JPH03146641A - Duplex stainless cast steel for manufacturing apparatus for wet type phosphoric acid - Google Patents

Abstract

PURPOSE:To obtain the duplex stainless cast steel having excellent corrosion resistance and erosion resistance to phospho-gypsum slurry advantageously from the viewpoint of economicity by forming it from the compsn. contg. each prescribed amt. of C, Si, Mn, P, S, Ni, Cr, Mo, Cu and N and in which the total content of Cr and Mo is regulated. CONSTITUTION:The above stainless cast steel contains, by weight, <=0.08% C, <=2.0% Si, <=2.0% Mn, <=0.04% P, <=0.03% S, 5 to 10% Ni, 28 to 36% Cr, 2 to 5% Mo, 0.5 to 3.0% Cu, 0.10 to 0.30% N and the balance substantial Fe. In this case, the formula of Cr(%)+2.5XMo(%)=>=38(%) shall be satisfied. Then, in addition to the above characteristics, the stainless cast steel has toughness and is inexpensive compared to an Ni-base high alloy.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to duplex stainless steel cast steel used in wet phosphoric acid production equipment that contains many corrosive impurities.

(Prior art) In the wet process of producing phosphoric acid, which is normally produced using sulfuric acid, many of the impurities contained in phosphate rock (chlorides, butrides, sulfates, oxides, etc.) are mixed into the phosphoric acid as they are. to form a phosphogypsum slurry. Phosphate gypsum slurry contains so-called corrosive impurities and abrasive impurities that adversely affect corrosion resistance and erosion resistance.
This accelerates corrosion and wear of materials for wet phosphoric acid production equipment, so a material that has both corrosion resistance and erosion resistance is required for phosphogypsum slurry.

In conventional wet phosphoric acid production equipment, phosphoric acid with many corrosive impurities is replaced with Ni-based alloy materials such as Hastelloy alloy and Illium alloy, or JIS 5CSI4 (18Cr-1
2Ni-2,5Mo-Fe), 5C523 (20C
A high Ni-based austenitic stainless steel alloy material such as r-28Ni-2Mo-3Cu-Fe) is used.

The former is not only expensive because it contains a large amount of Mo and Ni, but also has poor erosion resistance and short service life when used with phosphogypsum slurry that contains many abrasive impurities, while the latter uses phosphoric acid that contains many corrosive impurities. Since corrosion resistance and erosion resistance against gypsum slurry are insufficient, there is a need for the development of an economical stainless steel cast steel with excellent corrosion and wear resistance.

(Problems to be Solved by the Invention) The present invention solves the problems with conventional corrosion-resistant materials used in the manufacturing process of wet phosphoric acid, which contains many corrosive impurities. Our objective is to provide a duplex stainless steel cast steel that is economically advantageous and has excellent corrosion resistance and erosion resistance against phosphate gypsum slurry, which we discovered as a result of intensive research from the perspective of dealing with corrosion. .

(Means and effects for solving the problem) The present invention is based on C
In r-Ni-Mo-Cu-N-Fe stainless steel, the combined addition of Cr and Mo has a remarkable effect on improving the corrosion resistance and erosion resistance against phosphate gypsum slurry, and the addition of W and Co This addition was made based on the knowledge that it is more effective in improving corrosion resistance.

The duplex stainless steel cast steel of the present invention has C: 0.08wt%
Hereinafter, Si: 2.0wt% or less* Mn' 2.0w
t% or less.

P: 0.04wt% or less, S: 0.03wt% or less, N
i: 5-10wt%, Cr: 28-36wt%1
Mo: 2 to 5 wt%, however, Cr (wt%) + 2
．． 5 × Mo (wt%) = 38 (wt%) or more, C
u: 0.5-3. 0wtχ, N: 0.10-0.
30 wt%, the remainder consists of iron and unavoidable impurities, and if desired, W: 0.5 to 2.
Co: 0.1 to 1.0 wt%.

Hereinafter, the reasons for limiting the components of the duplex stainless steel cast steel of the present invention will be explained in detail.

C is an element that is inevitably mixed in during the melting process of stainless steel cast steel, but when the C content increases, chromium carbide (Cr
Since corrosion resistance and toughness are deteriorated due to precipitation of zsCh), the content is set to 0.08% or less.

Si: An element necessary as a deoxidizing agent for molten metal and effective in improving castability, but since it causes deterioration of material properties, the upper limit is set at 2.0%.

Mn: An element that has a deoxidizing and desulfurizing effect on molten metal and is effective in improving castability, but if it is included in a large amount, corrosion resistance will decrease, so the content should be 2.0% or less.

P: To increase corrosion resistance, the content should be 0.04%.
The following shall apply.

S: In order to increase corrosion resistance, its content should be 0.03%.
The following shall apply.

Ni is a niostenite forming element and is an element necessary for determining the phase ratio of ferrite phase and austenite phase. N to ensure excellent toughness and corrosion resistance
The amount of i is 5% or more. If it is included in a large amount, the amount of ferrite increases, the balance of the phase ratio between the ferrite phase and the austenite phase is lost, and the corrosion resistance deteriorates, so the upper limit is set at 10%. The amount of ferrite is set to 3 in order to maintain high levels of corrosion resistance and erosion resistance, and to prevent toughness deterioration.
It is preferably 0 to 70%, and needs to be adjusted according to the content of elements such as CR and base 0.

Cr: A ferrite-forming element, which is effective in improving the corrosion resistance and erosion resistance of the ferrite phase.

In addition, in order to ensure excellent corrosion resistance and erosion resistance, Cr1l is set to 28% or more, and the amount of ferrite is set to 3.
It is preferable to adjust it to 0 to 70%, and the C
In order to increase the value of r+2.5Mo to 38% or more, Cr1
l is defined as 28% or more. In order to maintain a high level of corrosion resistance and erosion resistance of duplex stainless steel cast steel, Cr
A combined increase in the amount of Cr and Mo is effective, and the effect of Mo is particularly large, and when the value of Cr+ 2.5 Mo is 38% or more, corrosion resistance and erosion resistance become noticeable. If it is included in a large amount, the toughness deteriorates, so the upper limit is set at 36%.

MO=a ferrite-forming element, which, like Cr, has an excellent effect on improving corrosion resistance and erosion resistance, and particularly has a remarkable effect on improving crevice corrosion resistance and pitting corrosion resistance. In highly corrosive phosphogypsum slurry, 2.0
% or more, and in the case of compound increase with Cr, the effect becomes remarkable when the value of Cr+2.5Mo is 38% or more.

If it is contained in a large amount, the toughness will deteriorate, so the upper limit should be set at 5.
Set to 0%.

Cu is a niostenite forming element and is effective in solid solution strengthening of the austenite phase and improving corrosion resistance. To obtain this effect, at least 0.5% is required. If it is contained in large amounts, it will reduce the strength and toughness, so the upper limit should be set at 3.
．． Set to 0%.

N is a niostenite-forming element and is effective in improving the corrosion resistance of austenite. It is also effective in solid solution strengthening of the austenite phase and improvement in erosion resistance.

In order to obtain this effect, 0.10% or more is required, but
If it exceeds 0.30%, blowholes will occur. Therefore 0
．． 1O~0.30%.

W: A ferrite-forming element, which is effective in improving corrosion resistance and erosion resistance. In order to obtain this effect, 0.5% or more is required, and if it exceeds 2.0%, the effect approaches a saturated state. Therefore, it is set at 0.5 to 2.0%.

It has the effect of improving the corrosion resistance and erosion resistance of the CO niostenite phase, and for that purpose it is necessary to use at least 0
．． 1% is required, but if it exceeds 1.0%, the effect approaches saturation. Therefore, it is set to 0.1 to 1.0%.

(Example) Next, the material properties of the duplex stainless steel cast steel of the present invention will be explained using examples.

Table 1 shows the chemical composition of the steels tested in the examples. Table 2 shows the mechanical properties. Table 3 shows the corrosion resistance in crude phosphoric acid, the corrosion abrasion resistance in gypsum slurry and the corrosion resistance in the reactor of a phosphoric acid production plant.

No. 1 to N[11B of the test steels in Table 1 are duplex cast stainless steels, Nα19 and Nα20 are J
IS SC514, 5CS23, IS21 are Hastelloy C
It is.

All sample steels were subjected to solution treatment.

Example 1 Corrosion tests were carried out using highly concentrated crude phosphoric acid (hos: 43.5%, HzSOi) obtained in the initial process of phosphoric acid production.
:5.35%, F:0.80%, ct-:0.3Q
%, Fe”: 1.17% 9M3°: 0.64%
) was heated to 90°C in a constant temperature water bath, and the test piece was immersed for 50 hours to determine the corrosion weight loss.

Calculate the corrosion rate (g/m”・hr) from this corrosion loss,
Corrosion resistance was evaluated.

Example 2 For the corrosion wear test, a method using a rotating disk was adopted. A gypsum slurry containing 28.7% by weight of fine-grained gypsum in the crude phosphoric acid was heated to 90°C in an abrasion test tank, and after mounting the test piece on a rotating disk, This rotating disk was continuously rotated at a circumferential speed of 1.17 m/s. This test was repeated eight times and the amount of wear on the test piece was measured. The weight change rate was determined by dividing the amount of wear by the weight of the initial test piece.

Example 3 In a corrosion test when immersed in a reaction tank of a manufacturing plant, the base metal Nα1. Nα3. Nα7 and conventional material Na
l9. k20. N1121 was used. These test pieces were immersed for 23 days in a reaction tank in which a crude phosphogypsum slurry having the same composition as in the corrosion wear test was circulated.

Note that the temperature of the slurry liquid is maintained at 90 to 93°C.

As is clear from the above test results, the alloy of the present invention NILI
~t4a7 has high strength and hardness, and at the same time has extremely good general corrosion resistance against crude phosphoric acid and extremely excellent corrosion wear resistance.
It can be seen that it has superior material properties compared to Hastelloy C.

Table 2 Mechanical Properties Table 3 Corrosion Resistance and Corrosion Wear Resistance (Effects of the Invention) The duplex stainless steel cast steel of the present invention has excellent corrosion resistance and erosion resistance, and is tougher than the Ni-based alloy. It also has the economical effect of being inexpensive and can be used in places with harsh operating conditions, such as wet phosphoric acid production equipment with many corrosive impurities, especially where the operating temperature is high and the concentration of corrosive impurities is high. It has the greatest effect on improving service life.

Claims (3)

[Claims] (1) C: 0.08 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, P: 0.04 wt% or less, S: 0.03 wt% or less, Ni: 5 to 10 wt%, C
r: 28-36 wt%, Mo: 2-5 wt%, however, C
r (wt%) + 2.5 × Mo (wt%) = 38 (wt%
) or more, Cu: 0.5 to 3.0 wt%, N: 0.10 to
A duplex stainless steel cast steel for wet phosphoric acid production equipment, characterized in that it contains 0.30 wt% and the remainder consists of iron and inevitable impurities. (2) C: 0.08 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, P: 0.04 wt% or less, S: 0.03 wt% or less, Ni: 5 to 10 wt%, C
r: 28-36 wt%, Mo: 2-5 wt%, however, C
r (wt%) + 2.5 × Mo (wt%) = 38 (wt%
) or more, Cu: 0.5 to 3.0 wt%, N: 0.10 to
Contains 0.30 wt%, W: 0.5 to 2.0 wt%,
A duplex stainless steel cast steel for wet phosphoric acid production equipment, characterized in that the remainder consists of iron and unavoidable impurities. (3) C: 0.08 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, P: 0.04 wt% or less, S: 0.03 wt% or less, Ni: 5 to 10 wt%, C
r: 28-36 wt%, Mo: 2-5 wt%, however, C
r (wt%) + 2.5 × Mo (wt%) = 38 (wt%
) or more, Cu: 0.5 to 3.0 wt%, N: 0.10 to
0.30wt%, W: 0.5-2.0wt%, Co: 0
．． 1 to 1.0 wt% of the duplex stainless steel cast steel for wet phosphoric acid production equipment, the remainder being iron and unavoidable impurities.