JPH049451A - Seawater corrosion resisting material - Google Patents

Abstract

PURPOSE:To obtain a seawater corrosion resisting material excellent in corrosion fatigue strength and erosion resistance by specifying a composition consisting of C, Si, Mn, Cr, Ni, Mo, Nb, Ti, and Fe and forming a structure in which the amount of retained austenite phase is regulated to a specific value. CONSTITUTION:This seawater corrosion resisting material has a composition consisting of <=0.08% C, 0.1-1.5% Si, 0.1-3% Mn, 17.1-19% Cr, 4.5-7.5% Ni, 0.5-3% Mo, 0.1-0.5% Nb, 0.05-0.5% Ti, usual impurities, and the balance Fe and also has a metallic structure of martensite+austenite+ferrite phase in which austenite phase is retained by 10-35%. This seawater corrosion resisting material has high mechanical strength, toughness, and corrosion resistance and also has superior erosion resistance as well as high corrosion fatigue strength.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to seawater materials such as marine propellers used in seawater. The present invention relates to stainless steel having corrosion fatigue strength, and particularly to a seawater-resistant material that can be advantageously used as a material for impellers for propeller pumps, pump casings, runners for water turbines, and hydrofoils for high-speed boats.

[Conventional technology]

Copper alloys with excellent corrosion resistance have traditionally been used in seawater materials such as marine propellers. Even with nickel, aluminum, and bronze, which are the most commonly used materials, in the case of marine propellers, as ships become faster, the propeller blade tips suffer erosion damage and must be repaired or replaced. . Erosion is caused by the collapse of cavitation bubbles that occur during propeller rotation, and it is difficult to avoid erosion that occurs on the trailing surface of the propeller blade tip. Creating a propeller shape that makes it difficult for cavitation erosion to occur will significantly reduce propulsion efficiency. and,
Erosion also occurs in pumps, impellers, runners, and hydrofoils.

Countermeasures against this erosion include: ■Replacement, ■Remove and reshape the damaged part, but even if these replacements and repairs are carried out, the cost of docking the damaged ship, the cost of replacement and repair, and the cost of the crew due to the body of the ship. There are labor costs, losses and burdens on shippers, etc., and problems arise in which costs increase.

Furthermore, in order to reduce the weight of equipment such as propellers and pump impellers, it is desired to develop materials with excellent corrosion fatigue strength in seawater. In response to this, 12% Cr martensitic cast stainless steel was considered as a stainless steel material, but although it lacked sufficient corrosion resistance and had good mechanical properties, it had drawbacks such as pitting corrosion. Ta.

Also, austenitic stainless cast steel with 18% C [-8 to 12% N1 has good corrosion resistance, but mechanical properties of 12%
%Cr-based stainless steel.

The 25Cr-2ONi two-phase alloy used for heat exchange equipment and the like has excellent corrosion resistance as a forged or rolled material. However, as large castings, corrosion resistance decreases due to slow cooling embrittlement during quenching and cooling.

As described above, there is no known stainless steel material that has all of corrosion resistance, strength, and erosion resistance.

[Problem to be solved by the invention]

In view of the above technical level, the present invention has been developed for large casting materials used in seawater, such as marine propellers and impellers, which not only have high mechanical strength, toughness, and corrosion resistance, but also have particularly high corrosion fatigue strength and excellent erosion resistance. The aim is to provide materials.

[Means to solve the problem]

The present invention includes C: 0.08% or less, Si: 0.1 to 15%,
Mn: 0.1 to 3%, Cr: 17.1 to 1
9%, Ni: 4.5-7.5%, Mo: 0.5
~3%, Nb:Oi~0.5%, Ti:0.05~0.
Contains 5% and normal impurities, the balance consists of iron,
It is a seawater-resistant material with excellent corrosion fatigue strength and erosion resistance, and has a metal structure with martensite, austenite, and ferrite phases, with an austenite phase content of 10 to 35% remaining, and has higher hardness than current copper alloy materials. It has excellent erosion resistance by approximately 1.5 times, and its corrosion fatigue strength in seawater is approximately 1.5 times higher, so it is smaller and more compact than the currently used prepared alloy material.
This makes it possible to produce lightweight, highly efficient seawater equipment (propellers, etc.) and contributes to energy savings.

Nickel, aluminum, and bronze, which have the best erosion resistance among conventional copper alloys, have a Brinell hardness of only 150 to 160, which affects lotionability.
Excellent for use in environments with severe cavitation in seawater.
I can't expect two lotions.

On the other hand, the Brinell hardness of the material of the present invention is 217 to 325.
In addition, due to the work hardening of the retained austenite phase that remains in the metallographic structure, the erosion resistance has been improved to approximately 1.5
This is an improvement of more than double. In addition, the tensile strength has been increased to over 95 kgf/mm2 compared to 66 kgf/mm2 for nickel, aluminum, and bronze, and the corrosion fatigue strength in seawater has been improved by more than 1.5 times. The material that made high corrosion fatigue strength possible was the addition of N1 equivalent (Ni +
WMn) and each component was adjusted so that the metallographic structure became a martensite, decaustenite + ferrite phase.

In the heat treatment of the material of the present invention, the material is air-cooled at 900 to 1000°C after being cast. The metal structure is mainly a martensitic phase, and the austenite content is adjusted within 10 to 35% (x-ray measurement) in order to lower the yield strength and improve erosion resistance. If the amount of austenite is less than 10%, no decrease in yield strength can be expected, and if it exceeds 35%, yield strength (0, 2
%) is as low as about 20 kgf/mm', and good mechanical properties cannot be obtained.

The ferrite phase is necessary to maintain good weldability of the material of the present invention, but if it exceeds 30%, the impact value will decrease, so it is preferable that the ferrite phase is less than 30%.

Furthermore, the presence of 10 to 35% of retained austenite phase significantly improves erosion resistance due to processing transformation of the retained austenite phase when used under cavitation.

In this way, the chemical composition and metal structure of the material of the present invention can be adjusted to optimal conditions to obtain excellent properties such as corrosion resistance, corrosion fatigue strength, erosion resistance, and weldability that have not been obtained previously. It has been made possible.

The reason why the alloy composition of the material of the present invention was specified is as follows.

Since C nichrome carbide is formed and precipitated at grain boundaries, reducing corrosion resistance in seawater and lowering erosion resistance and corrosion fatigue strength, its content is set to 0.08% or less.

Sl: It is necessary to add 0.1% or more as a deoxidizing agent during melting, but if it exceeds 1.5%, it will cause embrittlement, so the upper limit is set at 1.5%.

Like Mn:Si, it is necessary to add 0.1% or more as a deoxidizing agent, but if it exceeds 3%, it becomes brittle, so the upper limit is set at 3%.

Cr: The most important element for ensuring corrosion resistance in this invention, and the corrosion resistance improves as the amount of Cr increases. In addition, since 17.1% or more is necessary to maintain good corrosion resistance, the lower limit is set to 17.1% or more and the upper limit is set to 19%.

When the Nl nichrome addition is in the range of 17.1-19% as mentioned above, a minimum of 4.5% Ni is required to obtain good impact values. On the other hand, if the Ni content exceeds 7.5%, the austenite content will increase and cause a decrease in yield strength, so the upper limit is set to 7.5%.

MO=An element effective in improving corrosion resistance, especially preventing pitting corrosion, and needs to be added in an amount of 0.5% or more to exhibit its effect. However, if it exceeds 3%, the strength will be low and embrittlement will be significant, so the upper limit is set at 3%.

Nb: Nb fixes C as Nb carbide, suppresses the precipitation of Cr carbide that causes a decrease in corrosion resistance, and is effective in improving corrosion resistance. To exhibit this effect, it is necessary to add 0.1% or more, and if it exceeds 0.5%, the impact value decreases, so the upper limit is set at 0.5%.

Ti: Like Nb, it is an element that fixes C, and when added in combination with Nb, it has a great effect on preventing the precipitation of Cr carbides. Since it is added in combination with Nb, the minimum amount is 0.
0.05% is sufficient, and if the amount of Ti is too large, the castability deteriorates, so the upper limit is set to 0.5%.

〔Example〕

Next, the manufacturing method for the material of the present invention will be explained, and the present invention will be explained in detail.

Table 1 shows the chemical components of the inventive materials and comparative materials, where test materials Nα1 to Nα4 are inventive materials and test materials Nα5 to
Nα10 is a comparative material, and Nα11 is a copper alloy (copper alloy, aluminum, bronze) currently most commonly used as a seawater material, and its chemical composition is shown in Table 2. Table 3 also shows the mechanical properties, metallographic structure, and amount of retained austenite of these test materials. Furthermore, the seawater corrosion fatigue strength and erosion loss were evaluated in the fourth study, along with the present invention material, comparative material, and current material.
Shown in the table.

The corrosion fatigue test was carried out using a Wella rotary bending fatigue tester.
The test was carried out using a test piece with a diameter of 6° Omm, a rotation speed of 345 rpm, a test repetition rate of 2×101 times, natural seawater as the test liquid, and a test temperature of room temperature.

The erosion test was conducted using an ultrasonic erosion tester under the conditions of a frequency of 6.5 KHz, an amplitude of 95 μm, a test liquid of seawater, a test temperature of 25° C., and a test time of 2 hours.

The amount of austenite in the metal structure was measured by the xIl method.

The following is clear from Tables 1, 2, 3, and 4.

The corrosion fatigue strength of the No.5 to Nα10 test materials in seawater, which have chemical components other than the materials of the present invention, is 23 kgf/mm' or less, which is significantly lower than the 30 to 32 kgf/mm2 of the materials of the present invention. The Nα5 sample has Ni and Nb content, the NCL6 sample has C, Mo, Cr, and Mn content, the Nα7 sample has Cr and Ti content, and the Nu8 sample has [', Cr, T
i amount, No. 9 The sample material has Ni, Ti amount, N(11
The 0 sample material had both Cr and Nb contents other than those of the present invention material.

It is said that not only the hardness of the test material but also the metal structure has a large influence on the erosion resistance. 18%Cr-8%N1 Austenitic stainless steel, which is easily transformed into martensitic material due to work hardening, has good one-step zero-strength properties and is used for repairing water turbines and the like.

However, austenitic stainless steel has a tensile strength of 80
High corrosion fatigue strength cannot be expected at kgf/mm2 or less.

Also, although martensitic stainless steel has high strength, its erosion resistance is slightly inferior to austenitic stainless steel.

Therefore, in the material of the present invention, 10 to 35% of retained austenite is left in the base of the martensitic phase in order to solve the drawbacks of the austenitic and martensitic materials.

The reason why the austenite amount is limited to 10 to 35% is 10
If it is less than %, there is no effect of improving erosion resistance due to deformation of the austenite phase, and the yield strength is 70 kgf/+n.
This is because the austenite content is higher than 35% and cracks are likely to occur when cutting by fusing.Although the lotion properties are improved when the austenite content exceeds 35%, the yield strength is 22 kgf/mm2.
This is because high corrosion fatigue strength cannot be ensured.

Table 4 shows sample material Nα6 (100% martensite),
Sample material Nα8 (austenite 7 in martensite)
．． 1%) and the comparison material Nα7 (austenite in martensite) 40.5%) are shown in comparison with the austenite content and seawater fatigue strength.

The erosion amount of the test material Nα8 (austenite 7.1%) was 23.2■, compared to the present invention material (austenite content 10%).
~35%), which is 10.5~16.0■.

Above, the chemical composition and the metal structure are martensite, ten austenite and ferrite phases, and the amount of austenite is 1.
The rationale for limiting the rate to 0-35% has become clear.

As is clear from these results, the material of the present invention has a mechanical property of 95 kgf/mm2 or more and a corrosion fatigue strength of 30 k.
gf/nun'' and cavitation erosion resistance required as a seawater-resistant material, which is superior to the currently used nickel, aluminum, and bronze.

In addition, the present invention material can be used for marine propellers, various pump impellers,
Can be used for hydrofoils for high-speed boats, etc.

〔Effect of the invention〕

According to the present invention, a material having high corrosion fatigue strength and excellent erosion resistance in seawater can be obtained, so seawater equipment (propellers, impellers, hydrofoils, etc.)
By manufacturing this, it is possible to obtain equipment that exhibits high strength, excellent corrosion resistance, and erosion resistance, and is lightweight and capable of increasing efficiency, and its industrial effects are extremely large.

Claims (1)

[Claims] C: 0.08% or less, Si: 0.1 to 1.5%, Mn:
0.1-3%, Cr: 17.1-19%, Ni: 4.5
~7.5%, Mo: 0.5~3%, Nb: 0.1~0.
5%, Ti: 0.05-0.5% and normal impurities, the balance is iron, and the metal structure has martensite + austenite + ferrite phases, with an austenite phase amount of 10-35% remaining. A seawater-resistant material that is characterized by: