JPH07268552A - Precipitation hardening type duplex stainless cast steel for dehydrating roll of valve seat - Google Patents

Abstract

PURPOSE:To enable the efficient dehydrating operation of a valve sheat by constituting a stainless cast steel of specified % of C, Si, Mn, Cr, Ni, Mo, Cu and N, and the balance substantial Fe. CONSTITUTION:This stainless cast steel is constituted of a and the balance substantial Fe. two layer structure of a delta ferritic phase and an austenitic phase. The austenitic phase as a soft phase is dispersedly strengthened by the delta-ferritic phase as a hard phase. The componental compsn. of this stainless steel is constituted of 0.1 to 2% C, <=2% Si, <=2% Mn, 24 to 27% Cr, 3 to 7% Ni, 0.5 to 3% Mp, >2 to 4% Cu and 0.01 to 0.3% N, and the balance substantial Fe. Thus, by high corrosion fatigue strength, the durability of a press roll can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless cast steel having excellent corrosion fatigue strength which is a material for a body of a press roll for dehydrating thick sheet pulp.

[0002]

2. Description of the Related Art A press roll used for dewatering thick-walled sheet pulp that has been strained by a pulp machine has shaft members fitted in openings at both ends of a hollow cylindrical body (roll body member) made of a corrosion-resistant alloy. With the above structure, the moisture content of the pulp sheet is squeezed out of the sheet by the action of the pressing pressure of the roll. The body member of the press roll is stress (repetitive stress) due to the load of press pressure that squeezes out the moisture of the pulp sheet.
If, squeeze out the water the contact and of (SO ₄ ^{- -} or Cl such as a strongly acidic corrosive liquid containing) are placed in the use environment to be superimposed, required that those excellent in corrosion fatigue strength To be done. Conventionally, a material of the same type as a suction roll for dewatering wet paper in the paper making process, that is, cast iron is generally used as the body member, and in addition to this, JIS 512
1 SCS11 Duplex stainless cast steel (5Ni-25Cr-2
Mo-Fe), SCS14 austenitic stainless cast steel (12Ni-18Cr-2.5Mo-Fe), or 0.05C-19Cr-5Ni-2Mo-3Cu-F.
e-type duplex stainless cast steel (Japanese Patent Publication No. 61-58548) has been used.

[0003]

When the dewatering roll (press roll) of the pulp sheet is compared with the dewatering roll (suction roll) of the wet paper, the body member of the suction roll of the wet paper has many small holes (suction holes). In contrast to this, the body member of the press roll is, unlike this, a hollow cylindrical body having no holes. Therefore, in the suction roll, the squeezed water (strongly acidic corrosive liquid) circulates not only on the outer peripheral surface of the body member but also on the inner peripheral surface through the suction hole, whereas the dewatering roll of the pulp sheet, Only the outer peripheral surface of the barrel member comes into contact with the squeezed water, and the industrial water (or tap water) supplied as cooling water only comes into contact with the inner peripheral surface side. Therefore, from the viewpoint of the corrosive environment, the use condition of the press roll is milder than that of the suction roll. However, on the other hand, the pressing pressure applied for squeezing out water is about 80 KN / mm ^{2 for} the suction roll, while it is about 280 KN / mm ² or more for the press roll. It has a high load that exceeds three times that of suction rolls.

Further, comparing the modes of roll damage occurring during the actual use of the two, in the suction roll, cracks due to corrosion fatigue starting from the peripheral portion of the suction hole tend to occur, whereas in the press roll this is Differently, cracks due to corrosion fatigue are likely to occur on the inner peripheral surface of the body member (on the side where cooling water such as industrial water contacts), and damage to the inner peripheral surface side is the main cause of roll disposal. As described above, the dewatering roll of the pulp sheet is common with the dewatering roll of the wet paper in that it is used under the overlapping action of the repeated stress and the corrosion factor, but the specific conditions of use and the form of roll damage are , It is markedly different from that of the suction roll. Therefore, with regard to press rolls, which are dewatering rolls for pulp sheets, the roll barrel member for enhancing its durability and efficiently performing dewatering operation is based on a unique viewpoint different from that of suction rolls, depending on its specific use conditions. A rational composition design should be made. In view of the above, the present invention provides an alloy having a high yield strength and a high corrosion fatigue strength, which is suitable as a body material for a press roll for dehydrating a pulp sheet.

[0005]

Means and Actions for Solving the Problems The press roll alloy for dewatering a pulp sheet according to the present invention comprises C: 0.1 to 2
%, Si: 2% or less, Mn: 2% or less, Cr: 24 to 2
7%, Ni: 3-7%, Mo: 0.5-3%, Cu: 2
%, 4% or less, N: 0.01 to 0.3%, the balance being precipitation hardening type duplex stainless cast steel consisting essentially of Fe.

The present invention will be described in detail below. The reasons for limiting the components of the alloy of the present invention are as follows. C: 0.1 to 2% C enhances the strength of the alloy by solid solution strengthening and precipitation hardening of carbides. The lower limit is set to 0.1% in order to sufficiently exhibit this effect. One of the features of the present invention is that, unlike the conventional roll material (the body material of the suction roll for dewatering wet paper) having a low C composition, high strength is achieved with a high C content. Preferably, it is 0.5% or more. However, if the C content is excessively increased, the toughness of the alloy is lowered due to the increase of the Cr carbide content, and the C content in the vicinity of the Cr carbide particles is increased.
Since the corrosion resistance decreases with the decrease of the r concentration, the upper limit is 2%. It is preferably 1.7% or less.

Si: 2% or less Si is added as a deoxidizing element in the alloy melting step, and has the effect of improving the melt flowability. The addition amount up to 2% is sufficient for obtaining this effect, and if it is added in a large amount beyond that, the toughness and weldability of the alloy are deteriorated, so the upper limit is 2%.

Mn: 2% or less Mn is added as a deoxidizing / desulfurizing element in the alloy melting process and for improving castability. The addition amount for obtaining this effect is up to 2%, and if it exceeds this amount, the corrosion resistance of the alloy is deteriorated. Therefore, it is set to 2% or less.

Cr: 24-27% Cr is a ferrite-forming element, improves the corrosion resistance of the alloy (in particular, resistance to pitting corrosion, intergranular corrosion, etc.), and forms carbides to precipitate and harden it. Increase the strength of the alloy. At least 24% is required to secure the strength and toughness by the two-phase structure and to secure the corrosion resistance. The effect increases with increasing amount, but excessive addition causes deterioration of castability and deterioration of toughness due to excessive ferrite content ratio in the two-phase structure of the alloy. Therefore, the upper limit is 27%.

Ni: 3 to 7% Ni is a strong austenite-forming element, and has a predetermined amount of austenite phase formation and stabilization in a two-phase structure.
From the viewpoint of ensuring high toughness, at least 3% is required. On the other hand, if the amount exceeds 7%, the quantitative balance of the two phases in the two-phase structure is lost, so this is the upper limit.

Mo: 0.5-3% Mo forms a solid solution in the matrix and contributes to strengthening the corrosion resistance (resistance to pitting corrosion and crevice corrosion) of the alloy and improving the strength (especially yield strength). This effect is obtained by adding 0.5% or more. The effect is increased by increasing the addition amount, but the effect is almost saturated by adding up to 3%. In addition, addition in excess of that causes defects such as embrittlement and casting cracking of the alloy due to the formation of the σ phase. Therefore, the upper limit is 3%.

Cu: more than 2% and 4% or less Cu partially strengthens the austenite phase as a solid solution, and the rest precipitates from the ferrite phase as an α'phase to strengthen the ferrite phase. Also, the addition of a large amount of Cu is effective in enhancing the corrosion resistance. In particular, in the present invention, since the amount of C is increased to greatly enhance the intergranular corrosion resistance due to the precipitation of Cr carbide, the deterioration of the corrosion resistance due to the formation of a Cr-deficient layer due to the precipitation of Cr carbide is compensated, and Cu is indispensable as an element for ensuring the corrosion resistance of Al. This effect is obtained by adding a large amount exceeding 2%. However, if it exceeds 4%, the toughness of the alloy is impaired, so this is made the upper limit.

N: 0.01-0.3% N solid-solution strengthens the alloy. This effect is exhibited by the addition of 0.01% or more. The effect increases with increasing amount, and it is effective in improving corrosion resistance, particularly pitting corrosion resistance. However, 0.3%
If it exceeds, the corrosion resistance will be deteriorated due to the increase in the amount of nitrides formed, so this should not be exceeded.

The alloy of the present invention has a two-phase structure consisting of a δ-ferrite phase and an austenite phase, the austenite phase which is a soft phase is dispersion strengthened by the δ-ferrite phase which is a hard phase, and has a high corrosion fatigue strength. . The amount ratio (area ratio) of the δ-ferraate phase in the two-phase structure is preferably about 40 to 70%. The body member of the press roll made of the alloy of the present invention is manufactured by subjecting a hollow cylindrical body by centrifugal casting to a material, subjecting it to solution treatment, and then subjecting it to precipitation strengthening heat treatment. The solution heat treatment is performed at a temperature of about 1100 to 1250 ° C. as in the case of ordinary duplex stainless steel.
It is performed by heating and holding for a suitable time (about 1 to 5 hours per 1 inch of wall thickness). Cooling from the same temperature range is preferably water cooling or forced air cooling. The precipitation strengthening heat treatment is carried out by heating and holding at a temperature of about 600 to 700 ° C. for an appropriate time (about 0.5 to 4 hours per inch of wall thickness), and then air cooling. By this heat treatment,
A part of Cu dissolved in the ferrite phase is finely dispersed and precipitated as the α'phase, and a fine carbide (mainly Cr carbide) is dispersed and precipitated. As a result of these precipitation strengthening, the strength of the alloy is Greatly enhanced.

[0015]

EXAMPLES Centrifugal casting tubes having the chemical composition shown in Table 1 (tube size after machining: outer diameter 600 mm, wall thickness 50 m
m, length 2000 mm) is heat-treated to obtain a test tube material. In the table, Nos. 1 to 7 are invention examples, and Nos. 11 to 14 are comparative examples. Among the comparative examples, No. 11 is equivalent to SCS11 duplex stainless steel, No. 12 is equivalent to SCS14 austenitic stainless steel, No. 13 is 0.05C-19 Cr-5Ni-2Mo-3Cu.
-Fe type duplex stainless cast steel (corresponding to Japanese Patent Publication No. 61-58548), No. 14 is a cast iron material.

The heat treatment conditions for each test material are as follows. Specimen Nos. 1 to 7 (invention examples): solution heat treatment 1150 ° C × 2Hr → water cooling, precipitation strengthening heat treatment 650 ° C × 2Hr → air cooling. Specimen No. 11 (SCS11): Solution heat treatment 1100 ° C × 2Hr → water cooling, no precipitation strengthening heat treatment. Specimen No. 12 (SCS14): Solution heat treatment 1100 ° C × 2Hr → water cooling, no precipitation strengthening heat treatment. Specimen No. 13 (19Cr-5Ni-2Mo-3Cu-F
e): Solution heat treatment 1100 ° C. × 2 Hr → water cooling, no precipitation strengthening heat treatment, Specimen No. 14 (cast iron): As cast.

The test results of the material properties of each test material are shown in the right column of Table 1. In the table, the corrosion fatigue strength is based on the following test. [Corrosion fatigue test] Test piece: JIS Z2274 No. 1 smooth test piece (diameter 1
0 mm), test solution: A tap water (Cl ^- 10 ppm, CaCO ₃ 50 pp
m, pH6), B whitewater _{^{(Cl - 600 ppm, SO 4}} - 100
0 ppm, pH 3.5), Testing machine: Ono bending tester. Rotation speed 3000 rp
m. (Supply of the test solution is by a general dropping method) The endurance limit (N / mm ² ) after 10 ⁸ rotations is measured.

As shown in Table 1, Invention Examples Nos. 1 to 7 are
Of course, No. 14 (cast iron material), which is a typical conventional material, is No. 1.
1 (SCS11 Duplex Stainless Cast Steel), No.12 (SCS14
Austenitic stainless cast steel), and No.13 (0.05
C-19 Cr-5Ni-2Mo-3Cu-Fe type duplex stainless cast steel), it has higher strength and improved corrosion fatigue strength against repeated action under high load.

[0019]

[Table 1]

[0020]

INDUSTRIAL APPLICABILITY The alloy of the present invention is suitable as a material constituting the body of a press roll, which is a roll for dewatering pulp sheets used under high-load repetitive action, and due to its improved high corrosion fatigue strength, The durability of the press roll is increased, and it is effective in improving the efficiency of the dehydration treatment operation of the pulp sheet.

Claims (1)

[Claims] 1. C: 0.1 to 2%, Si: 2% or less, M
n: 2% or less, Cr: 24-27%, Ni: 3-7%,
Mo: 0.5-3%, Cu: over 2%, 4% or less,
N: 0.01 to 0.3%, the balance consisting essentially of Fe. A precipitation hardening type duplex stainless cast steel for a pulp sheet dewatering roll.