JP3720223B2 - Duplex stainless steel excellent in heat fatigue resistance, corrosion fatigue resistance, drilling workability, etc. - Google Patents

Description

【００３２】
【表２】

【００３３】
【発明の効果】
本発明の二相ステンレス鋼は、強度，靭性等の機械的性質、耐食性，耐腐食疲労性，耐熱疲労性等に優れていると共に、良好なドリル加工性を有し、製紙機サクションロール等の腐食と機械的応力が重畳作用する使用環境に供される部材料として好適である。殊に、サクションロール材として、抄紙速度の高速化に伴うロール胴部材の熱疲労の問題に効果的に対処することを可能とするものであり、しかもＮｉ，Ｍｏ等の高価な元素を節減された成分構成を有していることにより、経済性にも優れている。
【図面の簡単な説明】
【図１】製紙工程におけるサクションロール部を示す断面説明図である。
【符号の説明】
１：サクションロール胴部
１_１：サクションホール
２：プレスロール
３：サクションボックス
３_１：シール材
４：フェルト
５：湿潤紙[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ferritic-austenitic duplex stainless steel and a suction roll body member that are excellent in heat fatigue resistance and corrosion fatigue resistance useful as a papermaking suction roll material.
[0002]
[Prior art]
The papermaking suction roll is a porous roll used for the dehydration treatment of the wet paper that has been made, and its roll body contains water separated from the wet paper, so-called “white water” (Cl ⁻ , SO ₄ ^2−, etc.). Small holes “suction holes” (opening ratio ≈ 20 to 50 area%, the number of holes per roll reaches several tens of liters) are distributed and drilled. A hollow cylinder. A strong pressing force (nip pressure) of a press roll that squeezes moisture from wet paper acts on the surface of the roll body. Since the suction roll is exposed to such a severe corrosive environment and has a large load, it has been a problem that cracks due to corrosion fatigue are likely to occur. This problem has been addressed by material improvements based on duplex stainless steel materials, particularly by increasing the corrosion fatigue strength.
[0003]
[Problems to be solved by the invention]
In recent years, with the increase in papermaking speed, as a new problem in the actual use of the suction roll, the inner surface temperature of the roll body is, for example, 300 by frictional heat at the contact interface between the seal material of the suction box and the roll body. It has been pointed out that the temperature rises to 350 ° C., and cracks are likely to occur in the roll body due to thermal fatigue associated with the thermal cycle. In FIG. 1, reference numeral 1 denotes a body portion of the suction roll, and 3 denotes a suction box disposed therein. The suction box 3 is in contact with the inner peripheral surface of the roll body 1 via a sealing material 3 ₁ (a member made of phenol resin or graphite). The wet paper 5 held by the felt 4 passes between the suction roll 1 and the press roll 2 in synchronism with the rotational peripheral speed of the roll, and the squeezed moisture passes through the suction hole ₁₁ to the suction box 3. It is eliminated by the suction action.
[0004]
Heat generated by the friction of the sealing member 3 _1, insufficient supply of lubricating water for the sealing material, such as pressing of excessive sealant against the roll periphery is due mainly to maintenance issues. However, it is difficult to ensure complete maintenance to prevent the above-mentioned problems associated with higher papermaking speed, and the risk of rubbing of the sealing material is increasing. There is a need for new roll materials that can cope with reduced life.
The present invention has improved thermal fatigue to meet the above requirements, saves expensive and rare elements such as Cr, Mo, Ni, etc., improves economy, and is good in manufacturing a suction roll. The present invention provides a duplex stainless steel having drillability.
[0005]
[Means for Solving the Problems]
The ferrite-austenite duplex stainless steel of the present invention is
By weight, C: 0.05% or less, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 20.0-23.0%, Ni: 3.0 3.9%, Mo; 0.5-0.96%, Cu: 0.2-1.0%, W: 0.1-2.0%, N: 0.05-0.2% And satisfying the following formula [1], Ti: 0.5% or less, Nb: 0.5% or less, V: 1.0% or less, Al: 0.5% or less, Zr: 0 0.5% or less, B: 0.5% or less, rare earth element: 0.2% or less, Co: 1.0% or less, Ta: 1.0% or less, Bi: 1.0% or less It contains one or more elements and the balance has a chemical composition consisting of Fe and inevitable impurities , and the ferrite content (% α) in the microstructure is 45 to 80 area%.
% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%) [1]
[0006]
In duplex stainless steel, elements such as Cr and Mo have a great influence on corrosion resistance, and it is generally said that the greater the content, the higher the corrosion resistance. However, these are expensive and rare elements, and if the amount of addition (both are ferrite formers) is increased, the amount of addition of Ni (austenite former), which is also a rare element, is increased from the viewpoint of component balance. Need arises. Furthermore, an increase in these elements leads to a decrease in drill workability. The present invention is provided with improved heat fatigue resistance, corrosion fatigue resistance characteristics, good drill workability and the like while reducing these rare elements.
[0007]
First, the reasons for limiting the chemical composition of the present invention will be described. All element contents (%) are based on weight.
C: 0.05% or less C increases the strength of the alloy by the solid solution strengthening action, but if the content increases, precipitation of chromium carbide causes a decrease in toughness and a deterioration in corrosion resistance. Therefore, it should not exceed 0.05%.
[0008]
Si: 0.1 to 2.0%
Si is a deoxidizer during melting of the alloy, and is an element that improves the flow of molten metal during casting. For this reason, at least 0.1% is required, but if added in a large amount, the toughness and weldability of the alloy are reduced, so 2.0% is made the upper limit.
Mn: 0.1 to 2.0%
Mn is added as a deoxidizing / desulfurizing element. In order to obtain this effect, 0.1% or more is required. However, if it exceeds 2.0%, the corrosion resistance is lowered, so this is the upper limit.
[0009]
Cr: 20.0-23.0%
Cr forms a ferrite phase in the microstructure and increases the strength of the alloy. Further, it is an element necessary for enhancing the corrosion resistance of the alloy, particularly pitting corrosion resistance and intergranular corrosion resistance. For this reason, content of at least 20.0% is required. However, if added in a large amount, the toughness and weldability deteriorate, so 23.0% is made the upper limit.
[0010]
Ni: 3.0 to 3.9%
Ni is a strong austenite generating element and is an element necessary for balancing the ferrite-austenite in the microstructure, and forms an austenite phase to increase the toughness of the alloy. For this purpose, a content of at least 3.0% is required. However, Ni is an expensive element, and the upper limit is 3.9%.
[0011]
Mo:. 0.5~ 0 96%
Mo is a corrosion-resistant element and is particularly effective in improving pitting corrosion resistance and intergranular corrosion resistance. This effect is obtained by adding 0.5% or more. However, it said element is an expensive element similarly to Ni, and because it accompanied the increase in the attendant deterioration of toughness of the alloy, and 0.96% or less.
[0012]
Cu:. 0 2~1.0%
Cu improves corrosion resistance and intergranular corrosion resistance. However, if it exceeds 2.0%, the toughness and ductility will be insufficient, and the corrosion resistance will be lowered, so this should not be exceeded. Preferably, it is 0.2 to 1.0%.
[0013]
W: 0.1 to 2.0%
W has an effect of improving corrosion resistance, and contributes to improvement of thermal fatigue characteristics by solid solution strengthening of the ferrite phase. In order to obtain this effect, 0.2% or more is required. However, excessive addition impairs the toughness of the alloy and causes a decrease in corrosion resistance and corrosion fatigue strength, so it should not exceed 2.0%.
[0014]
N: 0.05-0.2%
N is an austenite-forming element and increases the distribution rate of corrosion-resistant elements such as Cr and Mo with respect to the austenite phase, thereby enhancing the corrosion resistance of the alloy. Further, as described later, fine precipitation of chromium nitride in the ferrite grains and in the grain boundaries contributes to the enhancement of resistance to thermal fatigue damage. This requires at least 0.05%. If it exceeds 0.2%, shrinkage cavities are likely to occur in the cast material, and the balance between the ferrite phase and austenite phase of the microstructure is impaired. Preferably, it is 0.1 to 0.2%.
[0015]
The alloy of the present invention is optionally added with one or more elements of Ti, Nb, V, Al, Zr, B, rare earth elements (REM), Co, Ta, Bi.
Ti, Nb, V, Al, Zr, B, REM (Y, La, Ce, Sm, etc.):
Each of these elements is added in a small amount to refine crystal grains and increase the strength of the alloy. It also has the effect of improving drill workability. Addition of a large amount not only impairs economic efficiency, but also causes a decrease in toughness. Therefore, Ti ≦ 0.5%, Nb ≦ 0.5%, V ≦ 1.0%, Al ≦ 0.5%, Zr ≦ 0.5%, B ≦ 0.5%, REM ≦ 0.2 %, Respectively.
[0016]
Co, Ta: Has an effect of improving corrosion resistance. Addition amounts are Co ≦ 1.0% and Ta ≦ 1.0%. Addition exceeding this amount is useless, and as the amount increases, drill workability decreases.
Bi: Has an effect of improving drill workability. This effect is obtained by addition of 1.0% or less, and addition exceeding this impairs economic efficiency.
[0017]
[1] Formula:% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%)
The content of Cr, Mo and N affects the drill workability. The drilling workability becomes better as the value of the formula [1] is lower, and it has been found that good drill workability can be secured by mutually adjusting the contents of these elements so that the value is 28 or less. did.
[0018]
Ferrite content ( % α): 45 to 80 area%
The amount ratio (area%) of the ferrite phase occupying the micro-rough structure (ferrite-austenite two-phase structure) is 45 to 80%. In addition to the balance effect of both phases, the strength and toughness are combined and the corrosion fatigue strength This is to improve the drilling workability and the drilling workability. If the amount of ferrite is less than this, the strength is insufficient and the drillability is lowered. An increase in the ferrite content ratio is advantageous in terms of increasing the corrosion fatigue strength, but if it exceeds 80%, the decrease in toughness increases.
[0019]
The duplex stainless steel of the present invention is preferably added with a component adjustment represented by the following formula [2].
Ferrite content ( % α) ≧ 0.2 × (% Cr /% N) +25 ... [2]
This ferrite grains and grain boundaries of the microstructure, in which more strengthens the fine precipitation of chromium nitride, defining the component adjustment for enhancing resistance to thermal fatigue damage.
[0020]
That is, since ferrite and austenite have different coefficients of thermal expansion (thermal expansion coefficient of austenite> thermal expansion coefficient of ferrite), the inner surface of the body part is heated by friction of the seal material of the suction box when using the actual suction roll. In the process, a microscopic thermal stress is generated in the austenite grains and tensile in the ferrite grains. In the cooling process, the austenite grains are pulled in tension, and the ferrite grains are loaded with a compressive thermal stress. By repeatedly raising and lowering the temperature accompanying the rotation of the roll, the thermal stress of compression / tension is repeatedly applied while being reversed. In addition, shearing thermal stress is superimposed on the grain boundary.
[0021]
Fracture starts due to the repetition of thermal stress on such grain boundaries and ferrite grains, and microcracks generated in the initial process gradually bond and grow, leading to breakage of the member. By satisfying the above formula [2], chromium nitride is precipitated in the slow cooling process in the solution heat treatment of the cast material, and as an effect of strengthening the ferrite grains and grain boundaries, the heat fatigue resistance can be further increased. Become.
[0022]
Solution heat treatment of cast material:
The duplex stainless steel of the present invention is subjected to a solution heat treatment for eliminating and homogenizing carbide solid solution and microsegregation in the cast structure. In this heat treatment, after heating and maintaining at a temperature of 900 to 1100 ° C. (α-γ two-phase temperature range) for an appropriate time (about 1 Hr / thickness of 1 inch), slow cooling (cooling in the furnace) of 5 ° C./min or less Achieved by doing.
[0023]
The solution heating temperature is set to 900 ° C. or more in order to cause sufficient solid solution and homogenization of the carbide, and there is a risk of sigma phase formation and embrittlement in a lower temperature range. Setting the upper limit temperature to 1100 ° C does not have the benefit of heating at a temperature higher than this, resulting in wasted heat economy and increased burden on furnace maintenance, and the balance in the quantity ratio of ferrite-austenite is impaired as the temperature increases. Because it is.
The reason why the cooling rate from the solution temperature is limited to 5 ° C./min or less (excluding 5 ° C./min) is to reduce the residual stress as much as possible. This reduction of residual stress by slow cooling, and the strengthening of the ferrite grains and grain boundaries by precipitation of the above chromium nitride, resistance to thermal fatigue resistance which is improved is provided. It should be noted that the cooling rate has no benefit of lowering than 0.5 ° C./min, and it is appropriate to set this as the lower limit in terms of productivity and the like.
[0024]
The roll drum member of the paper production suction roll made of the duplex stainless steel of the present invention is obtained by centrifugal hollow casting to obtain a hollow cylindrical body, subjected to the above solution heat treatment, drilling a suction hole, and finishing. Manufactured by.
[0025]
【Example】
A centrifugal cast pipe having the chemical composition and structure shown in Table 1 is subjected to solution heat treatment to obtain a test material. The solution heat treatment was performed by heating and holding at 1030 ° C. for 2 hours in a heating furnace, followed by cooling in the furnace (cooling rate: about 2 ° C./min). The test tube size (after machining) has an outer diameter of 250, a wall thickness of 50, and a length of 250 (mm).
Table 2 shows the physical properties of each specimen.
[0026]
[Corrosion test]
Measures pitting loss (g / m ² h) according to ASTM G48A method.
Corrosion test solution: ferric chloride solution (concentration 6%, solution temperature 50 ° C)
Test time: 72Hr
[0027]
[Corrosion fatigue test]
Measures the number of iterations (number of breaks) that lead to corrosion fatigue failure under the following test conditions using an Ono rotating bending fatigue tester.
Test piece: JIS Z2274 No. 1 (Parallel part φ10 × 35L)
Etchant ^{(TAPPI II): Cl - 1000ppm} , SO 4 2- 1000ppm, pH 3.5
Bending stress: 300 MPa
[0028]
[Thermal fatigue test]
The heat fatigue resistance is evaluated by repeating the heating and cooling of the test body and the number of repetitions of heating and cooling until cracks occur. The heating temperature was raised by heating the inner side surface of the test body with a high-frequency heating coil, and the cooling temperature was lowered by forcibly cooling the outer side surface via a cooling water pipe.
Specimen: Hollow cylinder (outer diameter 250 x wall thickness 50 x length 250 mm)
Maximum temperature: 400 ° C
Minimum temperature: 50 ° C
Temperature increase rate: 1 ° C / second Temperature decrease rate: 1 ° C / second
[Drill workability test]
The workability is evaluated based on the edge wear amount (mm) in the following drilling.
Drill: Gun drill (Cemented carbide, drill diameter 4.0mm)
Cutting length: 10m
Rotation speed: 4500rpm
Feed rate: 60 mm / min Cutting oil pressure: 50 kg / cm ²
[0030]
As shown in Tables 1 and 2, Comparative Examples Nos. 101 to 103 do not satisfy the chemical composition of the present invention, the formula [1], or the amount of ferrite, so that the strength and thermal fatigue characteristics are extremely low. No. 101 is inferior in corrosion fatigue characteristics, No. 102 is inferior in impact resistance, and No. 103 is inferior in corrosion resistance and corrosion fatigue characteristics. Moreover, all are inferior to drill workability.
On the other hand, the present invention has improved properties such as strength, toughness, corrosion resistance, thermal fatigue properties, corrosion fatigue properties, and drill workability.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
【The invention's effect】
The duplex stainless steel of the present invention is excellent in mechanical properties such as strength and toughness, corrosion resistance, corrosion fatigue resistance, heat fatigue resistance, etc. and has good drill workability, such as a paper machine suction roll. It is suitable as a material used in a use environment where corrosion and mechanical stress are superposed. In particular, as a suction roll material, it is possible to effectively cope with the problem of thermal fatigue of the roll body member accompanying an increase in papermaking speed, and it is possible to save expensive elements such as Ni and Mo. In addition, the composition is excellent in economic efficiency.
[Brief description of the drawings]
FIG. 1 is a cross-sectional explanatory view showing a suction roll portion in a paper making process.
[Explanation of symbols]
1: Suction roll body 1 ₁ : Suction hole 2: Press roll 3: Suction box 3 ₁ : Sealing material 4: Felt 5: Wet paper

Claims (7)

By weight, C: 0.05% or less, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 20.0-23.0%, Ni: 3.0 3.9%, Mo; 0.5-0.96%, Cu: 0.2-1.0%, W: 0.1-2.0%, N: 0.05-0.2% Containing and formula:
% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%)
And the balance has a chemical composition consisting of Fe and inevitable impurities , and the ferrite content (% α) in the microstructure is 45 to 80 area%. Excellent ferritic-austenitic duplex stainless steel. By weight, C: 0.05% or less, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 20.0-23.0%, Ni: 3.0 -3.9%, Mo; 0.5-0.96%, Cu: 0.2-1.0%, W: 0.1-2.0%, N: 0.05-0.2%, And Ti: 0.5% or less, Nb: 0.5% or less, V: 1.0% or less, Al: 0.5% or less, Zr: 0.5% or less, B: 0.5% or less, rare earth Element: not more than 0.2%, Co: not more than 1.0%, Ta: not more than 1.0%, Bi: not more than 1.0% and containing one or more elements selected from the group :
% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%)
And the balance has a chemical composition consisting of Fe and inevitable impurities , and the ferrite content (% α) in the microstructure is 45 to 80 area%, such as heat fatigue resistance, corrosion fatigue resistance, drill workability, etc. Excellent ferritic-austenitic duplex stainless steel. The ferrite content (% α) in the microstructure is given by the formula:
α ≧ 0.2 × (% Cr /% N) +25
The ferrite-austenite duplex stainless steel according to claim 1 or 2, wherein chromium nitride is precipitated in the ferrite grains and grain boundaries of the microstructure.   The ferrite-austenite duplex stainless steel according to any one of claims 1 to 3, which is a body member of a papermaking suction roll. By weight, C: 0.05% or less, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 20.0-23.0%, Ni: 3.0 3.9%, Mo; 0.5-0.96%, Cu: 0.2-1.0%, W: 0.1-2.0%, N: 0.05-0.2% Containing and formula:
% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%)
And the balance is 0.5 to 5 ° C./minute (however, 5 ° C./minute) after the solution of the centrifugally cast hollow cylinder made of Fe and inevitable impurities is heated in the temperature range of 900 to 1100 ° C. For a papermaking made of a ferrite-austenitic duplex stainless steel characterized in that the ferrite content (% α) in the microstructure is 45 to 80 area%, which is cooled to room temperature at a cooling rate of Suction roll body member. By weight, C: 0.05% or less, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 20.0-23.0%, Ni: 3.0 -3.9%, Mo; 0.5-0.96%, Cu: 0.2-1.0%, W: 0.1-2.0%, N: 0.05-0.2%, And Ti: 0.5% or less, Nb: 0.5% or less, V: 1.0% or less, Al: 0.5% or less, Zr: 0.5% or less, B: 0.5% or less, rare earth Element: not more than 0.2%, Co: not more than 1.0%, Ta: not more than 1.0%, Bi: not more than 1.0% and containing one or more elements selected from the group :
% Cr + 3.3 ×% Mo + 16 ×% N ≦ 28 (%)
The filled, after the remaining portion is a hollow cylindrical body which is cast centrifugal force consisting F e and unavoidable impurities, was heated solution at a temperature range of 900 to 1100 ° C., 0.5 to 5 ° C. / min (however, 5 ° C. Made of ferrite-austenitic duplex stainless steel characterized in that the ferrite content (% α) in the microstructure is 45 to 80 area%, which is cooled to room temperature at a cooling rate of (except / min). Suction roll body member. The ferrite content (% α) in the microstructure is given by the formula:
α ≧ 0.2 × (% Cr /% N) +25
The suction roll body member for papermaking according to claim 5 or 6, wherein chromium nitride is precipitated in ferrite grains and grain boundaries of the microstructure.

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