JPH08170153A - Highly corrosion resistant two phase stainless steel - Google Patents

Abstract

PURPOSE: To produce a two phase stainless steel having excellent corrosion resistance and hot workability and thermally stable structure and further capable of easily providing high strength and high toughness. CONSTITUTION: This steel is a highly corrosion resistant dual phase stainless steel having a composition consisting of, by weight, 5.0-9.0% Ni, 23.0-27.0% Cr, 5.0-10.0% W, 0.24-0.32% N, 0.0005-0.01% B, <=0.03% C, <=1.0% Si, <=1.5% Mn, <=0.040% P, <=0.010% S, <=0.040% sol.Al, <=2.0% Mo, and the balance Fe with inevitable impurities and satisfying 40<=PREW=Cr+1.65W+16N or 40<=PREW=Cr+1.65W+3.3Mo+16N (where each symbol of element represents the weight per cent of the element).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless steel, and more particularly, it has good corrosion resistance, hot workability, and a thermally stable structure, and further facilitates high strength and high toughness. It is a duplex stainless steel that can be provided, especially when it is used as pipes for the chemical industry or oil well line pipes, where corrosion is a problem, and where it is unavoidably affected by heat due to welding, etc. The present invention relates to a duplex stainless steel having high corrosion resistance suitable for use in applications requiring high strength materials.

[0002]

2. Description of the Related Art Duplex stainless steel containing a large amount of Cr has excellent resistance to pitting corrosion, crevice corrosion and stress corrosion caused by chlorides. , Has been used in a wide range since ancient times as a pipe material for the chemical industry and oil wells.

In recent years, as the usage environment has become more severe,
Further, there is a demand for a duplex stainless steel having good corrosion resistance, mechanical properties, and the like, and examples of inventions made to meet this demand include Japanese Patent Laid-Open Nos. 56-142855 and 62-50444. Same 62-18004
No. 3, JP-A-62-56556, JP-A No. 2-25
Some are disclosed in Japanese Patent Publication Nos. 8956, 5-132741 and the like.

As an index showing the pitting corrosion resistance of duplex stainless steel, the pitting corrosion resistance index (P
RE or PREW) is known.

PRE = Cr + 3.3Mo + 16N ..... PREW = Cr + 3.3 (Mo + 0.5W) + 16N .... Cr, Mo, N so that the PRE value or PREW value expressed by the formula becomes 40 or more.
And a so-called super duplex stainless steel having an increased W have been proposed (JP-A-62-56556 and JP-A-5-132741), which are superior in seawater resistance to conventional duplex stainless steels. It is attracting attention as a material.

The increase in the amounts of Cr, Mo, N and W is effective from the viewpoint of strengthening from the viewpoint of solid solution strengthening. However, the increase of these elements has the following problems.

When the contents of Cr, Mo and W in the steel are increased, precipitation of hard and brittle intermetallic compounds such as σ phase, χ phase and Laves phase is apt to occur. Precipitation is likely to occur, and a surface crack of the steel slab is likely to be induced due to an embrittlement mechanism called σ embrittlement during the cooling process during hot working, resulting in a problem that hot workability is significantly deteriorated.

Further, in steels with increased Cr, Mo and W, the precipitation of the σ phase is caused by the eutectoid reaction of δ → γ + σ at the ferrite / austenite interface (hereinafter referred to as the δ / γ interface), especially during welding. Thermal effects and stress relief (SR)
The precipitation of σ phase is accelerated during the heat treatment for
The δ / γ phase balance becomes unstable and collapses, and σ
Coagulation and coarsening of the phases occur, and not only corrosion resistance but also mechanical properties such as toughness and ductility deteriorate, and the original performance of the duplex stainless steel is impaired. In other words, since the thermal stability of the steel structure is impaired, strict control of hot working conditions, welding conditions, and other heat treatment conditions is required, resulting in reduced work efficiency in product manufacturing and piping work. There is also the problem of doing.

As means for improving the hot workability of duplex stainless steel, the above-mentioned δ / γ phase balance is defined, deoxidation and desulfurization are strengthened during steel melting, and hot workability improving elements are positively added. Addition and the like are considered, and among these, steel containing B as a hot workability improving element to improve the hot workability by strengthening the grain boundaries is proposed in JP-A-2-258956. However, it does not show any means for suppressing the precipitation of the σ phase itself. Further, as a strengthening means for steel, generally, a precipitation strengthening method in which fine intermetallic compounds, carbides, nitrides and the like are dispersed and precipitated is widely used, but a super duplex stainless steel with increased Cr, Mo, N and W is used. In particular, a duplex stainless steel obtained by applying the precipitation strengthening method to a duplex stainless steel having W higher than that of a conventional steel has not been found yet.

[0010]

SUMMARY OF THE INVENTION The object of the present invention was made in view of the above-mentioned circumstances. By completely suppressing the precipitation of the σ phase, the hot workability is dramatically improved and the structure is improved. It is an object of the present invention to provide a duplex stainless steel which is excellent in thermal stability and which has high corrosion resistance and high strength while maintaining corrosion resistance when an intermetallic compound is finely precipitated.

[0011]

Means for Solving the Problems The present invention includes the following (1) to
(4) Highly corrosion-resistant duplex stainless steel.

(1) C: 0.03% or less by weight%,
Si: 1.0% or less, Mn: 1.5% or less, P: 0.0
40% or less, S: 0.01% or less, sol-Al: 0.
040% or less, Ni: 5.0 to 9.0%, Cr: 23.
0-27.0%, W: 5.0-10.0%, N: 0.2
4 to 0.32%, B: 0.0005 to 0.01%, M
o: Highly corrosion-resistant duplex stainless steel containing 0 to less than 2%, the balance being Fe and inevitable impurities, and having PREW of 40 or more represented by the formula below.

PREW = Cr + 1.65W + 16N PREW = Cr + 1.65W + 3.3Mo + 16N However, the element symbols in the formula are Indicates the content (% by weight) of an element.

(2) In addition to the components described in (1) above,
Furthermore, 1 selected from the elements of the following first to third groups
A high corrosion resistance duplex stainless steel containing one or more components and having a PREW represented by the above formula or a formula of 40 or more.

(3) The metal structure is a structure in which an intermetallic compound is finely dispersed and precipitated in a δ phase. (1) to (2)
High corrosion resistance duplex stainless steel according to any one of 1.

(4) The aging heat treatment for holding the duplex stainless steel according to any one of (1) and (2) above in a temperature range of 800 to 900 ° C. for 5 to 30 minutes, according to (3). For producing high-corrosion duplex stainless steel.

[Group 1 element] Ta of 0.05 to 0.1% by weight and Hf of 0.05 to 0.1% by weight.

[Group 2 element] V of 0.1 to 1.0% by weight.

[Group 3 elements] 0.001 to 0.01% by weight of Ca and 0.001 to 0.01% by weight of Mg.

In the present invention described in the above (1) to (4), the steel may be a non-Mo-added steel, and when Mo is added and contained, 0.05 wt% or more, 2.0 wt% or more. %
It can be contained in the range of less than.

Conventional duplex stainless steel is made of Fe, Cr, N.
In contrast to the steel containing Mo and Mo as the main components, the duplex stainless steel of the present development contains Fe, Cr, N and W as the main components, which do not contain Mo, which was added in large amounts in the conventional steel. The largest difference is the steel containing a large amount of W in addition to the conventional steel.

In general, W is considered to have the same effect as Mo as an alloying element, and the weight ratio of Mo to Mo is 1/2.
W is treated as an equal amount. However, W is an extremely expensive element as compared with Mo, and because the addition of a large amount of W causes an increase in steel cost and is economically disadvantageous, the addition is limited to at most 5% by weight. Was not positively added.

Therefore, the present inventors have detailed the effect of Mo and W on the precipitation of intermetallic compounds such as σ phase of super duplex stainless steel, and the effect of various alloying elements on the hot workability. As a result of the investigation, the inventors have found the new facts described in (A) to (D) below, and made the present invention.

(A) Even with the addition of Mo alone or the combined addition of Mo and W, when the Mo content is high, the content is 800 to 10
By holding at 00 ° C for a short time, the σ phase preferentially precipitates at the δ / γ interface, and at the same time, the ferrite fraction significantly decreases. In addition, in the steel containing only Mo and the composite addition of Mo and W,
In the temperature range above 800 ° C, C precedes the precipitation of σ phase.
Since a large amount of Cr nitride containing r as a main component is precipitated, mechanical properties such as hot workability, strength and toughness, and corrosion resistance are significantly deteriorated.

(B) On the other hand, in the case of the super duplex stainless steel containing W alone, the precipitation of σ phase is significantly suppressed, and σ is increased even in the temperature range of 800 ° C. to 1000 ° C.
The precipitation of phases is suppressed, and the problems of hot workability and σ embrittlement during welding are alleviated.

(C) In addition, in the case of the steel containing W alone, 8
In the temperature range of 00 ° C to 900 ° C, intermetallic compounds other than σ phase (χ phase, μ phase, etc., hereinafter referred to as TCP phase) may precipitate in the ferrite phase, but the TCP phase is extremely finely dispersed and precipitated. Therefore, toughness and ductility are not reduced, and conversely, when aged for a short time in the temperature range of 800 ° C. to 900 ° C., the TCP phase other than the σ phase is uniformly finely dispersed and precipitated in the ferrite phase. Contributes to higher strength,
In this case, even if an appropriate amount of Mo is added and contained, there is no change in the precipitation form of the TCP phase, and conversely W in the ferrite phase is added.
+ The supersaturation of Mo atoms promotes the fine precipitation of the TCP phase, and further strengthening can be achieved.

(D) Further, in the case of adding W alone, M
o The thermal stability of the metal structure is excellent as compared with the case of single addition or the combined addition of Mo and W. That is,
In the steel containing only W, the movement of the δ / γ interface was suppressed, and δ / γ
While maintaining the phase balance, the TCP phase that precipitates in the temperature range of 800 to 900 ° C. does not aggregate and coarsen, and the fine T
The pinning action of the CP phase prevents the δ / γ interface from moving and stabilizes the δ / γ phase balance without causing deterioration of toughness.

(E) Even with W-added steel, there is no problem in forming a uniform film in a corrosive environment, and conversely, it is better to add a large amount of W, which exceeds the W addition amount of conventional steel. Improves corrosion resistance.

(F) In the W-added steel, precipitation of Cr nitride containing Cr as a main component which deteriorates hot workability is suppressed even in a temperature range of 800 ° C. or higher.

(G) Furthermore, the hot workability of the super duplex stainless steel in which a large amount of W is added to the conventional steel alone greatly differs depending on whether B is added or not.
If it is contained as an essential component, the hot workability is dramatically improved even though it is a W-added steel. That is, B has a deoxidizing effect and segregates at the grain boundaries or at the δ / γ interface to contribute to the improvement of hot workability by grain boundary strengthening. However, in Mo-added steel, regardless of whether B is added or not. Since the σ phase precipitates from the δ / γ interface,
When the phase precipitates, the effect of B is halved or disappears. On the other hand, in the steel containing only W, the grain boundary segregation of B acts effectively because the grain boundary precipitation of the σ phase is suppressed, and the hot workability jumps. To improve.

The reason why the effect of adding a large amount of W alone described in the above (A) to (F) is obtained is not clear in detail, but (a) δ → due to a slower diffusion rate than Mo.
It is presumed that it is due to the effects of (a) and (b) of delaying the γ + σ reaction, and (b) W having a larger atomic radius than Mo, thus suppressing the stable precipitation of the σ phase.

[0032]

The function and effect of each component of the duplex stainless steel of the present invention and the reason for limiting the content will be described in detail below. In addition,% regarding the content of each component is% by weight
Means

C: 0.03% or less C is effective in stabilizing the austenite phase, but if its content exceeds 0.03%, carbides are likely to precipitate and corrosion resistance deteriorates. Therefore, the content thereof is set to 0.03% or less.

Si: 1.0% or less Si is an element that is effective as a deoxidizing component of steel, but promotes the formation and precipitation of intermetallic compounds and promotes the agglomeration and coarsening thereof. Therefore, in the present invention, the content is 1.0
% Or less. Preferably, it is 0.5% or less.

Mn: 1.5% or less Mn improves hot workability due to desulfurization and deoxidation effects during melting of duplex stainless steel. It also has the effect of increasing the solubility of N, but if its content exceeds 1.5%, the corrosion resistance deteriorates. Therefore, the upper limit of the content is set to 1.5%.

P: 0.040% or less P, which is an unavoidable impurity, reduces the hot workability due to grain boundary segregation, and if its content exceeds 0.040%, corrosion resistance and toughness also deteriorate. Therefore, the upper limit of the content is set to 0.040%.

S: 0.010% or less S is also an unavoidable impurity and deteriorates the hot workability and corrosion resistance of steel. Therefore, in the present invention, the content is set to 0.
It is limited to 010% or less. It is preferably 0.005% or less.

Sol-Al: 0.040% or less Al is most effective as a deoxidizing agent for steel, and the deoxidizing effect improves as the amount of addition increases. However, when the amount of N in steel is high, it precipitates as AlN and deteriorates toughness and corrosion resistance. Therefore, in the present invention, the Al content is sol-
Al was suppressed to 0.040% or less.

Ni: 5.0 to 9.0% Ni is an austenite stabilizing element. However, if the content exceeds 9.0%, the ferrite content decreases,
On the other hand, if it is less than 5.0%, the amount of ferrite becomes too large and the basic characteristics of the duplex stainless steel are lost.

Cr: 23.0-27.0% Cr is a basic component effective in improving the corrosion resistance. If the content is less than 23.0%, the pitting corrosion resistance is insufficient. On the other hand, if it exceeds 27.0%, the precipitation of the σ phase and the coarsening of the agglomeration become remarkable, and it is difficult to secure the stability of the structure. Becomes

W: 5.0 to 10.0% As described above, W is a component that most characterizes the duplex stainless steel of the present invention and improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, Further, it is an element which forms a stable oxide and also improves the corrosion resistance in an environment of low pH.

Since W has a slow diffusion rate, δ → γ +
Suppresses movement of the δ / γ interface accompanying the σ-eutectoid reaction. Furthermore, the stability of the σ phase that causes embrittlement is governed by the atomic radius ratio between the atoms (Fe, Cr, W, Mo) that make up the σ phase, but W is the other element (Fe, Cr). , Mo) has a larger atomic radius, and thus also has an action of suppressing stable precipitation of the σ phase. That is, it effectively acts to suppress the stable precipitation of the σ phase and the coarsening of its aggregation.

Furthermore, W is the above σ in the ferrite phase.
Has a function of preferentially precipitating a fine TCP phase composed of a χ phase, a μ phase, etc. other than the phase, particularly 800 ° C to 90 ° C.
When aging is performed for a short time in the temperature range of 0 ° C, a structure in which the TCP phase is uniformly finely dispersed and precipitated in the ferrite phase,
It has the effect of increasing the strength of steel.

However, if the content is less than 5.0%, the above effects, particularly the effect of improving the corrosion resistance and the strength due to the uniform fine dispersion precipitation of the TCP phase, cannot be obtained, and conversely 10.0.
%, The precipitation of the .sigma. Phase is promoted, which causes the hot workability and the toughness to be lowered, and the TCP phase is excessively promoted to cause the toughness to be lowered. Therefore, the W content is 5.0 to
It was set at 10.0%.

N: 0.24 to 0.32% N is a strong austenite-forming element and is an element effective in improving the thermal stability and corrosion resistance of the duplex stainless steel. When a large amount of Cr and W, which are ferrite forming elements, is added as in the duplex stainless steel of the present invention, 0.24% or more is used in order to make the two phase balance of ferrite and austenite proper. It is necessary to include N. However, if its content exceeds 0.32%, the toughness and corrosion resistance of steel are significantly deteriorated due to the formation of nitrides. Therefore, the N content is set to 0.24 to 0.32%.

B: 0.0005 to 0.01% B, like W, is a component that characterizes the duplex stainless steel of the present invention. It fixes oxygen in the steel and segregates at grain boundaries. It is an element that strengthens grain boundaries and improves hot workability. However, if the content is less than 0.0005%, the effect of improving hot workability cannot be obtained. On the contrary, if the content exceeds 0.01%, the hot workability is deteriorated due to the formation of nonmetallic inclusions and the like. Therefore, the B content is 0.000
It was determined to be 5 to 0.01%. Preferably 0.0005
It is 0.008%.

The contents of Cr, N and W described above must be adjusted so that the PREW represented above becomes 40 or more. PRE represented by the above formula
The W value, that is, Cr + 1.65W + 16N, is obtained by deleting the term of Mo from the above-mentioned conventional formula, and itself is already disclosed as PREW in Japanese Patent Laid-Open No. 5-132741. However, in Japanese Unexamined Patent Publication (Kokai) No. 5-132741, a Mo content of 2.0 to 4.0% and a Mo content of more than 1.5% and 5.0
%, It is intended only for steels containing complex essential elements with W or less, and when the PREW value is set to 40 or more in the steel in which a large amount of W of 5.0 to 10.0% alone is added as a basic component, Similarly, there is no disclosure that the pitting corrosion resistance can be dramatically improved and maintained.

In addition to the components described above, the duplex stainless steel of the present invention further comprises the following Mo and the above-mentioned first
One or two or more of the elements of groups 3 to 3 can be contained as necessary.

Mo: less than 2.0% Mo has an action of improving pitting corrosion resistance and crevice corrosion resistance like W. From the viewpoint of improving corrosion resistance, Mo has the same effect as that of W W for compound precipitation
Is an element that has a completely different action from. That is, M
Since o is an element that promotes the stable growth of the σ phase, it not only causes the coarse σ phase to precipitate and causes a decrease in toughness, but also adversely affects the corrosion resistance.

Mo promotes the movement of the δ / γ interface,
Although it is also an element that makes the δ / γ phase balance unstable, in the case of composite addition with W, when W is added in an appropriate amount within the above range specified in the present invention, W is added alone. Not only does it exhibit the same TCP phase precipitation morphology as that of the case, but on the contrary, since TCP phase-forming elements Cr, W, and Mo in the ferrite phase become supersaturated, especially when a short-time aging treatment is performed, TCP It has the function of promoting uniform fine dispersion precipitation of phases and improving the strength of steel.

Therefore, to obtain this effect, Mo
However, if the content is less than 0.05%, the effect cannot be obtained. On the contrary, if the content exceeds 2.0%, precipitation of σ phase and disorder of δ / γ phase balance are caused, Since the thermal stability of the structure will be impaired, the Mo content when contained is preferably 0.05% or more and less than 2.0%.

In this case, a part of expensive W is replaced by Mo.
Since it can be replaced with, the cost can be reduced. Further, when Mo is contained, the PREW value is represented by the above-mentioned formula, which is the same as the above-mentioned formula shown in the above-mentioned JP-A-5-132741.

Group 1 elements (Ta, Hf): 0.05 to 0.1% for all elements Ta and Hf are elements which bring about the same action and effect as W with respect to the action of suppressing the precipitation of the σ phase. When it is desired to obtain the effect, one or two of Ta and Hf can be selected and contained. However, if the content of any element is less than 0.05%, the above effect cannot be obtained.
It must be at least%. On the other hand, if the content of any of the elements exceeds 0.1%, if the content of N is large, it precipitates as a nitride of these elements, resulting in a decrease in toughness. Therefore, when these elements are contained, the content of each element is set to 0.05 to 0.1%.

Group 2 element (V): 0.1 to 1.0% V is an element effective in improving crevice corrosion resistance when added in combination with W, and when this effect is desired to be obtained Can be added to. However, if the content is less than 0.1%, the above effect cannot be obtained, so when it is contained, the content must be 0.1% or more. On the other hand, if its content exceeds 1.0%, it precipitates as a V nitride, resulting in deterioration of hot workability and toughness. Therefore, the V content when contained is set to 0.1 to 1.0%.

Group 3 elements (Ca, Mg): 0.001 to 0.01% of all elements Ca and Mg are both effective in fixing oxygen in steel and improving hot workability. However, since the mechanism is different from that of B, it is possible to improve the hot workability in a wider temperature range by coexisting with B, and when this effect is desired. One or two of Ca and Mg can be selected and contained. However, the content of each element is 0.001%
If the content is less than the above, the above effect cannot be obtained. Therefore, in the case of containing any of the elements, it is necessary that the content of each element be 0.001% or more. On the other hand, if the content of each element exceeds 0.01%, non-metallic inclusions such as oxides and sulfides increase, leading to deterioration of corrosion resistance. Therefore, when these elements are contained, the content of each element is set to 0.001 to 0.01%.

It is to be noted that the Mo, the first group element, the second group element and the third group element may be contained by selecting one or more elements out of these Mo and each element in each group. Needless to say.

2 of the present invention having the component composition described above
The duplex stainless steel has excellent hot workability as will be apparent from the examples described below, and can be hot worked without any problems by a conventional method. After hot working, for example, 1050 After heating and holding at ˜1150 ° C. for 0.2 to 1 hour, it is subjected to solution heat treatment in which it is rapidly cooled using an appropriate means such as a water-cooling means to provide sufficient strength, toughness, corrosion resistance and excellent thermal stability of the structure. However, by performing the following aging treatment, a fine TCP phase other than the σ phase is uniformly dispersed and precipitated in the δ phase, that is, a so-called precipitation strengthening structure,
The thermal stability and strength of the tissue, especially strength, is further improved.

That is, when the duplex stainless steel of the present invention is subjected to an aging treatment in which it is heated and held at 800 to 900 ° C. for 5 to 30 minutes, a precipitation strengthening structure is obtained in which the TCP phase is uniformly dispersed and precipitated in the δ phase. , The thermal stability and strength of the tissue, especially the strength, are further improved. However, when the aging treatment temperature is lower than 800 ° C. or higher than 900 ° C., the precipitation nose for depositing the fine TCP phase of the duplex stainless steel of the present invention is in the temperature range of 800 to 900 ° C.
The TCP phase does not uniformly and finely precipitate in the δ phase. Further, when the aging time is less than 5 minutes, the precipitation of the TCP phase is insufficient, and even when the aging is performed for more than 30 minutes, the effect is saturated. Therefore, the aging heat treatment conditions when performing the aging treatment are set as described above.

If the aging time exceeds 100 minutes, σ
It should be noted that precipitation of intermetallic compounds, which adversely affects the toughness of phases, may start. Further, since the structure of the duplex stainless steel of the present invention is thermally stable, there is no problem even if the above aging treatment is used in combination with welding or SR treatment.

[0060]

【Example】

[Example 1] 36 types of duplex stainless steels having the chemical components shown in Table 1 and Table 2 were melted in a 20 kg vacuum melting furnace to prepare 100 mmφ ingots, and these ingots were heated to 1280 ° C. After forging into a plate material having a thickness of 50 mm, this plate material was subjected to solution heat treatment of 1100 ° C. × 30 minutes → water cooling to prepare a test material.

In Tables 1 and 2, Nos. 1 to 23 are steels according to the present invention, and Nos. 24 to 36 are comparative steels.

[0062]

[Table 1]

[0063]

[Table 2]

Next, various test pieces were cut out from these steel sheets and collected, and their hot workability, thermal stability of structure, tensile properties, toughness and pitting corrosion resistance were investigated by the following methods.

[Hot workability test] Diameter 10 mm, length 2
A 00 mm test piece was prepared, and after uniform heating and holding at 1300 ° C. using a greeble tester, it was cooled to 1000 ° C. and 900 ° C. at which precipitation of σ phase became remarkable, and after uniformly heating and holding at these temperatures, It was broken at a tensile speed of 350 mm / sec, and the drawing ratio of the broken portion at this time was measured and evaluated.

Some test materials (No. 2, 19, 2)
0, 31 and 36), 900-1300 ° C
The hot workability in the range was investigated and the results are shown in FIG.

[Thermal Structure Stability Test] An aging treatment of heating and holding at a temperature of 900 ° C. for 5 minutes to 200 hours was performed, and the surface was 20 mm in width, 3 mm in thickness, and 20 mm in length, and the surface thereof was emery paper or A test piece prepared by polishing the alumina powder to remove the processed layer was prepared.
A line diffraction experiment was carried out to measure the precipitation behavior of the σ phase (change in the diffraction intensity of the σ phase) with aging. For those whose precipitation phase could not be identified by the X-ray diffraction experiment, the precipitates were observed by electron microscopy. Observation and identification were carried out, and those in which precipitation of σ phase was not observed when heated and held at 900 ° C. for 5 to 30 minutes are shown as good thermal stability of the structure by a mark “◯”. When the precipitation of the σ phase was observed within the time, the thermal stability of the structure was unsatisfactory, and the mark “x” was shown.

For some of the test materials (Nos. 1, 8, 26 and 27), changes in the precipitation amount of the σ phase within the range of 5 minutes to 200 hours were investigated, and the results are shown in FIG. Shown in 2.

[Tensile Property Test] A round bar tensile test piece having a diameter of 6 mm and a gauge length of 40 mm was prepared and subjected to a tensile test at room temperature to obtain tensile strength (TS), 0.2% proof stress (YS) and elongation ( El) was measured.

[Impact Test] Thickness 5.0 mm, width 10 m
m, length 55 mm, and a test piece with a notch having a depth of 2.0 mm in the width direction formed in the center of the length direction, an impact test using a Charpy impact tester, and an impact value at break (Absorbed energy) was measured. The test temperature is 0
° C.

[Pit Corrosion Test] A thickness of 2 mm, a width of 25 mm, and a length of 50 based on the method specified in ASTM standard-G48.
mm test piece is prepared, and this test piece is subjected to a corrosion test solution (liquid:
10% FeCl ₃ -6H ₂ 0, liquid temperature: the 30 ° C.) in 24
After immersion for a period of time, the presence or absence of pitting corrosion was observed, and those without pitting corrosion were evaluated as “◯”, and those with pitting corrosion were evaluated as “x”.

The results of these tests are shown in Tables 3 and 4.

[0073]

[Table 3]

[0074]

[Table 4]

As is clear from the results shown in Tables 3 and 4, the duplex stainless steels of the examples of the present invention each have a PREW of 40 or more represented by the above formula or a formula and exhibit good pitting corrosion resistance. ing. Regarding hot workability,
The conventional super duplex stainless steel shows a high drawing ratio of 75% or more even at 900 ° C. where σ embrittlement is the most problematic, and the hot workability is good. Further, as shown in FIG. 1, a good hot workability is exhibited due to the synergistic effect with the addition of B, and when Ca and Mg are added in combination, a reduction ratio of 78% or more is obtained. Has excellent hot workability. On the other hand, the duplex stainless steels of Comparative Examples are inferior in hot workability except for Nos. 24 to 30, and particularly in hot workability in a temperature range of less than 1000 ° C.

The hot workability of Nos. 24 to 30 is excellent for the following reason. That is, with respect to No. 24, the W content is small and the precipitation of the σ phase does not occur. However, this No. 24 has poor corrosion resistance.
Further, for Nos. 24 to 27, the holding time was short, and the precipitation of the σ phase did not occur during this hot working test. The inter-workability deteriorates. Furthermore, No. 28-3
The reason for 0 is that the δ / γ phase balance is lost, which is outside the original properties of so-called super duplex stainless steel.

Further, as shown in FIG. 2, in the duplex stainless steel of the present invention, no precipitation of σ phase was observed within the aging time of 30 minutes at 900 ° C., and good thermal stability of the structure was observed. Is shown. On the other hand, in the duplex stainless steels of the comparative examples, the σ phase is precipitated in the holding time of 5 to 30 minutes, and especially those having a large Mo content are inferior in thermal stability of the structure.

Further, the duplex strength stainless steel of the present invention has a room temperature strength of 0.2% proof stress (YS) of 550 N / mm ² or more and a toughness of 0 ° C. according to the Charpy impact test.
Has an impact value of 20 kgf-m / cm ² or more, and has strength and toughness sufficiently comparable to that of the conventional super duplex stainless steel.

On the other hand, the duplex stainless steels of Comparative Examples are all inferior in hot workability, thermal stability of structure, toughness and pitting corrosion resistance.

[Example 2] In a vacuum melting furnace of 20 kg, Table 5
~ 55 types of duplex stainless steels having the chemical components shown in Table 7 are melted to make 100 mmφ ingots, these ingots are heated to 1280 ° C and forged into a plate having a thickness of 50 mm, and then this plate is formed. A solution heat treatment of 1100 ° C. × 30 minutes → water cooling was performed, and then an aging treatment was performed under each condition shown in Tables 8 to 10 to prepare a test material.

In Tables 5 to 7, Nos. 1 to 36 are steels according to the present invention, and Nos. 37 to 55 are comparative steels. Also,
Among Nos. 1 to 48, Nos. 1 to 14, Nos. 30 to 31, and Nos. 37 to 48 were Nos. Used in Example 1, respectively.
It is a steel having the same composition as Nos. 1 to 14, Nos. 22 to 23 and Nos. 24 to 35.

[0082]

[Table 5]

[0083]

[Table 6]

[0084]

[Table 7]

Next, various test pieces were cut out from these steel plates and collected, and the precipitation behavior of the TCP phase and σ phase, the thermal stability of the structure, the tensile properties, the toughness and the pitting corrosion resistance were investigated by the following methods. did.

[TCP phase and σ phase precipitation behavior test] Width 20
mm, thickness 3 mm, length 20 mm, the surface of which was polished by using emery paper and alumina to prepare a test piece, and an X-ray diffraction experiment was performed on the test piece.
The X-ray diffraction intensity was measured and the T
The fraction of the CP phase and the σ phase was expressed by the ratio of the γ phase strength and evaluated.

TCP phase fraction = TCP / {(γ ₂₀₀ + γ ₂₂₀ ) / 2} ... σ phase fraction = {(σ ₁₁₂ + σ ₂₁₂ + σ ₄₁₁ ) / 3} / {(γ ₂₀₀ + Γ ₂₂₀ ) / 2} ・ ・ ・ The greater the TCP phase fraction, the greater the T
The fact that the finely dispersed precipitation of the CP phase is remarkable means that the smaller the value of the σ phase fraction, the less the precipitation of the σ phase.

Also, some steels (No. 2 and No. 40)
The measurement results of the X-ray diffraction experiment for the are shown in FIGS. 3 and 4. FIG. 3 and FIG. 4 are diagrams showing typical X-ray diffraction patterns of duplex stainless steel subjected to a predetermined aging treatment, and the duplex stainless steel of the present invention (No. 2).
However, in the duplex stainless steel (No. 40) which is a comparative example, only the σ phase is precipitated and the X-ray peak is sharp, so that the σ phase is agglomerated and coarsened. Recognize.

[Thermal Structure Stability Test] From the test material which was subjected to the aging treatment in which it was heated and maintained at a predetermined temperature for a predetermined time, a width of 2 was obtained.
A test piece having a thickness of 0 mm, a thickness of 3 mm and a length of 20 mm, the surface of which was polished by using emery paper and alumina powder to remove the processed layer was prepared, and an X-ray diffraction experiment similar to the above was performed on the test piece. And TCP phase fraction and σ with aging
Change in phase fraction was measured, precipitation of σ phase was not observed within an aging time range of 5 to 30 minutes, and change rate of TCP phase within this aging treatment time was 5% or less. Is indicated by the mark "○", indicating that the thermal stability of the tissue is good.
When the precipitation of phases was observed and the rate of change of the TCP phase exceeded 5%, the thermal stability of the structure was unsatisfactory and indicated by a mark "x".

For some of the test materials (Nos. 1, 4, 8 and 39), T within the range of 0 to 6000 hours was used.
The examination results of the X-ray diffraction intensity ratio of the CP phase and the σ phase expressed by the above and the formula are shown in FIGS.

FIGS. 5 to 8 show the thermal stability of each alloy after aging treatment. In the case of No. 8 duplex stainless steel aged under the conditions defined by the present invention, the TCP phase is shown. Is stably deposited, and the σ phase is not deposited (see FIG. 5).

On the other hand, the steel composition is the same as that of No. 4 and is within the range of the present invention, but the aging temperature is 950 ° C. and the duplex stainless steel of No. 50 which has been aged under conditions outside the specified range of the present invention In the steel, the σ phase preferentially precipitates and the TCP phase does not precipitate (see FIG. 6). Similarly, the steel composition is the same as that of No. 1 and is within the range of the present invention, but the aging temperature is 600 ° C and the aging temperature of the No. 49 duplex stainless steel aged under conditions outside the specified range of the present invention. Is low, there is no precipitation of σ phase, but T
CP phase does not precipitate (see FIG. 7). In addition, although the aging conditions are within the scope of the present invention, in the duplex stainless steel No. 39 having a steel composition outside the scope of the present invention, precipitation of TCP phase is observed, but σ phase preferentially precipitates. Therefore, the thermal stability of the tissue cannot be guaranteed (see FIG. 8).

The "tensile property test", "impact test" and "pitting corrosion test" were carried out in the same manner as in Example 1, so detailed description thereof will be omitted.

The test results are also shown in Tables 8 to 10.

Nos. 49 to 55 in Table 10 are shown in Table 5
~ No. 1, No. 4, No. 8, No. 9, No. 1 in Table 7
It is a comparative example in which the steels of Nos. 7, No. 21 and No. 28 of the present invention were subjected to an aging treatment under the conditions outside the scope of the present invention.

[0096]

[Table 8]

[0097]

[Table 9]

[0098]

[Table 10]

As is clear from the results shown in Tables 7 to 10, the PREW value represented by the above or the formula is 40 or more, and the aging treatment under the predetermined condition is performed to obtain T
When the amount of CP phase is 0.1 or more in the X-ray diffraction intensity ratio with the γ phase, δ
Tissue uniformly finely dispersed precipitates in the phases, duplex stainless steel of the present invention, which is precipitation strengthening tissue are both 900 N / mm ² or more in tensile strength TS at room temperature, 650 N / mm ² or more in YS Has a high strength and maintains a high ductility of 35% elongation.

Also, the impact value is 20 kgf-m / cm ² or more, the toughness is excellent, the corrosion resistance is good, and the pitting corrosion resistance is excellent, and the thermal stability of the structure is also excellent. .

On the other hand, the duplex stainless steels of Comparative Examples are all inferior in strength, ductility, toughness, pitting corrosion resistance and thermal stability of structure.

[0102]

The duplex stainless steel of the present invention has excellent hot workability, thermal stability of structure and corrosion resistance, and a structure in which fine TCP phase is uniformly dispersed and precipitated in the δ phase,
That is, in the case of a precipitation strengthened structure, the steel has high strength while maintaining toughness, ductility and corrosion resistance. Therefore, this steel is used in applications where corrosion is a problem and applications in which the heat effect is inevitable due to welding, etc., specifically in equipment and instruments used in the marine environment, or line pipe and piping members for oil wells. Suitable as a material and 90
Since it has good hot workability even in the low temperature range around 0 ° C,
The number of man-hours required for the billet can be reduced, which contributes to the improvement of product yield. Further, when a precipitation-strengthened steel is used, it is suitable for use in a more severe corrosive environment than before, is suitable as a material for reducing the thickness and weight of products, and has excellent thermal stability. While maintaining the performance, it is possible to use welding work and stress relief heat treatment together.

[Brief description of drawings]

FIG. 1 shows typical invention steels and comparative steels 1300-900.
It is a figure which shows an example of the change of hot workability in the range of (degree C).

FIG. 2 is a diagram showing an example of the precipitation behavior of a σ phase by the aging treatment at 900 ° C. of a representative present invention steel and a comparative steel.

FIG. 3 is a diagram showing an example of a typical X-ray diffraction pattern showing precipitation behavior of an intermetallic compound in a typical precipitation-strengthened steel of the present invention.

FIG. 4 is a diagram showing an example of a typical X-ray diffraction pattern showing precipitation behavior of an intermetallic compound in a precipitation-strengthened steel of a representative comparative example.

FIG. 5 is a diagram showing an example of an X-ray diffraction intensity ratio representing the thermal stability of the structure of a typical precipitation strengthened structure steel of the present invention.

FIG. 6 is a diagram showing an example of an X-ray diffraction intensity ratio representing the thermal stability of the structure of a precipitation strengthened structure steel of a representative comparative example.

FIG. 7 is a diagram showing an example of an X-ray diffraction intensity ratio representing the thermal stability of the structure of a precipitation strengthened structure steel of a representative comparative example.

FIG. 8 is a diagram showing an example of an X-ray diffraction intensity ratio representing the thermal stability of the structure of a precipitation strengthened structure steel of a representative comparative example.

Claims (10)

[Claims] 1. C: 0.03% or less by weight%, Si:
1.0% or less, Mn: 1.5% or less, P: 0.040%
Hereinafter, S: 0.010% or less, sol-Al: 0.04
0% or less, Ni: 5.0 to 9.0%, Cr: 23.0 to
27.0%, W: 5.0 to 10.0%, N: 0.24 to
0.32%, B: 0.0005 to 0.01%, Mo: 0
A high corrosion resistance duplex stainless steel characterized by containing less than 2.0% and a balance of Fe and inevitable impurities, and having PREW of 40 or more represented by the formula below. PREW = Cr + 1.65W + 16N ..... PREW = Cr + 1.65W + 3.3Mo + 16N .. However, the element symbol in the formula is the content of each element. Indicates the amount (% by weight). 2. In addition to the components according to claim 1, further 0.
It is characterized by containing one or both of Ta of 0.05 to 0.1% by weight and Hf of 0.05 to 0.1% by weight, and PREW represented by the above or the formula is 40 or more. Highly corrosion resistant duplex stainless steel. 3. In addition to the components according to claim 1, further 0.
A highly corrosion-resistant duplex stainless steel containing 1 to 1.0% by weight of V and having PREW represented by the above formula or a formula of 40 or more. 4. In addition to the components according to claim 1, further 0.
001-0.01 wt% Ca and 0.001-0.01
A high corrosion-resistant duplex stainless steel containing one or both of Mg in a weight percentage and having a PREW represented by the above formula or a formula of 40 or more. 5. In addition to the components according to claim 1, further 0.
05-0.1 wt% Ta and / or 0.05-0.1 wt% Hf, or both, and 0.1-1.0 wt% V, and represented by the above or formula. PR
High corrosion resistance duplex stainless steel characterized by having an EW of 40 or more. 6. In addition to the components according to claim 1, further 0.
05 to 0.1 wt% Ta and 0.05 to 0.1 wt% Hf, or both, and 0.001 to 0.0
High corrosion resistance two phase containing 1% by weight of Ca and / or 0.001 to 0.01% by weight of Mg, or both, and PREW represented by the above or the formula being 40 or more. Stainless steel. 7. In addition to the components according to claim 1, further 0.
1 to 1.0 wt% V and 0.001 to 0.01 wt%
Of Ca and 0.001 to 0.01 wt% of Mg, or both, and PREW represented by the above or the formula is 40 or more, high corrosion resistance duplex stainless steel. 8. In addition to the components according to claim 1, further 0.
Either or both of Ta of 0.05 to 0.1% by weight and Hf of 0.05 to 0.1% by weight, V of 0.1 to 1.0% by weight, and 0.001 to 0.01% by weight. % Ca and 0.0
A highly corrosion-resistant duplex stainless steel containing 01 to 0.01% by weight of either or both of Mg and having PREW represented by the above formula or a formula of 40 or more. 9. The structure according to claim 1, wherein the structure is a structure in which an intermetallic compound is finely dispersed and precipitated in the δ phase.
High corrosion resistance duplex stainless steel according to any one of 1. 10. The aging heat treatment for holding the duplex stainless steel according to claim 1 in a temperature range of 800 to 900 ° C. for 5 to 30 minutes.
The method for producing a high-corrosion duplex stainless steel according to 1.