JP6286435B2 - Duplex stainless steel and duplex stainless steel structure using the same - Google Patents

Description

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

  A duplex stainless steel mainly composed of a two-phase metal structure of ferrite phase (α phase) and austenite phase (γ phase) has high strength and pitting corrosion resistance in chloride / sulfide environments. Excellent crevice corrosion resistance. Utilizing this property, it is widely used as a material for offshore structures and petrochemical industries. However, when exposed to high temperatures due to manufacturing conditions and use conditions, hard and brittle intermetallic compounds (σ phase, χ phase, Laves phase) and nitride / carbide embrittled phases mainly composed of Cr, Mo, etc. It is known that the toughness is reduced.

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

(PREW) =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 30 ×% N
(In the formula,% Cr,% Mo,% W and% N are values of each composition expressed in mass%.)
However, the higher the Cr, Mo, and W content, the easier the intermetallic compound precipitates. In addition, the higher the N, the easier it is for the nitride to precipitate, and if the amount added is excessive, defects due to blowholes occur during production.

  In the manufacturing process of the duplex stainless steel, in order to optimize the phase ratio between the ferrite phase and the austenite phase, after the solution heat treatment at 950 ° C. to 1200 ° C., avoid the precipitation of the aforementioned embrittlement phase and the 475 ° C. embrittlement. A quenching treatment such as water cooling is performed from the solution heat treatment temperature to room temperature. At that time, thin materials such as thin plates and pipes are not a big problem, but large structures, especially thick structures made by casting or forging, are caused by the difference in cooling rate between the surface and the interior. Since the embrittlement phase precipitates inside the material, there is a problem that stable production is difficult.

  In addition, there is a problem of toughness reduction due to the above-described embrittlement phase precipitation even in a place affected by heat due to welding or annealing for the purpose of removing residual stress.

  Until now, focusing on the material composition, a method for suppressing the embrittlement phase during production or use has been proposed.

  In Patent Document 1, in order to suppress the formation of intermetallic compounds that deteriorate the corrosion resistance and mechanical properties, for example, sigma (σ) phase and chi (χ) phase, Cr: 21.0% by weight. -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 0.0% 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 has 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, nitrides are easily formed, and the additive elements are not properly dissolved in the alloy, and embrittlement may proceed.

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

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

  According to the present invention, since the amount of nitrogen contained in the duplex stainless steel containing tantalum is small, formation of nitride can be suppressed. Furthermore, since this prevents tantalum metal tantalum from diffusing intermetallic compound-forming elements, it is possible to improve the corrosion resistance, embrittlement resistance, manufacturability, weldability and heat treatment properties of the duplex stainless steel. .

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 forged preparation material A. It is an external appearance photograph of the forged material B. It is an external appearance photograph of the forged material C. It is a gold phase observation result of the preparation material A. It is a gold phase observation result of the preparation material B. It is a gold 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 ferrite. It is a gold phase observation photograph of preparation material A after performing heat processing for 30 minutes at 800 ° C. It is a gold phase observation photograph of preparation material B after performing heat processing for 30 minutes at 800 ° C. It is the gold phase observation photograph of the preparation material C after performing the heat processing for 30 minutes at 800 degreeC. It is a graph which shows the measurement result of the Charpy impact value as it is in solution and after heat processing at 800 ° C. for 5 minutes. It is an electron micrograph of the preparation material A which is a comparative material. It is a graph which shows the result of having measured the density | concentration distribution of each element of the arrow direction in the analysis position (line segment) of FIG. 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 the concentration distribution of each element of the direction of an arrow in the 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 processing. It is a graph which compares and shows the pitting corrosion generation potential of an invention material and a comparative material. It is sectional drawing which shows the vertical axis diagonal flow seawater pump produced using invention material. It is sectional drawing which shows the flow control valve produced using invention material.

  TECHNICAL FIELD The present invention relates to a duplex stainless steel and a structure using the same, and more specifically, produced during the production of a highly corrosion resistant duplex stainless steel (in casting, forging, hot rolling or welding), during welding, and during heat treatment. By suppressing the formation of brittle phases (precipitates such as nitrides and carbides, intermetallic compounds such as sigma (σ) phase and chi (χ) phase), while maintaining high corrosion resistance, The present invention relates to a duplex stainless steel realizing embrittlement and manufacturability and a product using the same.

  The present application includes a plurality of means for solving the above problems. As an example, in order to suppress the formation of intermetallic compounds in the duplex stainless steel, Ta that inhibits diffusion of intermetallic compound forming elements is positively added. It is characterized by.

  That is, in mass%, 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. 0 to 12.0%, Cr: 20.0 to 40.0%, Mo: 7% or less, W: 6.5% or less, and Ta: 0.05 to 1.0% is added. Stainless steel. About N, More preferably, it is 0.3% or less.

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

  In addition, Si that promotes the formation of intermetallic compounds cannot be expected to inhibit diffusion due to Ta, and is preferably reduced to 0.5% or less. Further, from the viewpoint of corrosion resistance, it is preferable to limit the range of elements due to corrosion resistance and satisfy the pitting corrosion index (PREW) defined by the following formula of 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 in mass%.)
That is, in 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% And is a super duplex stainless steel satisfying a pitting corrosion index (PREW) of 40 or more.

  After being produced by forging or casting with an alloy of the above components, a 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. The structure made of a phase stainless steel can provide a product having good toughness, particularly the formation of an embrittled phase inside the structure is suppressed.

  Particularly useful as an alloy structure of the above components are pump impellers, pump casings, and flow control valves used in offshore structures, oil / gas environment structures, and chemical plant structures.

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

  In order to improve the manufacturability and embrittlement resistance of thick cast products, forged products and hot-worked products while maintaining high corrosion resistance, the present inventor has made embrittled phase precipitation by intermetallic compounds and carbonitrides. As a result of research on suppression technology, the following facts were found.

  First, a conventional embrittlement phase formation mechanism that does not contain Ta will be described.

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

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

  In the grain boundary region including the grain boundary 3, an intermetallic compound 6 and carbon / nitride 7 (carbide and nitride) are generated. These are also called embrittled phases. When this embrittlement phase is large, the material becomes brittle and the corrosion resistance, embrittlement resistance, manufacturability, weldability and heat treatment tend to be reduced.

  Next, an embrittlement phase formation suppression mechanism in the stainless steel of the present invention containing Ta will be described.

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

  In the case of this figure, the tantalum atoms 11 are more likely to occupy the vacancies 4 than the intermetallic compound forming elements 5, so that the diffusion of the intermetallic compound forming elements 5 is inhibited. Thereby, it is possible to prevent the nitride of the intermetallic compound forming element 5 from being generated in the grain boundary region 12.

  It is known that the intermetallic compound 6 is composed of a σ phase, a χ phase, etc., and is likely to precipitate on the α phase side starting from the α phase / γ phase interface. Elements constituting intermetallic compound 6 (intermetallic compound forming element 5), such as Cr, Mo, Si and W, are concentrated from the metal base material to the crystal grain boundary at the α-phase / γ-phase interface. To be deposited. 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 oversized elements having an atomic radius larger than the average atomic diameter of the elements constituting the stainless steel, and atomic vacancies (holes 4) in the metal matrix It is said that the vacancy 4 moves as a preferential diffusion path.

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

  There are several possible addition elements with large atomic radii, but generally, metal elements with large atomic radii have extremely low free energy for generating nitrides and carbides.

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

  From the above, Ta was selected as an additional element, which is relatively easy to be dissolved in the metal matrix during production and hardly precipitates as an intermetallic compound.

  Below, the reason for limiting the role and chemical composition range of the alloy element added to the duplex stainless steel according to the present invention will be described.

Chromium (Cr): 20.0-40.0%
Chromium is the most important basic element for maintaining the corrosion resistance of stainless steel. In the case of duplex stainless steel, since a two-phase structure of austenite and ferrite must be obtained, 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 set to 20% or more in consideration of the ratio (fraction). Further, if Cr _eq is increased, Ni _eq also needs to be increased, so the upper limit is set to 40% in consideration of economy. 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 in mass%. It is the value of each composition expressed.)
Ferrite phase fraction (volume%) = 55 × (Cr _eq / Ni _eq ) −66.1
Nickel (Ni): 3.0% to 12.0%
Nickel is a useful element that increases the overall corrosion resistance in relation to the corrosion resistance as an austenite stabilizing element, so it was made at least 3% or more. In consideration of the relationship between the chromium equivalent and the nickel equivalent, the phase ratio, and the economy, the upper limit is set to 12% or less. A more preferable range is 6% to 8%.

Molybdenum (Mo): 7.0% or less Molybdenum, together with chromium, is an important element for maintaining corrosion resistance and acts to stabilize the ferrite phase, but the addition promotes the formation of intermetallic compounds. 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 corrosion resistance, delays the precipitation rate of intermetallic compounds by substituting 1/2 of Mo, and improves corrosion resistance and mechanical properties. is there. However, tungsten is an expensive alloy element, and when added in a large amount, promotes the formation of intermetallic compounds and lowers the corrosion resistance of the welded portion, 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. Moreover, it is an element which increases the fluidity | liquidity of the molten steel at the time of manufacture and welding, and reduces the surface defect. However, since it is an element that increases the precipitation rate of intermetallic compounds and decreases the ductility of steel, 3.0% or less is preferable. 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 solubility of nitrogen and increases the deformation resistance at high temperature. In particular, when nitrogen is actively added to improve the corrosion resistance, it is essential to add an appropriate amount of manganese. Although it has a deoxidizing effect during melting and refining, if it is added in a large amount, the corrosion resistance is lowered and the formation of intermetallic compounds is promoted. 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. Its effect is one of the most important elements in relation to corrosion resistance, which is 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 supplement the strength. However, if added over 0.7%, cracks may occur due to blowholes during production. Therefore, it is preferable to set it as 0.7% or less. In particular, when Ta is added, a nitride containing Ta is formed and the effect is hindered. For this reason, it is more preferable to set it as 0.3% or less, and it is 0.05%-0.25% in order not to impair corrosion resistance in the α-phase and γ-phase in a solid solution It is even more preferable. Furthermore, the 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, it is preferable that the content is as small as possible because 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 set to 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 mentioned above, the atomic radius is large compared to the average atomic radius of the elements that make up duplex stainless steel, which prevents the diffusion of the main intermetallic compound-forming elements and reduces the precipitation rate of intermetallic compounds. There is. However, if the addition amount is too large, not only is it not economical, but the upper limit value is limited to 1.0% in order to destroy the balance of the ferrite / austenite phase ratio. On the other hand, if it is less than 0.05%, the addition effect cannot be expected. Further, it is more preferably in the range of 0.2 to 0.5% from the balance of the solid solution amount in the nitride phase and the ferrite phase.

Phosphorus (P): 0.1% or less Phosphorus is an impurity that is inevitably mixed in steel and not only deteriorates corrosion resistance but also segregates at grain boundaries and promotes precipitation of an embrittled phase. desirable. For this reason, 0.1% or less is desirable, 0.02% or less is more desirable, and 0.005% or less is particularly desirable. However, when P is excessively reduced, the manufacturing cost increases. Therefore, the addition amount of P is determined in consideration of this point.

  Examples will be described below.

  Table 1 shows the chemical composition (unit: mass%) of the duplex stainless steels of Example 1 (invention material (preparation material C)) and Comparative Examples 1 and 2 (comparative materials (preparation materials A and B)). Is.

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

  Ingots were obtained by melting 20 kg of duplex stainless steels having chemical compositions shown in Table 1 in a vacuum melting furnace. The preparation material A has the same components as the standard material S32750. The preparation material B has a reduced content of N, C and Si. The preparation material C is obtained by adding a small amount of Ta to an alloy having the same components as the preparation material B.

  The ingot was heated to 1250 ° C. and forged to obtain a plate material of 20 × 50 × 150 (mm). The forged plate material was subjected to a 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 with water to avoid precipitation of the embrittled phase.

  2A, 2B, and 2C show appearance photographs of the forged materials A, B, and C, respectively.

  From these photographs, it can be seen that the product can be produced without causing cracks or defects due to forging.

  3A, 3B, and 3C show the observation results of the gold phases of the manufactured materials A, B, and C after manufacture, respectively.

  In the gold phase observation, polishing was performed up to No. 2000 with SiC polishing paper, and final polishing was performed using 1 μm diamond abrasive grains, and then electrolytic etching was performed using a 10% NaOH aqueous solution. Thereby, the ferrite phase 31 is colored brown, and the intermetallic compound, carbide and nitride are colored black. The austenite phase 32 is white.

  The test piece was subjected to ultrasonic cleaning using acetone and distilled water, and then observed with an optical microscope. Subsequent observation of the gold phase is carried out in the same way. As a result of the observation of the gold phase shown in these figures, it can be seen that each of the fabricated materials has a two-phase structure of ferrite / austenite that can be clearly distinguished.

  In order to evaluate the embrittlement phase precipitation in the reheating conditions by cooling and welding at the time of manufacture, heat treatment was performed at 800 ° C., which is a temperature range in which the embrittlement phase is likely to precipitate.

  FIG. 4 is a graph showing the relationship between the heat treatment time at 800 ° C. and the amount of residual ferrite. The horizontal axis represents the heat treatment time, and the vertical axis represents the ferrite content. The amount of ferrite was measured with a ferrite scope using a magnetic induction method. Among the embrittlement phase precipitation, intermetallic compound precipitation proceeds by the decomposition of the ferrite phase into an intermetallic compound phase and an austenite phase at the precipitation temperature condition. Can be evaluated.

  From this figure, it can be seen that, compared with Comparative Examples 1 and 2, Example 1 has a slower decrease rate of the ferrite phase and suppresses the decomposition of the ferrite phase.

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

  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.

  Compared with the preparation materials A and B which are comparative materials, in the preparation material C which is an invention material, the precipitation amount of the embrittlement phase 53 is small, and it can be seen that precipitation of the embrittlement phase 53 is suppressed. In the preparation material B, the embrittlement phase 53 is particularly large.

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

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

  With respect to the plate material before and after the heat treatment, a 2 mm V notch Charpy test piece of 10 mm × 10 mm × 55 mm was taken from the longitudinal direction of the plate so that the center portion became 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 treatment is high. This shows that the toughness is improved by suppressing the formation of intermetallic compounds.

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

  FIG. 7A is an electron micrograph of the preparation 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 preparation material C which is the invention material, and FIG. 7D shows the result of measuring the concentration distribution of each element in the arrow direction at the analysis position (line segment) in FIG. 7C. It is.

  7A and 7C, the fine structure of the ferrite phase 71 and the austenite phase 72 is clearly shown. Grain boundaries are indicated by broken lines. At the analysis position 73 represented by a line segment, the concentration of each element was measured in the direction of the arrow (from the austenite phase 72 toward the ferrite phase 71).

  7B and 7D, the horizontal axis represents distance and the vertical axis represents density.

  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.

  In contrast, in the inventive material shown in FIG. 7D, a Ta concentration peak occurs in the vicinity of the grain boundary on the ferrite phase side, and the concentrations of Mo and Cr are lower than in FIG. 7B.

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

  From the above, it has been found that when Ta is added, Ta itself diffuses into the grain boundary, thereby inhibiting the diffusion of intermetallic compound-forming elements such as Mo and Cr and delaying the formation of intermetallic compounds.

(Effect of heat treatment on residual stress and impact value)
Assuming post-weld heat treatment (PWHT) for residual stress relaxation, the heat treatment was performed on the inventive material and the comparative material, and the influence of the heat treatment on the residual stress and impact value was evaluated.

  About the preparation materials A, B, and C, using a grindstone of particle size # 46, a tensile working stress was imparted by imparting a strong working layer by surface grinding to the plate material surface at a rotational speed of 1440 rpm and a cutting depth of 0.01 mm. A test material having a residual stress applied to the surface by surface grinding was subjected to heat treatment at 650 ° C. for 30 minutes assuming PWHT, and the influence of the heat treatment conditions on the residual stress and mechanical properties was evaluated.

  FIG. 8 shows the result of comparing the residual stress before and after the heat treatment. The residual stress was measured for each of the ferrite phase and austenite phase, and the average value multiplied by the volume ratio was evaluated as macro stress.

  By surface processing, a tensile stress of about 900 to 1100 MPa was applied, but the heat treatment at 650 ° C. × 30 minutes decreased to a tensile stress of about 200 MPa, and a stress relaxation effect of about 80% was obtained. .

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

From this figure, compared with the comparative material, the inventive material has improved impact value, and has left an impact value of about 100 J / cm ² even after heat treatment, and the residual stress is reduced by 80% by heat treatment at 650 ° C. × 30 minutes. However, the impact value of 100 J / cm ² or more is maintained.

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

  The pitting corrosion potential was measured according to JIS G0577 (2005).

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

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

  Fabrication material C (invention material)> Fabrication material B (comparative material)> Fabrication material A (comparative material, conventional material S32750 equivalent).

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

  From the above results, it was confirmed that the inventive material has pitting corrosion resistance higher than that of the conventional material, despite suppressing the 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 shaft mixed-flow seawater pump efficiently utilizes bell mouth 117 for rectifying seawater entering from the suction channel, shaft 111 for transmitting the rotational power of the prime mover, impeller hub 115 fixed to shaft 111, and rotational power for the prime mover efficiently. Impeller vane 113 applied to seawater, casing liner 114 having a spherical inner surface so that the outer peripheral clearance of impeller vane 113 is always constant, casing 112 converting velocity energy applied to seawater from impeller vane 113 into pressure energy, pressurization It consists of a column pipe 119 through which the seawater passes, an impeller cap 116, a cone 118, and the like.

  The casing liner 114 and the casing 112 were each made of cast steel of Example 1, and the impeller hub 115 and the impeller vane 113 were each made of forging material of Example 1. After casting and forging, a solution heat treatment at 1100 ° C. × 1 h was performed, followed by water cooling to obtain a two-phase composition with a ferrite content of 40-50%. Thereafter, the joint portion between the casing liner 114 and the casing 112 and the joint portion 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 was performed at that temperature for 30 minutes, and the mixture was rapidly cooled.

  When the 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, it was possible to produce a seawater pump with a long service life in which a decrease in toughness of the welded portion was suppressed and fatigue strength was improved.

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

  In this figure, the flow rate adjusting valve includes a casing 121 that supports the entire valve, a valve body 122 that adjusts the flow rate, a valve seat 123 that accommodates the valve body 122, a handle 125, and a shaft that adjusts the position of the valve body 122 by the rotation of the handle 125. 124 or the like.

  The casing 121 was made of the cast steel of Example 1. By using the steel material of Example 1, the corrosion resistance was high and a large flow control valve could be manufactured.

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

  1: Ferrite phase, 2: Austenite phase, 3: Grain boundary, 4: Void, 5: Intermetallic compound forming element, 6: Intermetallic compound, 7: Carbon / nitride, 11: Tantalum atom, 12: Grain boundary Region 31, 51, 71: ferrite phase, 32, 52, 72: austenite phase, 53: embrittlement 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 (2)

% By mass
N: 0.05-0.25%
C: 0.02% or less (C is not an essential component but includes 0%),
P: 0.02% or less (P is not an essential component but includes 0%),
Si: 0.5% or less (Si is not an essential component but includes 0%),
Mn: 1.2% or less (Mn is not an essential component but includes 0%),
Ni: 6.0 to 8.0%,
Cr: 24.0 to 26.0%,
(Mo + 0.5 × W): 3.0 to 5.0% (However, Mo: 7.0% or less, W: 6.5% or less, and either Mo or W is an essential component. Not 0%)
Ta: contains 0.2 to 0.5%, the balance is Fe and inevitable impurities,
Pitting index defined by the following formula (PREW) is you characterized in that 40 or more duplex stainless steel.
(PREW) =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 30 ×% N
(In the formula,% Cr,% Mo,% W and% N are values of each composition expressed in mass%.)   A duplex stainless steel structure using the duplex stainless steel according to claim 1.

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