JPH06184698A - Stainless steel with high corrosion fatigue strength - Google Patents

Abstract

PURPOSE:To produce a stainless steel excellent in strength, toughness, and corrosion resistance, having high corrosion fatigue strength, and suitable, e.g. for sleeve material for suction roll for papermaking. CONSTITUTION:This steel is a stainless steel having a composition, which consists of <=0.08% C, 0.2-2% Si, 0.2-2% Mn, 3-6% Ni, 18-26% Cr, 1-4% Mo, 2-4% Cu, 0.5-3% W, 0.1-0.3% N, and the balance essentially Fe and satisfies the relations in Cr+3.3Mo+16N<=30 (%), Cr+Mo+1.5Si<=27 (%), and Cu+W>=4 (%), and also having a structure which is composed essentially of austenite phase and delta-ferrite phase and where the area ratio of delta-ferrite phase and austenite phase is regulated to 30/70 to 60/40 and 5-15% (by area) of martensite phase is mixed. If necessary, proper amounts of one or >=2 elements among Ti, Nb, V, Al, Zr, and Co are added.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has high yield strength and high corrosion resistance, which are useful as a saxion roll material for papermaking machines.
The present invention relates to stainless steel having excellent corrosion fatigue strength.

[0002]

2. Description of the Related Art A body member (sleeve) of a saxion roll for dehydrating wet paper produced in a papermaking process is
There are about 20 to 50 small holes (suction holes) that suck and remove water (called white water) squeezed from wet paper.
It is a hollow cylindrical member that is dispersed and perforated with an opening ratio of% (area) (the number of holes per roll is several tens of millions). Sleeve Saku Chillon roll, white water squeezed from the wet paper as its material properties and corrosion resistance to (chlorine ions Cl ^{- -} and sulfate ions SO ₄ and the like is strongly acidic corrosive liquid containing), white water from the wet paper It is necessary to have strength that can withstand the repeated load of pressing force (nip pressure) of the press roll for squeezing the oil, and resistance to corrosion fatigue due to the weight effect of its corrosion factor and repeated stress. .

Conventionally, duplex stainless steel (JIS G4307) has been used as a sleeve material for the above-mentioned suction roll.
SUS329J1, G5121 SCS13A, S
CS14A etc.) have been used. Duplex stainless steel is a corrosion-resistant alloy having high strength and high toughness because it has a two-phase structure in which an austenite phase having high ductility and a δ-ferrite phase having high strength are mixed in substantially equal amounts. Regarding the material properties of the duplex stainless steel, for example, Co, Cu or the like is added for the purpose of improving strength, corrosion resistance, corrosion fatigue strength, etc. (Japanese Patent Publication No. 60-59291), M
There have been proposed those having an increased o content (Japanese Patent Publication No. 62-8505), those containing Co, Cu, N, etc. (Japanese Patent Publication No. 2-32343).

[0004]

[Problems to be Solved by the Invention] In recent years, in the papermaking process, the speed of the line has been increased more and more, and along with this, the load on the sleeve of the saxion roll has been further increased. In order to prevent breakage accidents and the like of the saxion roll in this high-speed line operation and ensure stable use, roll sleeves having higher corrosion fatigue strength are required. In order to realize high corrosion fatigue strength of the saxion roll, the corrosion resistance, especially the resistance to local corrosion such as pitting corrosion, is strengthened and the strength is increased, and the sleeve surface (hole wall of the saxion hole etc.) Even after a small initial crack due to local corrosion or fatigue occurs, it is necessary to improve the overall material properties such as toughness so as not to cause rapid growth of the crack originating from it. Therefore, the present invention is useful for various structural materials that are required to have high corrosion fatigue strength, including corrosion resistance, strength, and toughness, including sleeves of saxion rolls that are subjected to the above-mentioned high load operation. Is to provide.

[0005]

The stainless steel of the present invention has C: 0.08% or less, Si: 0.2 to 2%, Mn:
0.2-2%, Ni: 3-6%, Cr: 18-26%,
Mo: 1 to 4%, Cu: 2 to 4%, W: 0.5 to 3%,
N: 0.1 to 0.3%, the balance being substantially Fe, and the following formulas [I] to [III]: Cr (%) + 3.3Mo (%) + 16N (%) ≧ 30 (%) ... [ I] Cr (%) + Mo (%) + 1.5Si (%) ≦ 27 (%) ... [II] Cu (%) + W (%) ≧ 4 (%) ... [III] The austenite phase and δ-ferrite phase are the main phases, and the area ratio of δ-ferrite phase / austenite phase is 30/70 to 60/40, 5 to 15%.
It is characterized by having a structure in which the (area) martensite phase is mixed. In the stainless steel of the present invention, if desired, a part of Fe contains Ti: 1% or less, Nb: 1% or less, V: 1% or less, Al: 1% or less, Zr: 1% or less,
Co: A chemical composition substituted with one or more elements selected from 1% or less is provided.

[0006]

The stainless steel of the present invention has a high degree of corrosion resistance as an effect of containing Cr, Mo, N and the like, and also undergoes martensite transformation in a part of the austenite phase during the quenching process from the solution heat treatment temperature. The main phase composed of the austenite phase and the δ-ferrite phase has a structure in which a small amount of martensite phase is mixed, and the precipitation hardening heat treatment after the solution heat treatment strengthens the ferrite phase. It exhibits highly improved corrosion fatigue strength as a total effect such as high corrosion resistance due to the above corrosion resistant elements, high toughness due to the austenite phase, and high strength due to the strengthening of the ferrite phase and the mixing of the martensite phase.

The present invention will be described in detail below. The reasons for limiting the chemical composition of the steel of the present invention are as follows. % Indicating the element content is% by weight.

C: 0.08% or less C enhances the strength of the steel by the solid solution strengthening action and the precipitation strengthening action of carbides, but with the formation of chromium carbides, the toughness decreases and the Cr concentration in the vicinity of the Cr carbide particles. Therefore, the corrosion resistance such as pitting corrosion is reduced, so the content is made 0.08% or less.

Si: 0.2 to 2% Si is added in an amount of 0. 2 for the purpose of deoxidizing molten steel and improving the flowability of molten metal.
2% or more is required, but if added in a large amount, the toughness and weldability of the steel are deteriorated, so the upper limit is 2%.

Mn: 0.2-2% Mn is added as a deoxidizing / desulfurizing element of molten steel and 0.2% or more for improving castability, but if added in a large amount, corrosiveness decreases. The upper limit is 2%.

Ni: 3 to 6% Ni is a strong austenite forming element, and at least 3% is necessary to form a predetermined amount of austenite phase in the steel structure and secure toughness. But,
The addition of more than 6% not only impairs the economical efficiency but also loses the quantitative balance of the δ-ferrite phase and the austenite phase in the steel structure, so the upper limit is 6%.

Cr: 18-26% Cr is a ferrite-forming element and corrosion resistance of steel.
In particular, it is an element indispensable for securing resistance to pitting corrosion, intergranular corrosion, etc. in a strongly acidic corrosive environment. The amount requires at least 18%, and the effect increases with increasing amount.
However, if added in a large amount, the castability deteriorates, and the toughness decreases due to the lack of the austenite phase amount ratio in the steel structure, so the upper limit is 26%.

Mo: 1 to 4% Mo is an element effective for strengthening corrosion resistance, particularly resistance to pitting corrosion, crevice corrosion and the like. In order to obtain the effect of improving the corrosion resistance to the non-oxidizing acid and chloride ion-containing liquid, it is necessary to add 1% or more. However, if it exceeds 4%, the effect is almost saturated, and embrittlement and casting crack due to precipitation of σ phase are likely to occur, so the upper limit is 4%.

Cu: 2 to 4% Cu not only strengthens the austenite phase as a solid solution, but also precipitates from the ferrite phase as an α'phase to enhance the strength of the ferrite phase. This effect is obtained by adding 2% or more. In addition, the addition of Cu also enhances the pitting corrosion resistance of steel. However, if added in a large amount, the toughness of steel is impaired, so the upper limit is 4%.

W: 0.5-3% W enhances the strength of steel by solid solution strengthening the ferrite phase.
At least 0.5% is required to obtain this effect. Also, W improves the corrosion resistance of steel, and its effect is C
Increased by coexistence with u. However, if it exceeds 3%,
These effects are almost saturated and impair the economic efficiency, so the upper limit is set.

N: 0.1 to 0.3% N is a strong austenite-forming element, which strengthens the steel matrix by forming an interstitial solid solution, and distributes corrosion-resistant elements such as Cr and Mo to the austenite phase. It has the effect of increasing the corrosion resistance and improving the corrosion resistance of steel, especially the pitting corrosion resistance. This effect is obtained by adding 0.1% or more. However, 0.
If it exceeds 3%, the corrosion resistance is reduced due to the formation of nitrides, so this is the upper limit.

[I] Value of formula: 30 (%) or more Cr, Mo, and N in the formula are all elements effective in strengthening corrosion resistance, particularly pitting corrosion resistance. The corrosion resistance index is a standard for evaluating the pitting corrosion resistance of the steel of the present invention. By adjusting the contents of these elements within the respective specified ranges so that the calculated value becomes 30 (%) or more, high corrosion resistance for increasing the corrosion fatigue strength is secured.

Value of the formula [II]: 27 (%) or less Cr, Mo, and Si in the formula are all ferrite-producing elements, and the formula represents the Cr equivalent of the steel of the present invention. C
The corrosion resistance is increased by increasing the amounts of r and Mo, and the castability is improved by increasing the amount of Si. However, when the value of the formula [II] becomes large, the formation of martensite phase in the steel matrix is hindered and the strength of the steel is increased. Will not be fully achieved. Therefore, [I
I] The value calculated by the equation is limited to 27 (%) or less,
In the process of quenching (water cooling) from the solution heat treatment temperature, martensite transformation is caused to occur in part of the austenite phase.

Value of the formula [III]: 4 (%) or more In the formula, Cu and W are both ferrite-strengthening elements, and the precipitation strengthening action of Cu and the solid solution strengthening action of W improve the strength of the ferrite phase. Therefore, Cu is 2% or more,
As described above, it is necessary to add 0.5% or more of W, and the total amount of W is specified to be 4% or more in order to sufficiently enhance the ferrite phase by the coexistence of both elements. Is.

If desired, the stainless steel of the present invention has a T
The chemical composition is adjusted to contain one or more elements arbitrarily selected from i, Nb, V, Al, Zr, and Co. Ti: 1% or less Ti forms C and fixes C to form C
Prevents the precipitation of r-carbides (Cr ₂₃ C ₆ etc.) and the accompanying reduction in corrosion resistance. If the addition amount exceeds 1%, not only the effect is almost saturated, but also the toughness of the steel is lowered, so 1% is added.
Below. It is preferably 0.01 to 1%.

Nb: 1% or less Nb fixes C as NbC due to its strong affinity with C, and prevents precipitation of Cr carbide (Cr ₂₃ C ₆ etc.) and the accompanying reduction in corrosion resistance. If the added amount exceeds 1%, the effect is almost saturated, so the upper limit is 1%. Preferably, it is 0.04 to 1%.

V: 1% or less V enhances the strength of steel by the grain refining action. It also improves corrosion fatigue strength. If it exceeds 1%, the effect is almost saturated. It is preferably 0.02 to 1%.

Zr: not more than 1% Zr not only strengthens the austenite phase by solid solution but also forms stable MC type carbides due to its strong affinity with C to enhance the corrosion resistance of steel. In addition, it has a strong cleaning action and a structure refining action, and is effective in improving the material quality of steel. When added in large amounts,
Since it impairs the cleanliness of steel, it is set to 1% or less. It is preferably 0.02 to 1%.

Al: 1% or less Al forms carbides and enhances the strength of steel by the precipitation strengthening action. If added in a large amount, the toughness of steel will be impaired.
% Or less. It is preferably 0.2 to 1%.

Co: 1% or less Co is an element effective for solid solution strengthening the austenite phase and improving the corrosion fatigue strength of steel. The effect of improving the corrosion fatigue strength is strengthened by coexistence with Cu and W. If the addition amount exceeds 1%, the effect is almost saturated and the economy is impaired, so this is made the upper limit. Preferably 0.1-1%
Is.

The stainless steel of the present invention has a structure in which the austenite phase and the δ-ferrite phase are the main phases and a small amount of martensite phase is mixed therein. The amount ratio (δ-ferrite phase / austenite phase) of the main phase, austenite phase and δ-ferrite phase, is 30/70 to 60/4.
The reason why the range of 0 (area ratio) is defined is that if the strength is less than 30/70, the strength of the steel is insufficient, and if it exceeds 60/40, the toughness is insufficient.
A balance of strength and toughness is secured as a ratio of / 40. The amount of the mixed martensite phase accompanying the main phase is set to 5% (area) or more in order to sufficiently enhance the strength improving effect of the steel by the in-phase, 15%.
The reason why the (area) is set to the upper limit is that if it exceeds the upper limit, it becomes difficult to maintain high toughness and the corrosion resistance is deteriorated.

The stainless steel of the present invention is produced by subjecting the cast material to a solution treatment and then a precipitation strengthening heat treatment. As in the case of ordinary duplex stainless steel, the solution heat treatment is performed by heating and holding at a temperature of about 1000 to 1200 ° C. for an appropriate time (about 1 to 5 Hr per 1 inch of the wall thickness of the cast material), and then rapidly cooling (water cooling is performed). Is appropriate). By this solution treatment, the cast structure becomes a clean two-phase structure consisting of an austenite phase and a δ-ferrite phase, and in the quenching process from the same temperature, martensitic transformation occurs in a part of the austenite phase, thereby forming an austenite phase. It has a structure in which a martensite phase is mixed with a main phase composed of a δ-ferrite phase. Precipitation strengthening heat treatment
It is carried out by heating and holding at about 600 to 700 ° C. for an appropriate time (about 0.5 to 4 hours per 1 inch of wall thickness), and then air cooling. By this heat treatment, Cu dissolved in the ferrite phase is precipitated as an α'phase, and the strength of the steel is further enhanced by the precipitation strengthening action.

[0028]

EXAMPLE A centrifugal cast material having the chemical composition shown in Table 1 was heat-treated to obtain a test material, and the mechanical properties and corrosion test were performed on each material to obtain the results shown in Table 2. In the table, Nos. 11 to 15 are invention examples, Nos. 21 to 27 are comparative examples, and Comparative examples No. 21 to 24 are similar to the invention examples, but of formulas [I] to [III] Either rule (in the table,
An example having a chemical composition that does not satisfy (underline), No. 25 is C
This is an example lacking the inclusion of u. Each of the test materials except No. 21 (duplex stainless steel) was subjected to solution heat treatment of water cooling (about 50 ° C./min) after heating and holding at 1100 ° C. for 2 hours.
After heating and holding at 600 ° C. for 1 hour, precipitation strengthening heat treatment of air cooling was performed.

(A) Pitting corrosion test According to the ASTM G48A method, pitting corrosion loss (g / m ² ·
hr) is measured. Corrosion test solution: ferric chloride solution (concentration 6
%, Liquid temperature 50 ° C.), test time: 72 Hr. (B) Corrosion fatigue test etchant _{^{(Cl -: 100ppm, SO 4}} -: 1000p
^{_{pm, S 2 O 3 -:}} 10ppm, pH: 3.5, solution temperature:
Ono-type rotary bending test (bending stress: 19.6N) at room temperature
/ Mm ² , rotation speed: 3000 rpm), and the endurance limit (N / mm ² ) was measured after 5 × 10 ⁸ rotations.

As shown in Table 2, invention examples No. 11 to 1
Sample No. 5 has high strength and high toughness, and has significantly high corrosion fatigue strength, and its characteristics are far superior to those of other Comparative Examples No. 21 to 25. The pitting corrosion resistance of Comparative Example No. 21 is poor because it does not satisfy the formula I, and therefore, the corrosion fatigue strength is also low. No. 22 has low strength and toughness. The low strength is the formula II C
This is because the formation of the martensite phase was insufficient because the r equivalent value was excessive. Therefore, while having good corrosion resistance,
Corrosion fatigue strength is hardly improved. Since No. 23 and No. 24 do not satisfy the formula III,
There is a lack of strength. Due to its insufficient strength, the corrosion resistance and toughness are good, but the corrosion fatigue strength is also low. Since No. 25 does not contain Cu, its strength is low and its corrosion resistance is not sufficient, so that its corrosion fatigue strength is hardly improved.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

The stainless steel of the present invention has strength, toughness,
Since it has excellent corrosion resistance and a high degree of corrosion fatigue strength that cannot be obtained with conventional duplex stainless steel, etc., it can be used as a sleeve material for saxion rolls in the paper manufacturing process, and it can be used in high-load operations in recent years. The service life of the saxion roll is improved and roll maintenance is improved. INDUSTRIAL APPLICABILITY The stainless steel of the present invention is useful not only as a sleeve material for a sacion roll but also as various structural material used in an environment in which a corrosion factor and repeated stress act on weight.

Claims (2)

[Claims] 1. C: 0.08% or less, Si: 0.2 to 2
%, Mn: 0.2 to 2%, Ni: 3 to 6%, Cr: 18
.About.26%, Mo: 1 to 4%, Cu: 2 to 4%, W: 0.
5 to 3%, N: 0.1 to 0.3%, the balance substantially consisting of Fe, and the following formulas [I] to [III]: Cr (%) + 3.3Mo (%) + 16N (%) ≧ 30 (%) ... [I] Cr (%) + Mo (%) + 1.5Si (%) ≦ 27 (%) ... [II] Cu (%) + W (%) ≧ 4 (%) ... [III] The composition has an austenite phase and a δ-ferrite phase as main phases, and the area ratio of the δ-ferrite phase / austenite phase is 30/70 to 60/40, which is 5 to 15%.
High-corrosion fatigue strength stainless steel having a structure in which (area) martensite phase is mixed. 2. A part of Fe is Ti: 1% or less, Nb:
1% or less, V: 1% or less, Al: 1% or less, Zr: 1%
The high corrosion fatigue strength stainless steel according to claim 1, which has a chemical composition substituted with one or more elements selected from Co: 1% or less.