JPH09209087A - Duplex stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a duplex stainless steel combining excellent impact resistance with corrosion resistance. SOLUTION: This duplex stainless steel has a composition consisting of, by weight, 20-35% Cr, 3-12% Ni, 0.5-10% Mo, 2-8% W, 0.01-2% Co, 0.1-5% Cu, 0.05-0.5% N, <=0.12% C, <=1% Si, <=2% Mn, one or more elements among Ti, Nb, and V or one or more kinds among rare earth elements by <=0.2%, and the balance iron with inevitable impurities.

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 used for a device member or the like which requires high strength and high corrosion resistance.

[0002]

2. Description of the Related Art There are many types of applications that require materials having high strength and high corrosion resistance, such as water turbine runners used for hydroelectric power generation in rivers and excavator parts used under severe conditions in various mines.

Conventionally, austenitic stainless steels such as SUS304 and SUS316 have been used for hydraulic turbine runners used for hydroelectric power generation in rivers and the like.

By the way, in recent years, deterioration of water quality such as acidification of river water and increase of chlorine concentration has been remarkable due to acid rain and domestic wastewater. Along with such environmental changes, austenitic stainless steels that have hitherto been used are often subject to corrosion called stress corrosion cracking leading to fracture.

Ferritic stainless steel is known as a stainless steel having excellent resistance to stress corrosion cracking. However, although ferritic stainless steel hardly undergoes stress corrosion cracking, its impact resistance is extremely poor.

Most of the duplex stainless steels composed of a ferritic phase and an austenitic phase have been developed so far for the purpose of improving tensile strength, proof stress and corrosion resistance. Has intermediate properties of steel. For example, the stress corrosion cracking resistance is slightly inferior to ferritic stainless steel, but is superior to austenitic stainless steel. The impact resistance is superior to ferritic stainless steel but inferior to austenitic stainless steel.

Therefore, there has been a demand for the development of a duplex stainless steel having excellent impact resistance as well as corrosion resistance for a turbine runner material.

[0008] Conventionally, ordinary steel that has been subjected to cathodic protection and inhibitor treatment has been used for excavators used under severer conditions in various mines. However, by performing the anticorrosion treatment as described above, the cost becomes high, and the cost is comparable to the cost of using expensive stainless steel. Therefore, the motivation for using stainless steel for excavator parts has increased, and particularly duplex stainless steel having excellent mechanical properties has come to be used.

However, the excavation environment for mine development in recent years has become more severe, and for example, it is often used in a hydrogen sulfide environment such as a hot spring and a chloride ion environment such as seawater.

Therefore, it is required to develop a duplex stainless steel having excellent impact resistance and corrosion resistance even for excavator parts.

However, as a duplex stainless steel excellent in corrosion resistance such as stress corrosion cracking resistance and crevice corrosion cracking resistance,
Cr: 20-35% by weight, Ni: 3-12% by weight, M
o: 0.5 to 10% by weight, W: 1 to 3% by weight, Co:
0.01 to 2% by weight, Cu: 0.1 to 5% by weight, N:
0.05-0.5% by weight, C: 0.12% by weight or less, S
Some of the inventors of the present invention have previously proposed a material containing i: 1% by weight or less and Mn: 2% by weight or less, with the balance consisting of iron and unavoidable impurities (Japanese Patent Laid-Open No. 8-13094; The previous proposal). In the following,% indicated by showing the composition means% by weight.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a duplex stainless steel having both excellent impact resistance and corrosion resistance.

[0013]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of earnest research, in addition to adding a large amount of W to the previously proposed duplex stainless steel, one of Ti, Nb and V was added.
They have found that the addition of one or more species is effective in further significantly improving the impact resistance without lowering the corrosion resistance, and arrived at the first present invention. That is, the first aspect of the present invention is: Cr: 20 to 35%, Ni: 3 to 12%, M
o: 0.5-10%, W: 2-8%, Co: 0.01-
2%, Cu: 0.1 to 5%, N: 0.05 to 0.5%,
C: 0.12% or less, Si: 1% or less, Mn: 2% or less, and one or more of Ti, Nb and V: 0.
It is a duplex stainless steel containing 2% or less and the balance iron and unavoidable impurities.

In addition, the present inventors have found that the addition of W
The addition of one or more rare earth elements is similar to one or more of Ti, Nb and V in the first aspect of the present invention.
The present invention has been found to be effective in further significantly improving impact resistance without lowering corrosion resistance, and has reached the second present invention. That is, the second aspect of the present invention is Cr: 20 to 35.
%, Ni: 3 to 12%, Mo: 0.5 to 10%, W: 2
~ 8%, Co: 0.01-2%, Cu: 0.1-5%,
N: 0.05 to 0.5%, C: 0.12% or less, Si:
A duplex stainless steel containing 1% or less, Mn: 2% or less, and one or more rare earth elements: 0.2% or less, and the balance being iron and inevitable impurities.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components and composition of the duplex stainless steel of the present invention will be described.

Cr is an element that contributes to the improvement of corrosion resistance and the strength due to the formation of a ferrite phase. If it is less than 20%, the corrosion resistance decreases, and if it exceeds 35%, the toughness decreases.

Ni is an element that stabilizes the austenite phase and improves toughness and general corrosion resistance. If it is less than 3%, the austenite phase disappears, and if it exceeds 12%, the amount of the austenite phase increases and the strength decreases.

Mo is an element that contributes to pitting corrosion resistance,
If it is less than 0.5%, sufficient pitting corrosion resistance cannot be obtained, and if it exceeds 10%, it becomes brittle.

W is an element that contributes most to the improvement of impact resistance, and is an especially important element in the present invention. In particular, the impact resistance is significantly improved in the coexistence of one or more of Ti, Nb, and V described later, or in the presence of one or more rare earth elements. It also improves pitting corrosion resistance in the coexistence with Mo. 2
If it is less than 8%, it has no effect on impact resistance, and if it exceeds 8%,
Corrosion resistance such as stress corrosion cracking resistance and crevice corrosion resistance decreases.

Co is an element that improves crevice corrosion resistance and concentrates in the austenite phase to improve elongation. If it is less than 0.01%, the crevice corrosion resistance is not sufficient, and if it exceeds 2%, the yield strength decreases.

Cu improves the corrosion resistance in a non-oxidizing environment, especially the general corrosion resistance. If it is less than 0.1%, its contribution is not sufficient, and if it exceeds 5%, it causes cast segregation and causes a decrease in strength. .

N dissolves in the austenite phase as a solid solution and remarkably improves pitting corrosion resistance and crevice corrosion resistance. If less than 0.05%, the effect is not sufficient, and if more than 0.5%, Cr
It forms a compound with and, on the contrary, reduces corrosion resistance.

C is an austenite phase forming element.
If it exceeds 0.12%, Cr carbides are precipitated and ductility, toughness, and corrosion resistance are reduced.

Si is an element necessary for deoxidation and desulfurization of molten steel during steel melting. If it exceeds 1%, the strength and corrosion resistance decrease.

Mn is an element necessary for deoxidation and desulfurization of molten steel during steel melting. If it exceeds 2%, the corrosion resistance decreases.

Ti, Nb and V fix C, which has a bad influence on corrosion resistance and toughness, as a carbide and improve the corrosion resistance and toughness. At least one of Ti, Nb and V is preferably added in an amount of 0.05% or more and 0.2% or more.
If it exceeds 1.0, precipitates will be formed, and both toughness and corrosion resistance will decrease.

The rare earth element fixes S, which deteriorates toughness and corrosion resistance, as a sulfide and improves toughness and corrosion resistance. One or more rare earth elements are preferably added in an amount of 0.05% or more, and when it exceeds 0.2%, precipitates are formed,
Both toughness and corrosion resistance decrease.

[0028]

【Example】

[Examples 1 to 7 and Comparative Examples 1 to 4] The molten metal melted to a predetermined composition in a high-frequency atmospheric melting furnace was cast into a cylindrical mold of a vertical centrifugal casting device having a diameter of 50 cm rotating at a rotation speed of 750 rpm. A ring-shaped ingot having a wall thickness of 15 cm was prepared. Then, the ingot was heat-treated at 1150 ° C. for 6 hours. Tensile test pieces, impact test pieces, and corrosion resistance test pieces were cut out from the obtained ingot after the heat treatment.

The tensile test is conducted according to JIS Z2241.
According to JIS, a No. 14A test piece of JIS Z 2201 was used. From the results, 0.2% proof stress and elongation were measured.

The impact test conforms to JIS Z 2242 and uses JIS Z 2202 No. 4 test piece, and
The Charpy impact value at ° C was measured.

The corrosion resistance test evaluates crevice corrosion resistance. Two test pieces cut out to a size of 30 mm × 10 mm × 5 mm are bolted together to form an aqueous solution of 5% sulfuric acid and 5% hydrogen sulfide at 30 ° C. After immersing in and artificial seawater for 120 hours, the presence or absence of crevice corrosion was observed.

The chemical analysis values (%, balance: iron) of each test piece are shown in Tables 1-1 and 1-2, and the test results obtained are shown in Table 2.

[0033]

[Table 1]

[0034]

[Table 2]

From Tables 1-1, 1-2 and 2, the following can be seen. That is, in each of Examples 1 to 7,
It contains at least one of Ti, Nb and V. And in Examples 1-7, all contain W, Ti,
Compared with Comparative Example 1 containing no Nb and V (previously proposed duplex stainless steel) and Comparative Example 2 containing Ti and V and containing a small amount of W, the corrosion resistance was the same, but the impact value was It has improved significantly.

Further, in Comparative Example 3 in which the amount of W added was too large, the impact value was high, but the corrosion resistance was poor. Furthermore, in Comparative Example 4 in which the amount of Nb added is large, both the impact value and the corrosion resistance are lowered.

[Examples 8 to 11 and Comparative Examples 5 to 7] Example 1 except that the ingot was heat-treated at 1150 ° C for 2 hours.
A tensile test, an impact test and a corrosion resistance test were conducted in the same manner as in. In order to obtain a molten metal having a predetermined composition, a rare earth element such as misch metal (Ce: 45%,
La: including 30%) was used.

The chemical analysis values (%, balance: iron) of each test piece are shown in Table 3-1 and Table 3-2, and the test results obtained are shown in Table 4.

[0039]

[Table 3]

[0040]

[Table 4]

The following can be seen from Table 3-1, Table 3-2 and Table 4. That is, Examples 8 to 11 all contain one or more rare earth elements. Then, in each of Examples 8 to 11, as compared with Comparative Example 5 containing W and containing no rare earth element (the previously proposed duplex stainless steel), the corrosion resistance was not changed, but the impact value was significantly increased. Has improved.

Comparative Example 6 containing a large amount of rare earth element
In, the impact value and the corrosion resistance are both reduced. Furthermore, W
In Comparative Example 7 in which the addition amount of Al is too large, the impact value is high, but the corrosion resistance is poor.

[0043]

According to the present invention, it is possible to provide a duplex stainless steel having both excellent impact resistance and corrosion resistance.

Claims (2)

[Claims] 1. Cr: 20 to 35% by weight, Ni: 3-1
2% by weight, Mo: 0.5-10% by weight, W: 2-8% by weight, Co: 0.01-2% by weight, Cu: 0.1-5% by weight, N: 0.05-0. 5% by weight, C: 0.12% by weight
Hereinafter, Si: 1 wt% or less, Mn: 2 wt% or less, and one or more of Ti, Nb and V: 0.2 wt%
Duplex stainless steel containing the following, with the balance consisting of iron and inevitable impurities. 2. Cr: 20 to 35% by weight, Ni: 3-1
2% by weight, Mo: 0.5-10% by weight, W: 2-8% by weight, Co: 0.01-2% by weight, Cu: 0.1-5% by weight, N: 0.05-0. 5% by weight, C: 0.12% by weight
Hereinafter, a duplex stainless steel containing Si: 1 wt% or less, Mn: 2 wt% or less, and one or more rare earth elements: 0.2 wt% or less, with the balance being iron and inevitable impurities.