CN105492641A

CN105492641A - 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

Info

Publication number: CN105492641A
Application number: CN201380079140.6A
Authority: CN
Inventors: 冲崎直也; 丸野裕策; 能岛雅史
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2013-08-28
Filing date: 2013-08-28
Publication date: 2016-04-13
Also published as: JP6286435B2; EP3040434B1; JPWO2015029167A1; EP3040434A1; WO2015029167A1; EP3040434A4

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 use it two-phase stainless steel works, marine structure, oil/atmosphere surrounding works, pump impeller, pump case and flow control valve valve body

Technical field

The present invention relates to duplex stainless steel and use its works.

Background technology

The duplex stainless steel formed primarily of the metal structure of 2 phases of ferrite phase (α phase) and austenite phase (γ phase) has high intensity, and resistance to pitting under muriate/sulfide environment and the excellent of resistance to crevice corrosion.Utilize this characteristic, the material as marine structure or petrochemical industry etc. is widely used.But, when being exposed to high temperature according to manufacturing condition or working conditions, there will be a known the brittle phase formed with the hard and crisp intermetallic compound (σ phase, χ phase, Laves phase) that is principal constituent such as Cr, Mo or nitride/carbide, toughness reduces.

In duplex stainless steel, the index of resistance to pitting (PREW) that following formula represents is higher, and erosion resistance more improves.

(PREW)＝％Cr+3.3×(％Mo+0.5×％W)+30×％N

(in formula, %Cr, %Mo, %W and %N are the value of each composition represented with quality %.)

But the content of Cr, Mo and W is higher, and intermetallic compound is more easily separated out.In addition, N is higher, and nitride is more easily separated out, and when addition is superfluous, produces the defect caused by bubble during fabrication.

In the manufacturing process of duplex stainless steel, in order to ferrite phase and austenite phase compared optimization, after 950 DEG C ~ 1200 DEG C are carried out solution thermal treatment, in order to avoid precipitation or 475 DEG C of embrittlement of above-mentioned brittle phase, implement the quenching process that water-cooled etc. causes from solution thermal treatment temp to room temperature.Now, very large problem is not become to the thin-walled material such as thin plate or pipe arrangement, but in large-scale structure product, in the works of the heavy wall particularly made by casting or forge, result from the difference of surface and inner speed of cooling, separate out mutually in material internal embrittlement, therefore, there is the problem of stable manufacture difficulty.

In addition, in the place being subject to welding the heat affecting caused or annealing for the purpose of residual stress removing etc., also there is above-mentioned embrittlement and separate out the problem that the toughness that causes reduces mutually.

Up to now, be conceived to material composition, propose when suppressing to manufacture or the method for the brittle phase in the time of use.

In patent documentation 1, with suppress to make the intermetallic compound of erosion resistance and mechanical properties deterioration, such as Sigma (σ) mutually and open (χ) phase be formed as object, disclose containing counting Cr:21.0% ~ 38.0%, Ni:3.0% ~ 12.0%, Mo:1.5% ~ 6.5%, W:0 ~ 6.5%, below Si:3.0%, below Al:1.0%, below Mn:8.0%, N:0.2% ~ 0.7%, below C:0.1% with % by weight; And below B:0.1%, below Cu:3.0%, below Co:3.0% the super duplex stainless steel of at least one.Also this super duplex stainless steel is described further preferably containing the element be selected from by more than one in the group that below Ca:0.5%, below Mg:0.5%, below Ta:0.5%, below Nb:0.5%, below Ti:1.5%, below Zr:1.0%, below Sn:1.0% and below In:1.0% are formed.

Prior art document

Patent documentation

Patent documentation 1: JP 2011-174183 publication

Summary of the invention

Invent problem to be solved

When patent documentation 1, because the content of nitrogen is many, therefore, easily form nitride, Addition ofelements has in the alloy solid solution inadequately and carries out brittle worry.

The object of the invention is to, in duplex stainless steel, suppress the formation of intermetallic compound (σ phase, χ phase, Laves phase) and nitride, improve erosion resistance, brittle resistance, manufacturing, weldability and heat-treatability.

For solving the means of problem

The feature of duplex stainless steel of the present invention is, containing is below N:0.3%, below C:0.1%, below P:0.1%, below Si:3.0%, below Mn:8.0%, Ni:3.0 ~ 12.0%, Cr:20.0 ~ 40.0%, below Mo:7.0%, below W:6.5%, Ta:0.05 ~ 1.0% in mass %; Surplus is Fe and inevitable impurity.

Invention effect

According to the present invention, because the amount containing nitrogen contained in the duplex stainless steel of tantalum is few, therefore, it is possible to suppress the formation of nitride.And then thus, the metal tantalum not becoming nitride hinders the diffusion of intermetallic compound forming element, therefore, it is possible to the erosion resistance of raising duplex stainless steel, brittle resistance, manufacturing, weldability and heat-treatability.

Accompanying drawing explanation

Figure 1A is the concept map of the brittle phase Forming Mechanism represented in the existing steel of existing two-phase stainless.

Figure 1B represents that the embrittlement in the existing steel of two-phase stainless of the present invention forms the concept map of suppression mechanism mutually.

Fig. 2 A is the outward appearance photo of the manufactured materials A of forging.

Fig. 2 B is the outward appearance photo of the manufactured materials B of forging.

Fig. 2 C is the outward appearance photo of the manufactured materials C of forging.

Fig. 3 A is the metallographic observation results of manufactured materials A.

Fig. 3 B is the metallographic observation results of manufactured materials B.

Fig. 3 C is the metallographic observation results of manufactured materials C.

Fig. 4 is the coordinate diagram representing the heat treatment time of 800 DEG C and the relation of remaining ferrite amount.

Fig. 5 A is the metallographicobservation photo of the manufactured materials A after enforcement 800 DEG C, the thermal treatment of 30 minutes.

Fig. 5 B is the metallographicobservation photo of the manufactured materials B after enforcement 800 DEG C, the thermal treatment of 30 minutes.

Fig. 5 C is the metallographicobservation photo of the manufactured materials C after enforcement 800 DEG C, the thermal treatment of 30 minutes.

Fig. 6 is the coordinate diagram of the measurement result of Charpy's impact value after representing the solution and thermal treatment of 800 DEG C, 5 minutes same as before.

Fig. 7 A is the electron micrograph of the manufactured materials A as comparative material.

Fig. 7 B be represent determine Fig. 7 A analysis position (line segment) on the coordinate diagram of result of concentration distribution of each element of the direction of arrow.

Fig. 7 C is the electron micrograph of the manufactured materials C as invention material.

Fig. 7 D be represent determine Fig. 7 C analysis position (line segment) on the coordinate diagram of result of concentration distribution of each element of the direction of arrow.

Fig. 8 is the coordinate diagram of the result representing the residual stress compared before and after thermal treatment.

Fig. 9 is the coordinate diagram of the result of the result representing the charpy impact test compared before and after thermal treatment.

The pitting of invention material and comparative material is produced current potential to compare and the coordinate diagram that represents by Figure 10.

Figure 11 is the sectional view representing the vertical shaft oblique flow sea-water pump using invention material to make.

Figure 12 is the sectional view representing the flow control valve using invention material to make.

Embodiment

The present invention relates to duplex stainless steel and use its works, more specifically, when relating to by suppressing (when casting, forging, hot rolling or welding), welding when the manufacture of high corrosion resistance duplex stainless steel and thermal treatment time the formation of brittle phase (precipitate such as nitride, carbide, Sigma (σ) phase, open (χ) equal intermetallic compound) that generates, while maintain high corrosion resistance, realize the duplex stainless steel of more excellent brittle resistance and manufacturing and use its goods.

The application comprises the means of the above-mentioned problem of multiple solution, as one example, it is characterized in that, in order to suppress the intermetallic compound in duplex stainless steel to be formed, actively adds the Ta of the diffusion hindering intermetallic compound forming element.

Namely, for a kind of stainless steel, it is characterized in that, adding is below N:0.7%, below C:0.1%, below P:0.1%, below Si:3.0%, below Mn:8.0%, Ni:3.0 ~ 12.0%, Cr:20.0 ~ 40.0%, below Mo:7%, below W:6.5%, Ta:0.05 ~ 1.0% in mass %.About N, more preferably less than 0.3%.

More preferably adding to not damage Ta the effect caused, being preferably set to N:0.05 ~ 0.25%, below C:0.02%, suppress the formation of nitride and carbide.N particularly preferably 0.05 ~ 0.19%.

In addition, promote that the Si of the formation of intermetallic compound can not expect therefore, to be preferably reduced to less than 0.5% by the diffusion blocking that Ta causes.In addition, from the view point of erosion resistance, preferably restriction results from the scope of element of erosion resistance, and meeting with the index of resistance to pitting (PREW) of following formula definition is more than 40.

(PREW)＝％Cr+3.3×(％Mo+0.5×％W)+30×％N

Namely, for one surpasses duplex stainless steel, it is in the scope being N:0.05 ~ 0.25%, below C:0.02%, below P:0.02%, below Si:0.5% below Mn:1.2%, Ni:6.0 ~ 8.0%, Cr:24.0 ~ 26.0%, Mo:3.0 ~ 5.0%, below W:4.0%, Ta:0.2 ~ 0.5% in mass %, and meeting the index of resistance to pitting (PREW) is more than 40.

With the alloy of mentioned component by after forging or casting making, implement the solution thermal treatment of 30 minutes ~ 2 hours the temperature of 950 DEG C ~ 1200 DEG C and austenite/ferrite compared the two-phase stainless steel works being set to 0.2 ~ 0.8, the goods suppressing the embrittlement of the inside of special works to form, have good toughness mutually can be provided.

As the thing that the works of the alloy of mentioned component is particularly useful be marine structure or oil/atmosphere surrounding works, the pump impeller, pump case and the flow control valve that use in chemical plant's works.

Below, accompanying drawing is used to be described to embodiments of the present invention.

The present inventor is in order to keep manufacturing and the brittle resistance of the former state of high erosion resistance, the cast product improving heavy wall, forged article and hot-work product, the embrittlement caused intermetallic compound and carbonitride is separated out suppression technology mutually and is studied, and found that the fact as described below.

First, the brittle phase Forming Mechanism of the conventional example not containing Ta is described.

In this figure, duplex stainless steel comprises: ferrite mutually 1, austenite mutually 2, be formed at grain circle 3 between them.In ferrite mutually 1, Cr, Mo, W etc. as the element (intermetallic compound forming element 5) forming intermetallic compound spread via emptying aperture 4, move to grain circle 3.

In the grain boundary region comprising a boundary 3, produce intermetallic compound 6 and carbon/nitride 7 (carbide and nitride).These are also referred to as brittle phase.When more than this embrittlement mutually, there is the tendency that material roughening, erosion resistance, brittle resistance, manufacturing, weldability and heat-treatability reduce.

Then, be described forming suppression mechanism mutually containing the embrittlement in the of the present invention stainless existing steel of Ta.

When this figure, a side of tantalum atom 11 easily occupies emptying aperture 4 compared with intermetallic compound forming element 5, therefore, hinders the diffusion of intermetallic compound forming element 5.Thereby, it is possible to prevent the nitride etc. producing intermetallic compound forming element 5 in grain boundary region 12.

Between known metal compound 6 by σ phase, χ is equal forms, with α phase/γ phase interface for starting point is easily separated out in α phase side.Cr, Mo, Si and W as the element (intermetallic compound forming element 5) forming intermetallic compound 6 concentrate at the grain boundary of α phase/γ phase interface from base material metal, separate out as intermetallic compound 6.Therefore think, if the velocity of diffusion of these intermetallic compound forming elements 5 can be reduced, then can postpone the precipitation of intermetallic compound 6.Can think, the average atom footpath of Cr, Mo and W in these elements and the stainless element of formation is in a ratio of the oversize element with large atomic radius, strong with the interaction of the atom emptying aperture (emptying aperture 4) in base material metal, emptying aperture 4 is moved as preferential diffusion pathway.

Owing to having such phenomenon, to add compared with these elements atomic radius large and with emptying aperture 4 more strongly interactional element, make emptying aperture 4 be added element to catch, hinder the diffusion of intermetallic compound forming element 5 to be important.Thus, particularly become in the temperature province of 650 DEG C ~ 950 DEG C of problem in the precipitation of brittle phase the velocity of diffusion of intermetallic compound forming element 5 is reduced.When stainless steel (steel) that size is little, the embrittlement in this temperature province can be avoided by quenching, but large and become problem quenching difficulty is carried out in the inside of steel in size.The present invention is solved by the composition of adjustment steel for this problem.

As the candidate of the large Addition ofelements of atomic radius, consider several, but generally speaking, the metallic element that atomic radius is large, the free energy level generating nitride or carbide is low.

The result of heat calculation, when with the addition of the element that the nitrogen such as Zr, Ti, Hf/carbide formation can be high, particularly when improving the super duplex stainless steel of erosion resistance by adding nitrogen, form nitride in the stage of liquid phase during fabrication, to the solid solution difficulty in parent phase.In addition, nitride formed can be lower element among, Nb is considered to himself easy group of element entering the σ phase of intermetallic compound.

Consider from above situation, in metal parent phase, solid solution ratio is easier to during fabrication, and the selected Ta being difficult to separate out as intermetallic compound is as Addition ofelements.

Below, the reason being limited to alloys producing and the chemical composition range added in duplex stainless steel of the present invention is described.

Chromium (Cr): 20.0 ~ 40.0%

Chromium is most important fundamental element in the maintenance of stainless erosion resistance.When duplex stainless steel, austenite and ferritic 2 phase constitutions must be obtained, therefore, the chromium equivalent (Cr that consideration defines by following formula _eq) and nickel equivalent (Ni _eq) and the ratio (point rate) of ferrite phase that determines thus, be set to the chromium amount of more than 20%.In addition, Cr is made _eqduring increase, Ni _eqalso must increase, therefore, consider economy and higher limit is set to 40%.Preferred scope is 24% ~ 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

(in formula, %Cr, %Si, %Mo, %W, %V, %Al, %Nb, %Ti, %Ni, %Mn,

%C, %Cu, %N and %Co are the values of each composition represented with quality %.)

Point rate (volume the %)=55 × (Cr of ferrite phase _eq/ Ni _eq)-66.1

Nickel (Ni): 3.0% ~ 12.0%

Nickel associates with erosion resistance as austenite stabilizer element and makes the useful element that general corrosion patience increases, and therefore, is at least set to more than 3%.Consider chromium equivalent and the relation of nickel equivalent, the ratio of phase and economy, higher limit is set to less than 12%.Preferred scope is 6% ~ 8%.

Molybdenum (Mo): less than 7.0%

Molybdenum and chromium are element important in the maintenance of erosion resistance simultaneously, have the effect making ferrite phase stabilization, but promote that by adding intermetallic compound is formed.Therefore, measured and be limited in less than 7.0%.Preferred scope is 3.0% ~ 5.0%.

Tungsten (W): less than 6.5%

Tungsten is that the Mo by making erosion resistance improve, be replaced into the amount of 1/2 makes the speed of separating out of intermetallic compound postpone, improves the element of erosion resistance and mechanical properties.But tungsten is the alloying element of high price, in addition, when adding in large quantities, promote the generation of intermetallic compound, the erosion resistance of weld part is reduced, therefore, content is limited in less than 6.5%.Preferred scope is less than 4.0%.

Silicon (Si): less than 3.0%

Silicon is the element making ferrite tissue stabilization, for manufacture time the effective element of deoxidation.In addition, for when making manufacture or welding time the mobility of molten steel increase and reduce the element of surperficial defect.But, due to the element that the ductility for making the speed of separating out of intermetallic compound increase, make steel reduces, therefore, preferably less than 3.0%.Be more preferably less than 0.5%.

Manganese (Mn): less than 8.0%

Manganese is the austenite stabilizer element of the nickel that can replace high price, is the element that the resistance to deformation making the solid solubility of nitrogen increase, make high temperature increases.When particularly erosion resistance being improved when adding nitrogen energetically, the manganese of appropriate amount must be added.When dissolving refining, there is deoxidation effect, but when adding in large quantities, erosion resistance reduces, and promotes the generation of intermetallic compound.Therefore, its higher limit is limited in less than 8%.Preferred scope is less than 1.2%.

Nitrogen (N): less than 0.7%

Nitrogen is the useful element making to improve relative to the patience of pitting.Its effect associates with the erosion resistance of about 30 times that reach chromium, is one of most important element.In addition, in case tablet circle acumen turns to object and reduces carbon content, nitrogen can be added and fills up intensity.But, when adding more than 0.7%, sometimes produced by bubble during fabrication and ftracture.Therefore, preferably less than 0.7% is set to.Particularly when adding Ta, forming the nitride containing Ta, hindering effect.Therefore, in order to balancedly be solid-solution in α phase and γ phase, not damage erosion resistance, be preferably set to less than 0.3% further, be further preferably set to 0.05% ~ 0.25%.And then, the particularly preferably scope of 0.05 ~ 0.19%.

Carbon (C): less than 0.1%

The element of grain circle acumenization when carbon is formation carbide and brings out welding.Particularly when adding Ta, forming the carbide containing Ta, hindering the effect that Ta adds, therefore, more few more preferred.But the reduction due to C causes the rising of manufacturing cost, therefore, be set to less than 0.1%.Preferred scope is less than 0.02%.

Tantalum (Ta): 0.05% ~ 1.0%

Tantalum is for making one of characteristic element of band of the present invention.As mentioned above, because atomic radius is large compared with the average atom radius of the element of formation duplex stainless steel, therefore, there is the diffusion stoping main intermetallic compound forming element, the effect that the speed of separating out of intermetallic compound is reduced.But, when addition is too much, not only uneconomical, and make ferrite/austenitic balance collapse compared, therefore, higher limit is limited in 1.0%.On the other hand, when its lower than 0.05% time, its additive effect can not be expected.In addition, from the viewpoint of the balance of the solid solution capacity to Nitride Phase and ferrite phase, the scope of 0.2 ~ 0.5% is more preferably.

Phosphorus (P): less than 0.1%

About phosphorus, for being mixed into inevitable impurity in steel, not only make erosion resistance deterioration, and segregation in grain circle, promote the precipitation of brittle phase, therefore, more few more preferred.Therefore, be preferably less than 0.1%, more preferably less than 0.02%, be particularly preferably less than 0.005%.But, when excessively being reduced by P, cause the rising of manufacturing cost.Therefore, the addition of P is also considered this point and determines.

Below, embodiment is described.

Embodiment

Table 1 represents chemical constitution (unit: quality %) for the duplex stainless steel of embodiment 1 (invention material (manufactured materials C)) and comparative example 1 and 2 (comparative material (manufactured materials A and B)).

The thermal treatment of the cooling that enforcement simulates when manufacturing in these materials and the reheating that welding causes, compares.

In vacuum melting furnace, the duplex stainless steel of the chemical constitution shown in molten tabulation 1 20kg respectively, obtains blank.Manufactured materials A has the equal composition with gauge material S32750.Manufactured materials B makes the content of N, C and Si reduce.Manufactured materials C adds the Ta of trace relative to the alloy with the equal composition of manufactured materials B.

[table 1]

Table 1

※ 1PREW value=Cr+33 (Mo+0.5W)+30N (mass%)

By above-mentioned blank heating to 1250 DEG C, forge, obtain the sheet material of 20 × 50 × 150 (mm).In order to obtain comparing of suitable ferrite phase/austenite phase, the sheet material of forging after 1100 DEG C × 1 hour implements solution thermal treatment, in order to avoid the precipitation of brittle phase, the quenching by water-cooled.

Fig. 2 A, 2B and 2C represent the outward appearance photo of manufactured materials A, B and C of forging respectively.

Known from these photos: can not to produce and forge the cracking that causes or defect and manufacture.

Fig. 3 A, 3B and 3C represent the metallographic observation results of manufactured materials A, B and C after manufacture respectively.

In metallographicobservation, be polished to No. 2000 with SiC polishing paper, use after the diamond abrasive grain of 1 μm carries out precision work polishing, use the 10%NaOH aqueous solution to implement electrolytically etching.Thus, ferrite 31 is colored as brown mutually, and intermetallic compound, carbide and nitride are colored as black.In addition, austenite phase 32 is white.

For test film, after using acetone and distilled water enforcement ultrasonic cleaning, observe with opticmicroscope.Later metallographicobservation also uses the same method enforcement.The result of the metallographicobservation shown in these figure is known: any one manufactured materials also has the ferrite/austenitic duplex structure that can clearly distinguish.

In order to the embrittlement under the heat condition again that cooling when manufacturing relative to above-mentioned manufactured materials evaluation and welding cause is separated out mutually, implement the thermal treatment of 800 DEG C as the brittle temperature province easily separated out mutually.

Fig. 4 is the coordinate diagram representing the heat treatment time of 800 DEG C and the relation of remaining ferrite amount.Being heat treatment time with transverse axis, take the longitudinal axis as ferrite amount.Ferrite amount is measured by the ferrite analyser employing magnetic induced method.It is that intermetallic compound and austenite phase are carried out that intermetallic compound in brittle precipitation is mutually separated out by ferrite phase decomposition under Precipitation Temperature condition, therefore, by evaluating remaining ferrite amount, can evaluate the precipitation tendency of intermetallic compound.

From this figure: compared with comparative example 1 and 2, the minimizing speed of the ferrite phase of embodiment 1 is slow, suppresses the decomposition of ferrite phase.

Fig. 5 A, 5B and 5C be respectively enforcement 800 DEG C, 30 minutes thermal treatment the metallographicobservation photo of manufactured materials A, B and C.

In this figure, known: except ferrite phase 51 and austenite mutually 52, brittle 53 increases mutually.

Known: compared with manufactured materials A and B as comparative material, as in the manufactured materials C of invention material, brittle mutually 53 amount of precipitation few, suppress brittle mutually 53 precipitation.The brittle 53 special changes mutually of manufactured materials B are many.

Fig. 6 represents the measurement result of the Charpy's impact value after the thermal treatment of 800 DEG C × 5 minutes.

About Charpy's impact value, measure according to JISZ2242 (2005).The outline of determination step is as described below.

Sheet material before and after thermal treatment, the mode that becomes notch portion with central part are taked to the 2mmV recess summer coomb's test Coomb sheet of 10mm × 10mm × 55mm from the longitudinal direction of plate, measure impact value.

From this figure: compared with manufactured materials A and B as comparative material, as in the manufactured materials C of invention material, the Charpy's impact value after thermal treatment raises.Thus, toughness is improved by suppressing the formation of intermetallic compound.

Fig. 7 A ~ 7D represents the result that the EDX in grain circle (α/γ border) after the thermal treatment of 800 DEG C × 1 minute measures.

Fig. 7 A is the electron micrograph of the manufactured materials A as comparative material, Fig. 7 B represent determine Fig. 7 A analysis position (line segment) on the result of concentration distribution of each element of the direction of arrow.

In addition, Fig. 7 C is the electron micrograph of the manufactured materials C as invention material, Fig. 7 D represent determine Fig. 7 C analysis position (line segment) on the result of concentration distribution of each element of the direction of arrow.

In Fig. 7 A and 7C, clearly represent the microtexture of ferrite phase 71 and austenite phase 72.Grain circle is represented by dashed line.In the analysis position 73 represented by line segment, carry out the concentration determination of each element in the direction (from austenite phase 72 to ferrite phase 71) of arrow.

In Fig. 7 B and 7D, being distance with transverse axis, take the longitudinal axis as concentration.

In the comparative material shown in Fig. 7 B, the concentration of Mo and Cr in the vicinity of grain circle of ferrite phase side raises.

On the other hand, in the invention material shown in Fig. 7 D, near grain circle of ferrite phase side, produce the peak of the concentration of Ta, concentration step-down compared with Fig. 7 B of Mo and Cr.

In other words, known: Ta preferentially spreads in α/γ grain circle, hinder the diffusion of Mo and Cr as intermetallic compound forming element.

As known from the above: when with the addition of Ta, by Ta from spreading in grain boundary, hindering the diffusion of the intermetallic compound forming elements such as Mo, Cr, the formation of intermetallic compound is postponed.

(impact that thermal treatment brings residual stress and impact value)

Imagine the welding postheat treatment (PWHT) for the purpose of residual stress relaxes, thermal treatment is implemented to invention material and comparative material, evaluates the impact that thermal treatment brings residual stress and impact value.

About manufactured materials A, B and C, use the grinding stone of granularity #46, the strong machined layer caused plate surface imparting plane grinding with speed of rotation 1440rpm, approach 0.01mm, gives stretching residual stress thus.With respect to plane grinding effects on surface give residual stress for examination material, imagination PWHT and implement the thermal treatment of 650 DEG C × 30 minutes, evaluates the impact on the heat-treat condition that residual stress and mechanical characteristics are brought.

Fig. 8 represents the result that the residual stress before and after by thermal treatment compares.Measure residual stress mutually with ferrite phase, austenite respectively, the mean value being multiplied by volume ratio gained is evaluated as microstress.

Stress is paid in the stretching of being given about 900 ~ 1100MPa by surface working, but the thermal treatment passed through 650 DEG C × 30 minutes is all reduced to the tensile stress of about about 200MPa, can obtain the stress alleviation effects of about 8 one-tenth.

The result that the result of the charpy impact test that Fig. 9 represents before and after by thermal treatment compares.

From this figure: relative to comparative material, the impact value of invention material is improved, and also remains 100J/cm after thermal treatment ²the impact value of left and right, relaxes 8 one-tenth residual stresss, while keep 100J/cm by the thermal treatments of 650 DEG C × 30 minutes ²above impact value.

(impact that thermal treatment brings pitting generation current potential)

The result that pitting before and after thermal treatment (650 DEG C × 30 minutes) produces potential measurement is shown in following.

About Pitting Potential, measure according to JISG0577 (2005).

The pitting of invention material and comparative material is produced current potential and compares and represent by Figure 10.

As shown in this figure, pitting patience order (after thermal treatment) of each material is as described below.

Manufactured materials C (invention material) > manufactured materials B (comparative material) > manufactured materials A (comparative material, be equivalent to material S32750 in the past).

That is, invention material has the high pitting generation current potential of material than ever.

Results verification by above: invention material not only suppresses brittle, also has the resistance to pitting of more than material in the past.

(use has the goods 1 of invention material)

Figure 11 is the sectional view of the vertical shaft oblique flow sea-water pump that the present invention relates to.

In this figure, vertical shaft oblique flow sea-water pump comprises: rectification is from the bellmouth 117 sucking the seawater that water route enters, pass on the main shaft 111 of the rotary power of primover, be fixed on the impeller hub 115 of main shaft 111, the rotary power of primover is given effectively the impeller vane 113 of seawater, often becoming certain mode with the gap of the periphery of impeller vane 113 makes inner side for the casing liner 114 of sphere, the velocity energy being given to seawater by impeller vane 113 is transformed into the housing 112 of pressure energy, seawater through pressurization passes through inner column pipe 119, bead cover(ing) 116, Tapered Cup 118 etc.

The cast steel of casing liner 114 and each personal embodiment 1 of housing 112 makes, and the forged material of impeller hub 115 and each personal embodiment 1 of impeller vane 113 makes.Cast and after forging, carry out the solution thermal treatment of 1100 DEG C × 1h, carry out water-cooled thereafter, become 2 phase composites of ferrite amount 40 ~ 50%.Thereafter, casing liner 114 and the junction surface of housing 112 and the junction surface of impeller hub 115 and impeller vane 113 are engaged by MIG, is wound around band heater, welding heat affected zone is warming up to after 650 DEG C, implement the thermal treatment of 30min in this temperature, carry out quenching.

Measured by X-ray residual stress and determine the residual stress of welding heat affected zone, then tensile stress is reduced to 80MPa.By using the steel of embodiment 1, the sea-water pump of the long service life that toughness reduction is also inhibited, fatigue strength also improves of weld part can be made.

(employing the goods 2 of invention material)

Figure 12 is the sectional view of the flow control valve that the present invention relates to.

In this figure, flow control valve is made up of the main shaft 124 etc. of the valve seat 123 of the valve body 122 of the shell 121 of sutaining valve entirety, adjust flux, storage valve body 122, bearing circle 125, the rotation of direction of passage dish 125 and the position of regulation valve body 122.

Shell 121 is formed by the cast steel of embodiment 1.By using the steel of embodiment 1, the flow control valve that erosion resistance is high, large-scale can be made.

This flow control valve can use in seawater, oil and chemical plant's environment.

Nomenclature

1: ferrite phase, 2: austenite phase, 3: grain circle, 4: emptying aperture, 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: brittle phase, 73: analysis position, 111: main shaft, 112: shell, 113: impeller vane, 114: casing liner, 115: impeller hub, 116: bead cover(ing), 117: bellmouth, 118: Tapered Cup, 119: column pipe, 121: shell, 122: valve body, 123: valve seat, 124: main shaft, 125: bearing circle.

Claims (amendment according to treaty the 19th article)

1. duplex stainless steel, it is characterized in that, containing is N:0.05 ~ 0.25%, below C:0.02%, below P:0.02%, below Si:0.5%, below Mn:1.2%, Ni:6.0 ~ 8.0%, Cr:24.0 ~ 26.0%, Mo:3.0 ~ 5.0%, below W:6.5%, Ta:0.2 ~ 0.5% in mass %; Surplus is Fe and inevitable impurity.

2. duplex stainless steel according to claim 1, is characterized in that, the index of resistance to pitting (PREW) defined by following formula is more than 40.

(PREW)＝％Cr+3.3×(％Mo+0.5×％W)+30×％N

In formula, %Cr, %Mo, %W and %N are the value of each composition represented with quality %.

3. two-phase stainless steel works, is characterized in that, it uses the duplex stainless steel described in claim 1 or 2.

4. two-phase stainless steel works, is characterized in that, after forging or casting making, implements the solution thermal treatment of 30 minutes ~ 2 hours, make austenite/ferrite be in a ratio of 0.2 ~ 0.8 the temperature of 950 DEG C ~ 1200 DEG C.

5. marine structure, is characterized in that, is the two-phase stainless steel works described in claim 3 or 4.

6. oil/atmosphere surrounding works, is characterized in that, is the two-phase stainless steel works described in claim 3 or 4.

7. pump impeller, is characterized in that, is the two-phase stainless steel works described in claim 3 or 4.

8. pump case, is characterized in that, is the two-phase stainless steel works described in claim 3 or 4.

9. the valve body of flow control valve, is characterized in that, is the two-phase stainless steel works described in claim 3 or 4.

Claims

1. duplex stainless steel, it is characterized in that, containing is below N:0.3%, below C:0.1%, below P:0.1%, below Si:3.0%, below Mn:8.0%, Ni:3.0 ~ 12.0%, Cr:20.0 ~ 40.0%, below Mo:7.0%, below W:6.5%, Ta:0.05 ~ 1.0% in mass %; Surplus is Fe and inevitable impurity.

2. duplex stainless steel, it is characterized in that, containing is N:0.05 ~ 0.25%, below C:0.02%, below P:0.02%, below Si:0.5%, below Mn:1.2%, Ni:6.0 ~ 8.0%, Cr:24.0 ~ 26.0%, Mo:3.0 ~ 5.0%, below W:6.5%, Ta:0.2 ~ 0.5% in mass %; Surplus is Fe and inevitable impurity.

3. duplex stainless steel according to claim 1 and 2, is characterized in that, the index of resistance to pitting (PREW) defined by following formula is more than 40,

(PREW)＝％Cr+3.3×(％Mo+0.5×％W)+30×％N

4. two-phase stainless steel works, is characterized in that, it uses the duplex stainless steel according to any one of claims 1 to 3.

5. two-phase stainless steel works, is characterized in that, after forging or casting making, implements the solution thermal treatment of 30 minutes ~ 2 hours, make austenite/ferrite be in a ratio of 0.2 ~ 0.8 the temperature of 950 DEG C ~ 1200 DEG C.

6. marine structure, is characterized in that, is the two-phase stainless steel works described in claim 4 or 5.

7. oil/atmosphere surrounding works, is characterized in that, is the two-phase stainless steel works described in claim 4 or 5.

8. pump impeller, is characterized in that, is the two-phase stainless steel works described in claim 4 or 5.

9. pump case, is characterized in that, is the two-phase stainless steel works described in claim 4 or 5.

10. the valve body of flow control valve, is characterized in that, is the two-phase stainless steel works described in claim 4 or 5.