 ° C. for 30 minutes. While maintaining an impact value of 100 J / cm ² or more.

(The effect of heat treatment on the pitting potential)
The results of the pitting corrosion potential measurement before and after heat treatment (650 ° C. × 30 minutes) are shown below.

The pitting potential was measured in accordance with JIS G0577 (2005).

FIG. 10 shows the pitting corrosion potential of the inventive material and the comparative material in comparison.

As shown in the figure, the pitting resistance of each material (after heat treatment) is as follows.

Preparation material C (inventive material)> Preparation material B (comparative material)> Preparation material A (comparative material, equivalent to conventional material S32750).

That is, the inventive material has a higher pitting potential than conventional materials.

From the above results, it was confirmed that the inventive material has pitting corrosion resistance higher than that of the conventional material despite suppressing embrittlement.

(Product 1 using invention material)
FIG. 11 is a cross-sectional view of a vertical axis mixed flow seawater pump according to the present invention.

In this figure, the vertical mixed flow seawater pump efficiently uses bellmouth 117 to rectify seawater entering from the suction channel, shaft 111 to transmit the rotational power of the prime mover, impeller hub 115 fixed to shaft 111, and rotational power to the prime mover Impeller vanes 113 given to seawater, casing liner 114 whose inner surface is spherical so that the clearance on the outer periphery of impeller vanes 113 is always constant, casing 112 which converts velocity energy given to seawater from impeller vanes 113 into pressure energy, pressurization It consists of a column pipe 119, an impeller cap 116, a cone 118, etc. through which the stored seawater passes.

The casing liner 114 and the casing 112 were each made of the cast steel of Example 1, and the impeller hub 115 and the impeller vane 113 were each made of the forging of Example 1. After casting and forging, solution heat treatment at 1100 ° C. for 1 h was performed, and then water cooling was performed to obtain a two-phase composition with a ferrite content of 40 to 50%. Thereafter, the junction between the casing liner 114 and the casing 112 and the junction between the impeller hub 115 and the impeller vane 113 are joined by MIG welding, a band heater is wound, and the welding heat affected zone is heated to 650 ° C. A heat treatment of 30 min was carried out at that temperature to rapidly cool it.

When residual stress in the weld heat affected zone was measured by X-ray residual stress measurement, the tensile stress was reduced to 80 MPa. By using the steel material of Example 1, the toughness fall of a welding part was also suppressed and the sea water pump with a long service life which fatigue strength improved was able to be manufactured.

(Product 2 using invention material)
FIG. 12 is a cross-sectional view of a flow control valve according to the present invention.

In the figure, the flow rate control valve includes a casing 121 for supporting the entire valve, a valve body 122 for adjusting the flow rate, a valve seat 123 in which the valve body 122 is fitted, a handle 125, and a shaft for adjusting the position of the valve body 122 And the like.

The casing 121 was made of the cast steel of Example 1. By using the steel material of Example 1, it was possible to manufacture a large flow rate control valve with high corrosion resistance.

The flow control valve can be used in seawater, petroleum and chemical plant environments.

1: ferrite phase 2: 2: austenite phase 3: 3: grain boundary 4: 4: interstitial compound forming element 6: 6: intermetallic compound 7: 7: carbon / nitride 11: tantalum atom 12: grain boundary Area: 31, 51, 71: ferrite phase, 32, 52, 72: austenite phase, 53: embrittled phase, 73: analysis position, 111: shaft, 112: casing, 113: impeller vane, 114: casing liner, 115 : Impeller hub, 116: impeller cap, 117: bell mouth, 118: cone, 119: column pipe, 121: casing, 122: valve body, 123: valve seat, 124: shaft, 125: handle.

Claims (10)

1. N: 0.3% or less, C: 0.1% or less, P: 0.1% or less, Si: 3.0% or less, Mn: 8.0% or less, Ni: 3.0 to 10% by mass 12.0%, Cr: 20.0 to 40.0%, Mo: 7.0% or less, W: 6.5% or less, Ta: 0.05 to 1.0%, the balance being Fe and A duplex stainless steel characterized by being an unavoidable impurity.
2. N: 0.05 to 0.25%, C: 0.02% or less, P: 0.02% or less, Si: 0.5% or less, Mn: 1.2% or less, Ni: 6 in mass% .0 to 8.0%, Cr: 24.0 to 26.0%, Mo: 3.0 to 5.0%, W: up to 6.5%, Ta: 0.2 to 0.5% And the balance is Fe and unavoidable impurities.
3. The duplex stainless steel according to claim 1 or 2, wherein a pitting resistance index (PREW) defined by the following formula is 40 or more.
   (PREW) =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 30 ×% N
   (In the formula,% Cr,% Mo,% W and% N are values of each composition expressed by mass%.)
4. A duplex stainless steel structure using the duplex stainless steel according to any one of claims 1 to 3.
5. After preparing by forging or casting, solution heat treatment is performed at a temperature of 950 ° C. to 1200 ° C. for 30 minutes to 2 hours to make the austenite / ferrite phase ratio 0.2 to 0.8. Stainless steel structure.
6. A marine structure comprising the duplex stainless steel structure according to claim 4 or 5.
7. An oil and gas environment structure comprising the duplex stainless steel structure according to claim 4 or 5.
8. A pump impeller comprising the duplex stainless steel structure according to claim 4 or 5.
9. It is a duplex stainless steel structure of Claim 4 or 5, The pump casing characterized by the above-mentioned.
10. It is a duplex stainless steel structure of Claim 4 or 5, The valve body of the flow control valve characterized by the above-mentioned.

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