JPH09256109A - High toughness and high corrosion fatigue strength duplex stainless steel with excellent drill workability - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a duplex stainless steel having excellent corrosion resistance, toughness, corrosion fatigue strength and the like, which is used as a material for a body of a suction roll for a papermaking machine and has excellent drilling workability. To do. SOLUTION: This duplex stainless steel has C: 0.04%
Below, Si: 0.2-2.0%, Mn: 0.2-2.0
%, Cr: 23.0 to 25.0%, Ni: 5.0 to 6.
5%, Mo: 1.0 to 2.0%, Cu: 0.9% or less,
N: 0.03 to 0.08%, W: 0.5 to 2.0%, C
r equivalent / Ni equivalent: 2.0 to 2.3, the balance being substantially Fe
And the amount of ferrite in the two-phase structure is 70-85 area%
It is.

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 having high toughness, high corrosion fatigue strength and the like, which is useful as a body member of a suction roll for a papermaking machine and has excellent drilling workability.

[0002]

2. Description of the Related Art A body member (sleeve) of a saxion roll for dehydrating wet paper made in a papermaking process is
Moisture squeezed from the wet paper, the so-called "white water" number of small holes for sucking eliminate (chlorine ions Cl ^{- -} and sulfate ions SO ₄ such strongly acidic corrosive liquid containing) (Saku Chillon holes) are distributed bored It is a hollow cylindrical member, and its material properties include corrosion resistance to white water, mechanical strength that can withstand repeated loading of pressing force (nip pressure) of a press roll that squeezes white water from wet paper, and corrosion fatigue strength. It is required to have both. Conventionally, duplex stainless steel ((JIS G4307 S
US329J1, G5121 SCS13A, SCS14A, etc.) have been used.
Duplex stainless steel is a corrosion-resistant alloy having strength, toughness, corrosion resistance, etc. due to a mixed-phase structure of δ-ferrite phase and austenite phase.
Precipitation hardening type materials having a high content of Cu and the like, low C type materials and the like have also come into practical use.

[0003]

The body sleeve material of a suction roll for a papermaking machine has a hollow cylindrical tube body (centrifugal force casting tube, etc.) that has been subjected to a predetermined heat treatment and then a suction hole is bored. It is manufactured through the steps. The suction holes are dispersedly formed in the roll body at an opening ratio of about 20 to 50% by area, and the number of holes per roll reaches hundreds of thousands.
The suction hole is drilled. The body member of the conventional suction roll has been variously devised in alloy design for the purpose of improving the corrosion resistance and the corrosion fatigue strength, but it cannot be said to be satisfactory from the viewpoint of drill workability. The present invention provides a duplex stainless steel having improved drilling workability while maintaining material properties such as corrosion resistance, toughness, strength, and corrosion fatigue strength required for a body member of a suction roll.

[0004]

The duplex stainless steel of the present invention has C: 0.04% or less, Si: 0.2 to 2.0%,
Mn: 0.2-2.0%, Cr: 23.0-25.0
%, Ni: 5.0 to 6.5%, Mo: 1.0 to 2.0
%, Cu: 0.9% or less, N: 0.03 to 0.08%,
W: 0.5 to 2.0%, Cr equivalent / Ni equivalent: 2.0
2.3, the balance consists essentially of Fe, and the amount of ferrite in the two-phase structure is 70 to 85 area%.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The duplex stainless steel of the present invention has a δ
-It has a two-phase structure in which the ferrite phase occupies 70 to 85 area%, the hardness (Brinell) is 200 to 240, and the work hardening coefficient (n) is as low as 0.30 or less. The duplex stainless steel of the present invention has various properties such as corrosion resistance, mechanical properties, and corrosion fatigue strength required for a body member of a suction roll as an effect of the above chemical composition and duplex composition, and has work hardening ability. It has a low material property. Abrasion of the tool (drill) blade edge during drilling is promoted by contact with work-hardened chips and component blade edges. The dual-phase stainless steel of the present invention has a low work hardening ability, so that the wear rate of the tool edge is slow, and therefore the increase in cutting resistance with the increase in cutting length is small, and efficient drilling is possible. .

The reasons for limiting the components of the present invention are as follows. C: 0.04% or less C is an element that forms and stabilizes the austenite phase,
As the C content increases, the toughness decreases due to the precipitation of Cr carbide, and the Cr concentration near the Cr carbide particles is diluted (sensitization).
Causes deterioration of corrosion resistance. Therefore, 0.04%
The following is assumed.

Si: 0.2-2.0% Si is a deoxidizing agent during the melting of steel, and is an element that improves the melt flowability in the casting process, and is at least 0.2.
%, But if added in a large amount, the toughness and weldability of steel deteriorate, so 2.0% is made the upper limit.

Mn: 0.2-2.0% Mn is added as a deoxidizing / desulfurizing element for molten steel. To obtain this effect, 0.2% or more is required, but if it exceeds 2.0%, the corrosion resistance is deteriorated, so this is made the upper limit.

Cr: 23.0 to 25.0% Cr is an element necessary for ensuring the corrosion resistance, forms a ferrite phase, and enhances the strength of the alloy. To obtain this effect, 23.0% or more is required. However, if the amount is too large, the toughness is impaired and the corrosion resistance becomes insufficient due to the relative decrease in the austenite phase. Therefore, the upper limit is 25.0%.

Ni: 5.0-6.5% Ni is an austenite phase formation / stabilization element,
It is an element necessary to secure the quantitative balance between the austenite phase and the ferrite phase of the two-phase structure, and to secure the toughness and corrosion resistance. To obtain this effect, it is set to 5.0% or more. However, if the amount is excessively increased, the austenite phase increases and the work hardening coefficient of the alloy increases, so that the drill workability decreases. Further, Ni is an expensive element, and excessive addition impairs economic efficiency. Therefore, 6.5%
Is the upper limit.

Mo: 1.0 to 2.0% Mo is an element contributing to the improvement of corrosion resistance, and this effect is obtained by adding 1.0% or more. But 2.0
If it exceeds%, the effect of improving the corrosion resistance is almost saturated and the toughness is deteriorated. Further, Mo is an effective element, and excessive addition impairs economic efficiency. Therefore, the upper limit is 2.0%.

Cu: 0.9% or less Cu is a corrosion resistance improving element, but if added in a large amount over 0.9%, toughness is lowered and corrosion fatigue strength is lowered, so this is the upper limit. To do.

N: 0.03 to 0.08% N is an austenite-forming element, and also has pitting resistance,
It is an element effective in improving corrosion resistance such as crevice corrosion resistance. This effect is obtained by adding 0.03% or more. However, the duplex stainless steel of the present invention has a low amount ratio of austenite phase and a small solid solution amount of N in steel. Therefore, when the content exceeds 0.08%, nitrides such as Cr and Mo are generated. In addition to causing deterioration in toughness, corrosion resistance, etc., drill workability also deteriorates. Therefore, the upper limit is 0.08%. N is dissolved in the austenite phase in a large amount and is hardly dissolved in the ferrite phase. Therefore, in the alloy steel of the present invention having a high ferrite phase content ratio, the effect can be obtained by adding such a small amount of N. .

W: 0.5 to 2.0% W improves corrosion resistance and corrosion fatigue strength, and solid-solution strengthens the ferrite phase. This effect is obtained by adding 0.5% or more. The effect of W is enhanced by coexistence with Cu. However, if added in a large amount, the corrosion resistance, toughness, corrosion fatigue strength and the like are deteriorated, so 2.0% is made the upper limit.

Cr equivalent / Ni equivalent: 2.0 to 2.3 Cr equivalent and Ni equivalent are values calculated by the following equations. This value is limited to the above range because δ−
By forming a two-phase structure in which the amount ratio of the ferrite phase is 70 to 85 area% as described above, the work hardening coefficient is reduced to a low value while maintaining the strength required for the suction roll and the like by the balance of the two phases. This is to suppress and secure good drill workability. That is, if the above equivalent ratio is less than 2.0, not only the austenite phase amount ratio becomes excessive, resulting in insufficient strength, but also the work hardening coefficient becomes high, while the equivalent ratio exceeds 2.3. In this case, contrary to the above, the amount ratio of the ferrite phase becomes excessive, so that the hardness of the alloy becomes unnecessarily high, and in any case, the cutting resistance during drilling becomes large.

[Equation 1] Cr equivalent =% Cr + 1.5 ×% Si + 1.4% Mo-4.99 Ni equivalent =% Ni + 30 ×% C + 0.5% Mn + 26 (% N-0.02) +2.77

The heat treatment for heat treatment of the duplex stainless steel of the present invention is carried out by heating and holding in a temperature range of about 1000 to 1100 ° C. for an appropriate time (about 1 to 5 hr per 1 inch of the wall thickness of the cast material).
This is done by cooling. Cooling from the solution temperature is about 5 to 15 ° C./in order to suppress the generation of residual stress.
A relatively slow cooling rate of the order of minutes is desirable. In the manufacture of sleeves for body members of suction rolls for papermaking machines, a hollow cylinder is centrifugally cast, and after the solution heat treatment described above, a suction hole is formed by drilling and finishing machining is performed. A suction roll is finished by fitting and assembling the members.

[0017]

EXAMPLE A centrifugal cast material (hollow cylinder) of duplex stainless steel having the chemical composition shown in Table 1 is subjected to solution heat treatment to obtain a test material. “Eq.Cr/Ni ratio” in Table 1 is Cr equivalent /
The value of Ni equivalent is shown. Table 2 shows the material properties obtained for each test material. Cast material size: Outer diameter 600, Wall thickness 72, Length 3000 (mm) Heat treatment (solution heat treatment): 1050 ℃ × 6 Hr → Air cooling (10 ℃
/ Min)

[Corrosion test] A test piece is immersed in a corrosive liquid to determine a corrosion rate (g / m ² h) (in accordance with ASTM G48A method). Corrosion test liquid: ferric chloride solution (FeCl _3, concentration 6%), liquid temperature
50 ℃ test time: 72hr

[Corrosion Fatigue Test] The durability limit (MPa) was measured by an Ono-type rotary bending fatigue tester. Test piece: φ10 × 30 ^L (parallel part) Corrosion test liquid: Cl ^- 100 ppm, SO ₄ ^2- 1000 ppm, pH 3.5 Rotation speed: 3000 rpm Number of cycles: 10 ⁸

[Drillability] Test material (however, plate thickness 50
mm) is drilled with a gun drill to form 100 through holes.
Cutting resistance (torque, kg ・
cm) is measured. Drill diameter: φ4.3 mm Rotation speed: 5000 rpm Feed rate: 0.008 mm / revolution

[0021]

[Table 1]

[0022]

[Table 2]

Invention Example (No. 1 to 3) and Comparative Example (No. 11 to 1)
In comparison with 9), Comparative Example No. 11 has the same mechanical properties such as strength and elongation, and drilling workability as those of the invention example, and the corrosion fatigue strength is relatively good, but the Ni content Insufficient corrosion resistance and toughness due to lack and excess ferrite phase ratio (insufficient austenite phase ratio). Comparative example No.1
No. 2 has high toughness, but due to lack of austenite phase, strength, corrosion resistance, corrosion fatigue strength are low, and drill workability is also poor. Comparative Example No. 13 has a high toughness, but due to the lack of the ferrite phase, the yield strength is low, the corrosion fatigue strength is lowered, and the drill workability is also poor. In addition,
The reason why the corrosion resistance is low is that the N content is relatively insufficient with respect to the high content ratio of the austenite phase. Comparative example No.1
No. 4 has good strength, ductility and toughness, and is excellent in drilling workability, but due to insufficient N content, it has low corrosion resistance, and since corrosion resistance is low, corrosion fatigue strength is also low. ing.

Comparative Examples No. 15 and No. 16 have high strength, but due to the precipitation of a large amount of nitride, the toughness, corrosion resistance and corrosion fatigue strength are low. This is due to a large amount of precipitation, and the drillability is poor due to the hardening due to the precipitation of nitrides. Comparative Example No. 17 is
Corrosion resistance and toughness are good due to the large content of Mo and the high austenite content ratio, and it has high strength due to the solid solution strengthening action of the austenite phase due to the large content of N. Due to lack of / Ni ratio (increased austenite amount ratio, increased work hardening coefficient), drill workability is extremely poor. Further, the content of each element of No. 18 and No. 19 satisfies the regulations of the present invention, but the former is Eq.C.
The r / Ni ratio and the ferrite content ratio are too high, the toughness is inferior, the corrosion resistance, the corrosion fatigue strength, etc. are at a low level.
Insufficient Ni ratio and ferrite amount ratio, low strength, and poor drill workability. On the other hand, the invention example has strength, mechanical properties such as ductility and toughness, corrosion resistance and corrosion fatigue strength, etc. necessary for the body member of the suction roll, etc. It has low resistance and improved drillability.

[0025]

INDUSTRIAL APPLICABILITY The duplex stainless steel of the present invention has good mechanical properties, corrosion resistance, and corrosion fatigue strength, and at the same time, has improved drill workability. It is preferably used as a material for a body of a suction roll. Needless to say, the application of the duplex stainless steel of the present invention is not limited to this, and is useful as various structural part materials that require the above-mentioned properties.

Claims (1)

[Claims] 1. C: 0.04% or less, Si: 0.2-
2.0%, Mn: 0.2 to 2.0%, Cr: 23.0 to
25.0%, Ni: 5.0-6.5%, Mo: 1.0-
2.0%, Cu: 0.9% or less, N: 0.03 to 0.0
8%, W: 0.5 to 2.0%, Cr equivalent / Ni equivalent:
A high-toughness / high-corrosion fatigue strength duplex stainless steel with 2.0 to 2.3, the balance being essentially Fe, and having a ferrite content of 70 to 85 area%, which is excellent in drill workability.